1
•A
E.BIBL . RADCL.
•
THORNTO
Books
11 The
THE LIBRARY
OF
THE UNIVERSITY
OF CALIFORNIA
PRESENTED BY
PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID
TWO ESSAYS,
LONDON :
I'HINTLJ) TIY THOMAS DAMSON, WHITEFRIAUS.
TWO ESSAYS:
ONE
UPON SINGLE VISION WITH TWO EYES;
THE OTHER
ON DEW.
A LETTER
TO THE
RIGHT HON. LLOYD, LORD KENYON
AND
AN ACCOUNT
OF
A FEMALE OF THE WHITE RACE OF MANKIND,
PART OF WHOSE SKIN RESEMBLES THAT OF A NEGRO;
WITH
. SOME OBSERVATIONS ON THE CAUSES OF THE DIFFERENCES IN
COLOUR AND FORM BETWEEN THE WHITE AND NEGRO
RACES OF MEN.
BY THE LATE WILLIAM CHARLES WELLS,
M.D. F.B.S. L. & E.
WITH
A MEMOIR OF HIS LIFE,
WRITTEN BY HIMSELF.
LONDON :
PRINTED FOR ARCHIBALD CONSTABLE AND CO. EDINBURGH,
LONGMAN, HURST, REES, ORME, AND BROWN,
AND HURST, ROBINSON, AND CO. LONDON.
1818.
rr
QII3
TO
MATTHEW BAILLIE, M.D. F.R.S. L.&E.
THIS VOLUME,
A MEMORIAL OF THEIR COMMON FRIEND,
IS INSCRIBED,
WITH MUCH RESPECT, ESTEEM, AND AFFECTION,
BY THE EDITOR.
MEMOIR OF THE LIFE
OF
WILLIAM CHARLES WELLS, M.D.
WRITTEN BY HIMSELF.
Al&v dp&evsiv, xou vifeipoxpv
MEMOIR.
I WAS born in Charles town, South Carolina,
in May, 1757, being the second son, but
fourth child, of Robert and Mary Wells,
both natives of Scotland. My mother bore
many children afterwards, none of whom
lived more than a few years, except one,
a daughter, who now resides in London ;
my brother died about twenty years ago;
my two eldest sisters survive.
My father and mother came to Carolina
in 1753 ; but a mercantile scheme which
he was then pursuing having failed, he
took to the business of a bookseller and
bookbinder, to which he had been bred
when a youth in Dumfries. He soon after-
wards added to these occupations, that of
VH1 MEMOIR.
a printer of a newspaper, for which he was
well qualified from his previous educa-
tion, being a good Latin scholar, and par-
ticularly well read in history and the
belles lettres; he had, besides, studied his
own language grammatically, and wrote
it with great correctness and purity. In
these new employments, both character
and ease of circumstances were acquired
by him ; in consequence of the latter, he
was enabled to send my elder brother,
nearly five years older than myself, to a
considerable grammar-school at Dumfries,
which was then kept by a Mr. George
Chapman.
I was always my father's favourite, and
he, fearing that I should become tainted
with the disloyal principles which began
immediately after the peace of 1763 to
prevail throughout America, obliged me
to wear a tartan coat, and a blue Scotch
bonnet ; hoping, by these means, to make
MEMOIR. IX
me consider myself a Scotchman. The
persecution I hence suffered produced this
effect completely.
This object was afterwards promoted
by sending me to Dumfries school before
I was eleven years old. I remained at it
nearly two years and a half, at the expira-
tion of which time, I had finished the
course of studies usually pursued there.
His correspondent in Scotland then sent
me to Edinburgh, in the autumn of 1770.
I attended there several of the lower classes
of the University, and went also to the
school of a drawing master; I mention
this latter circumstance, particularly, be-
cause it was in this school that I first
formed an acquaintance with two of my
present most intimate friends, Mr. David
Hume, and Mr. William Miller, now better
known by the title of Lord Glenlee.
I returned to Carolina in the summer of
1771, and a few months afterwards, was
X MEMOIR.
placed as an apprentice with Dr. Alex-
ander Garden, the chief practitioner of
physic in Charlestown, and well known to
naturalists by his communications to the
Royal Society. My manners from my in-
fancy had always been rude and rough,
but after I went to Edinburgh, and fell
into the company of Mr. Hume and Mr.
Miller, and other }7oung men of superior
rank to myself, they became considerably
softened; but I had always from my ear-
liest boyhood a strong desire to act agree-
ably to truth. Dr. Garden had been ac-
customed to apprentices of a very different
character, and in consequence, frequently
suspected me of falsehood ; and upon one
of these occasions, he attempted to strike
me with his hand, but I eluded the blow.
From this time, however, I became in my
conduct towards him reserved and indig-
nant, and finding little or no entertain-
ment in the society of the young men of
MEMOIR. XI
the place, I betook myself seriously to
study, and in the course of three years ac-
quired, perhaps, more knowledge, though
unassisted, than in any three subsequent
years of my life. When I had resided
with him somewhat more than three years,
the American rebellion first broke out in
New England.
My father, whose conduct as the printer
of a newspaper had become extremely
offensive to the people of Carolina from
his constantly maintaining the cause of
royalty, found it prudent to leave that
country and to return to Great Britain.
Soon after he went away, public matters
became worse, and I was desired with
others to sign a kind of state paper there,
" the association/' which as it appeared
to me to be an open act of rebellion, I
positively refused to do. I therefore de-
termined to leave the country also, but
my services were now of considerable
importance to my master, who was at the
Xll MEMOIR.
same time one of my father's attornies;
my mother's brother was also one of his
attornies; and these two, along with my
elder brother, strongly resisted the execu-
tion of my design; but my mother who
was a third attorney, a woman of an en-
thusiastic turn of mind, declared, that the
first public act of my life should never
disgrace me; she, therefore, in spite of
the attempts of the others, sent me off to
England about three months after my
father had parted from her. I arrived
in this country in the autumn of the same
year, 1775, and was most kindly received
by my father, and applauded by him for
my conduct.
In the beginning of the winter of the
same year, I went to Edinburgh, and
commenced my regular medical educa-
tion ; strengthening at the same time, my
former friendships with Mr. Hume and
Mr. Miller, with whom indeed I had
kept up a correspondence whilst I was in
MEMOIR.
Carolina. I studied three winters in Edin-
burgh, and in the course of that time
gained a third intimate friend, the present
Dr. Robertson Barclay. I passed my pre-
paratory trials for the degree of doctor in
medicine in the summer of 1778; but did
not at that time completely graduate. In
the autumn I returned to London, and
attended a course of Dr. William Hunter's
lectures, and took instructions in prac-
tical anatomy.
Having been about this time offered a
surgeoncy in a Scotch regiment, in the ser-
vice of Holland, to fit myself in some slight
degree for it, I became a surgeon's pupil
at St. Bartholomew's hospital for three
months. The only excuse I can offer for
my boldness in accepting this office, is,
that I was told by my friend who pro-
posed it to me, that the battalion to which
I should belong, did duty in the same gar-
rison with another battalion of the same
XIV MEMOIR.
regiment, and that I should consequently
enjoy the assistance of the surgeon to it,
a man of considerable experience, but of
no school education. On this adventure
I embarked early in 1779, and for some
time felt myself pleasantly situated in the
regiment ; but the colonel, who had been
promoted to his command, merely from
being an officer in the Dutch guards, and
bearing a Scotch name, Hamilton, but
scarcely able to speak English, soon began
to find fault with my conduct, and in con-
sequence once confined me for two days,
in the main guard of the garrison, and #.
second time, for several days, in its pre-
vost, or military -prison. This behaviour,
it may well be supposed, could not be
borne by a high-spirited Englishman. I
therefore resigned my commission, and
upon the very day of receiving my dis-
missal from the service, I attacked him
openly in the street, and dared him to
MEMOIR. XV
fight me ; he became furious, and ordered
a file of musqueteers to seize me, and
carry me to prison. Upon the same day
he dispatched an express to the Hague,
reporting to the Marshal Duke of Bruns-
wick, that he had been openly attacked
in the streets by one of the surgeons of
his regiment. This was held by the mar-
shal as so violent a breach of discipline,
that it was thought the least punishment
I should receive, would be confinement for
several years in a remote military prison.
Fortunately, colonel Hamilton was not the
senior officer of the garrison, and two
days after, there arrived, in the ordinary
course of business at the marshal's office
at the Hague, a report from the com-
manding officer, in which it was stated,
that surgeon Wells was no longer an officer
in the service. This completely altered
the state of affairs, and colonel Hamilton
was desired to seek redress for his injury,
XVI MEMOIR.
in some other way than by complaining
to the marshal. There is no doubt that
the natural warmth of my temper urged
me to do, what might have appeared to
another person rash, but that I could not
have been much in fault is evident from
this, that, the present Dr. Storer of Not-
tingham, and the present Dr. Stewart of
Perth, both previous surgeons to the same
regiment, and the Rev. Mr. Pearson its
chaplain, now residing in the neighbour-
hood of Chelsea, had all abandoned their
situations in the regiment from the treat-
ment they had received from colonel Ha-
milton.
Immediately afterwards, that is in the
beginning of 1780, I went to Leyden,
where I remained for about three months,
chiefly occupied in preparing a Thesis
upon " Cold/' a paltry affair, and having no
other recommendation, than that its Latin
was altogether my own. From thence I
MEMOIR.
returned to Edinburgh, and in the autumn
of the same year, published my Thesis,
and received the honour of being made
Doctor in Medicine. While I was at
Edinburgh at this time, I formed a fourth
intimate friendship, namely, one with the
present Dr. Lister of London.
Carolina had now been lately conquered
by the king's troops; in consequence of
which, soon after my arrival in London,
in the autumn of that year, 1780, my
father requested me to go to that country,
to look after his affairs, which had been
greatly injured during the war; and as he
had not been at all satisfied with my bro-
ther's conduct of them, who was the only
one of the family that was now there. I
arrived in Carolina in the beginning of the
year 1781, and found his property there
much diminished from various causes;
amongst others, the burning of two valu-
able houses which had not been insured.
I still however was able to render him
b
XV111 MEMOIR.
some service. Having accomplished this,
I next thought it would be doing a grateful
thing to my family, to produce a recon-
ciliation between my father and my bro-
ther. I therefore told the latter, that if
he would go to England to see my father,
I would remain in Carolina till his return,
for the purpose of managing his affairs
there. He readily accepted of my pro-
posal, and I in consequence became a
printer, a bookseller, and a merchant. I
had previously become an officer of vo-
lunteers, and had been entrusted by some
of my father's friends in England, with the
management of affairs in Carolina of con-
siderable importance. All these concerns
might have been supposed sufficient to
occupy the time and attention of a young
man of twenty-four (practically unac-
quainted with any other employment be-
sides that, which arose out of his parti-
cular profession). But some insubordina-
tion having been shown by a company
MEMOIR. XIX
of volunteers which was composed of the
principal merchants of the place, it was
judged necessary that the chief delin-
quents should be tried by a general court
martial of militia officers, and I was ap-
plied to by the colonel commandant of
the militia, who was my particular friend*
to conduct the prosecution as Judge Ad-
vocate. I, foolishly enough perhaps, con-
sented, and had as my opponents the
two principal lawyers in the place, who
acted as counsel for the accused. In the
course of the trial I suffered considerable
obloquy in consequence of their violence ;
but the natural firmness of my mind, and
a consciousness of doing what appeared to
me to be right, enabled me to resist all
their attempts to browbeat me; and the
sentence given by the court martial was
altogether in conformity to my advice.
Not long after this, orders were received
from the commander-in-chief at New York,
to evacuate the garrison. Every thing was
XX MEMOIR.
now to be performed in hurry and bustle,
and 1 immediately began to prepare for
my departure.
I embarked in December, 1782, for St.
Augustine, in East Florida, carrying with
me as much of my brother's moveable
property as I could ; amongst other things,
a printing press, and a considerable quan-
tity of printers' types. When I arrived at
St. Augustine, I determined to put up
the press there, and print a newspaper.
But here a considerable difficulty arose;
the press had been easily taken to pieces
in Carolina, and I naturally thought that
it might be readily put together again;
more especially as I had brought with me
a regular pressman: but to my surprise
he told me that he knew nothing of the
matter ; that he could work a press as well
as any person, when it was put together,
but that the putting it together consti-
tuted the particular business of a press-
joiner. In this dilemma I recollected that
.
MEMOIR. XXI
there was amongst my brother's books,
one entitled " the Printer's Grammar/' con-
taining rude cuts of a printer's press. I
studied this book for several days with
the greatest diligence; and at length by
means of the information derived from it,
and with the assistance of a common
negro carpenter, completely succeeded in
my attempt to put the press in working
order. Immediately afterwards, I began
to publish a weekly newspaper in my bro-
ther's name; the first thing of the kind
ever attempted in that country. I still
however had much leisure upon rny hands ;
(for from the time that I left London, I
had scarcely ever read a book, and had
always resolved never to exercise my pro-
fession except in Great Britain ;) I there-
fore became a captain of volunteers, as
some threats had been made by the Ame-
ricans to attack East Florida, before ac-
counts had been received of the signing of
XV111 MEMOIR.
some service. Having accomplished this,
I next thought it would be doing a grateful
thing to my family, to produce a recon-
ciliation between my father and my bro-
ther. I therefore told the latter, that if
he would go to England to see my father,
I would remain in Carolina till his return,
for the purpose of managing his affairs
there. He readily accepted of my pro-
posal, and I in consequence became a
printer, a bookseller, and a merchant. I
had previously become an officer of vo-
lunteers, and had been entrusted by some
of my father's friends in England, with the
management of affairs in Carolina of con-
siderable importance. All these concerns
might have been supposed sufficient to
occupy the time and attention of a young
man of twenty-four (practically unac-
quainted with any other employment be-
sides that, which arose out of his parti-
cular profession). But some insubordina-
tion having been shown by a company
MEMOIR. XIX
of volunteers which was composed of the
principal merchants of the place, it was
judged necessary that the chief delin-
quents should be tried by a general court
martial of militia officers, and I was ap-
plied to by the colonel commandant of
the militia, who was my particular friend^
to conduct the prosecution as Judge Ad-
vocate. I, foolishly enough perhaps, con-
sented, and had as my opponents the
two principal lawyers in the place, who
acted as counsel for the accused. In the
course of the trial I suffered considerable
obloquy in consequence of their violence ;
but the natural firmness of my mind, and
a consciousness of doing what appeared to
me to be right, enabled me to resist all
their attempts to browbeat me; and the
sentence given by the court martial was
altogether in conformity to my advice.
Not long after this, orders were received
from the commander-in-chief at New York,
to evacuate the garrison. Every thing was
XX MEMOIR.
now to be performed in hurry and bustle,
and I immediately began to prepare for
my departure.
I embarked in December, 1782, for St.
Augustine, in East Florida, carrying with
me as much of my brother's moveable
property as I could ; amongst other things,
a printing press, and a considerable quan-
tity of printers' types. When I arrived at
St. Augustine, I determined to put up
the press there, and print a newspaper.
But here a considerable difficulty arose;
the press had been easily taken to pieces
in Carolina, and I naturally thought that
it might be readily put together again;
more especially as I had brought with me
a regular pressman : but to my surprise
he told me that he knew nothing of the
matter ; that he could work a press as well
as any person, when it was put together,
but that the putting it together consti-
tuted the particular business of a press-
joiner. In this dilemma I recollected that
MEMOIR. XXI
there was amongst my brother's books,
one entitled " the Printer's Grammar," con-
taining rude cuts of a printer's press. I
studied this book for several days with
the greatest diligence; and at length by
means of the information derived from it,
and with the assistance of a common
negro carpenter, completely succeeded in
my attempt to put the press in working
order. Immediately afterwards, I began
to publish a weekly newspaper in my bro-
ther's name; the first thing of the kind
ever attempted in that country. I still
however had much leisure upon my hands ;
(for from the time that I left London, I
had scarcely ever read a book, and had
always resolved never to exercise my pro-
fession except in Great Britain ;) I there-
fore became a captain of volunteers, as
some threats had been made by the Ame-
ricans to attack East Florida, before ac-
counts had been received of the signing of
XX11 MEMOIR.
the preliminary articles of peace; I also
accepted of the management of a com-
pany of young officers, who had agreed to
perform plays for the advantage of the
poorest loyal refugees from Carolina and
Georgia. In the course of my manage-
ment, a considerable difficulty arose, about
finding a person to undertake the cha-
racter of Lusignan, in Zara. As I had
once seen Garrick in this character, about
seventeen years before, (the only character
indeed, which I had ever seen him per-
form), I determined to attempt it myself.
My success was great. This induced me
soon afterwards to appear in the character
of Old Norval, in Douglas; and here my
success was still greater. I was foolish
enough not to stop, but afterwards ap-
peared in the part of Castalio, in the
Orphan. My exertions here were mere
common place ; and I failed still more, in
an attempt to appear in comedy.
MEMOIR. XX111
About this time I received a letter from
my father, in which he requested me, now
that preliminaries of peace were signed,
to go again to Carolina, for the purpose
of looking after his affairs. I answered,
that I should immediately comply with
his desire, though I was confident it would
be at considerable risk in some way or
other. I accordingly went to Charlestown
about Midsummer, 1783, furnished with a
flag of truce from General Tonyn, which,
in the previous intercourse between the
two countries during the war, had always
been regarded as a sufficient security
against any attempt upon the liberty of
the person who bore it, though he might
be held indebted to any person in the
country to which he was going. Imme-
diately upon my arrival in Charlestown, I
was arrested upon a private suit, originat-
ing out of a transaction of my brother's.
I refused to give bail, on the ground that
XXIV MEMOIR.
this would be deserting the security I had
obtained by the flag of truce received
from General Tonyn. My refusal proved
most fortunate. For the night after my
committal to prison, a numerous mob as-
sembled before the house of Mr. and Mrs.
Harleston, (who had insisted upon my
staying with them as their guest, whilst I
remained in Carolina,) demanding, that I
should be surrendered to them. I had
looked forward to an outrage of this kind,
and had therefore provided myself with
arms, being determined never to surrender
myself to a mob. A Mr. Simpson, a
young English barrister, the son of the
late attorney-general for Carolina, who
had never done any thing to excite the
animosity of the people of that country,
was, notwithstanding, upon the same night
seized in the house of a lady with whom
he was residing, and flung into the har-
bour in deep water, where he only escaped
MEMOIR. XXV
death by being able to swim. I remained
in prison upwards of three months, during
which time I was robbed by another pri-
soner, and in consequence of my com-
plaining of this, was most grossly abused
by the jailor in the common news-
paper. Learning that an Irish gentle-
man, a Mr. .ZEdanus Bourke, was one of
the judges of that country, I immediately
complained to him of the jailor's usage,
which he directly put a stop to, by a
severe letter to him, a copy of which was
sent to me. Immediately upon my having
been confined, I wrote to General Tonyn,
acquainting him with my situation, and
saying, I should be ready to undergo any
suffering, rather than that his flag of truce
should be tarnished while in my posses-
sion. I received no positive answer, for
he was a very dilatory man, till upwards
of two months after my application had
been made to him. A government vessel
XXVI MEMOIR.
then arrived at Charlestown, bringing a
Captain Wyllie as a commissioner from
General Tonyn, to demand my release ;
Captain Wyllie was, at the same time, pri-
vately instructed to inform me, that if this
could not be obtained in consequence of
his interference, I was completely at liberty
to regain my freedom, by paying the un-
just demand which had been made upon
me; this last measure it was at length
found necessary to have resort to.
As soon as this affair was terminated, I
embarked with Commissioner Wyllie in the
vessel which had brought him to Charles-
town, and proceeded towards St. Augus-
tine. The master of the vessel was king's
pilot for the harbour. This probably
made him fool-hardy; for in weather a
little windy, but not stormy, he ran his
vessel aground upon breakers which had
previously occasioned the loss of many
vessels. She immediately bulged, and lost
MEMOIR. XXV11
her masts, and it was expected that her
deck would separate from her ribs, and
be carried out to sea, as the tide was now
falling. The wind, however, became mo-
derate, and the accident which we dreaded
did not happen. With some others of the
passengers, I had stripped myself com-
pletely naked, and lashed myself to the
capstan, in order that I might have some-
thing firm to abide by, and not be washed
away by the waves. Some hours after
this, the tide having begun to turn, and
set in towards the harbour, and the even-
ing becoming dusky, it was determined
by those who could swim, to make their
way through the breakers, as we saw boats
waiting for us in smooth water at their
edge. Commissioner Wyllie preceded me,
and when taken up, told an intimate friend
of mine, who had come down in his boat
to assist me, that I should certainly be
drowned, as I was unable to swim. Shortly
XXV111 MEMOIR.
after he had left the wreck, I determined
upon making the same experiment myself,
and with the assistance of a stout sailor
got through the breakers, sometimes swim-
ming and sometimes wading. The weather
having become still more moderate in the
night, those who were left upon the wreck
were easily saved the next morning; but
in the course of a few hours afterwards,
the vessel went entirely to pieces. It may
be mentioned here, that the master of the
government vessel who had brought us
into this situation, was, a few months
afterwards, drowned among the same
breakers.
In the course of a few months, my
brother arrived from England ; in conse-
quence of which, I embarked at St. Au-
gustine for Great Britain, i. e. in the
month of May, 1784.
During the time I had been in America,
which was nearly four years, I completely
MEMOIR. XXIX
gave up all study, not even reading the
magazines and pamphlets which had been
sent me by my father. I have already
said in what manner much of my time
had been employed ; I shall now add, that
another considerable portion of it was
spent in female society, which I had for-
merly much neglected. As soon as I re-
turned to London, I began to think se-
riously of studying my profession, to fit
myself for the exercise of it ; and in con-
sequence, cultivated the acquaintance of
medical persons. In this way I became
acquainted with the present Dr. Baillie,
and soon after contracted with him an in-
timate friendship, which now constituted
the fifth, and has been the last I have
ever formed.
The next spring I spent three months
in Paris, more with the view however of
seeing the place, than for improvement in
my profession.
XXX MEMOIR.
About Midsummer 1785, I returned to
London, and in the autumn, had the name
of Dr. Wells affixed upon the door of a
lodging which I had hired. During the
war my father's affairs had prospered, and
at the end of it he regarded himself worth
about .£20,000. But as soon as peace
took place, his principal correspondents
became dilatory in their remittances, but
were still urgent for additional supplies of
goods ; he was weak enough to comply ;
so that when 1 returned to this country
in 1784, I found him considerably em-
barrassed in his circumstances. He told
me, however, that he regarded this em-
barrassment as only temporary, and there-
fore urged, that I should exercise my
profession in London, expecting, that he
should hereafter be enabled to afford me
all necessary assistance, though at pre-
sent, he could give rne nothing more than
the use of his table. I was obliged, in
MEMOIR. XXXI
consequence, to borrow £130 from one of
my friends, to enable me to commence my
career.
I soon found, after a very trifling ex-
perience, that I was a good deal unfit for
early success in my profession in London ;
for I entertained a very high notion of its
dignity, and felt a great contempt for
most of the apothecaries with whom I be-
came accidentally acquainted; in conse-
quence, I passed several years almost with-
out taking a single fee. I fortunately
then was chosen one of the physicians to
the Finsbury Dispensary ; for I now was
furnished with the means of studying me-
dicine practically, and received from the
institution a gratuity of £50 annually;
some few fees also were the consequence
of my appointment ; but I had resided in
London fully ten years, before my in-
come from every source amounted to £250
per annum. To supply the consequent
XXX11 MEMOIR.
deficiency, I was frequently obliged to
make further loans from my friends, until
the whole of my debt amounted to about
£600. I think it right in justice to myself
however, to mention, that these were my
only debts ; for I never allowed a trades-
man to call for money and go away with-
out it.
About 1795, my professional receipts
became equal to my expenditure; agree-
ably to the rigid, and almost sordid manner,
in which it was conducted. In the next
five years, I wras enabled to pay off a little
of my debt.
At the expiration of that time, in 1800,
I was suddenly seized with a slight fit of
apoplexy. From this, however, I did not
recover so far as to be enabled to return
to the exercise of my profession, for several
months ; and I never afterwards regained
the complete possession of my memory.
I became, too, much more unfit for the
MEMOIR. XXX111
pursuit of any difficult train of thought,
which was the production of another per-
son. I did not however, as well as I could
ascertain, become less equal than I had
been, for the pursuit of my own trains of
thought; in proof of which, I may perhaps
be allowed to say, that in the fourteen
years following this illness, I made more
literary efforts than I had done, during the
whole preceding period of my life. Dread-
ing however another attack of apoplexy,
or one of palsy, warnings of which I had
almost daily since that time received, I
determined to live most abstemiously, and
in consequence, took not more food when
I was at home (I dined there about four or
five times a week) than was sufficient for
a child of seven years old, and that con-
sisting of vegetable matter. I was the
more induced to adopt this manner of
proceeding, as my 'father and one of his
brothers had previously died of apoplexy ;
XXXIV MEMOIR.
and a younger sister of my own had been
attacked with the same disease when in a
state of parturition. It was successful as
far as the disease of my head was con-
cerned; for I never suffered a second
attack of it; but my health became infirm
in other respects, and I was seized at dif-
ferent times with several dangerous dis-
eases, having no apparent relation to my
great ailment.
I had long meditated making some in-
quiry into the nature of Dew, which I
thought would not occupy me more than
a few nights, at the house of one of my
friends in the country. I commenced it
in the autumn of 1812, but soon found
that I had greatly miscalculated the time
which it would employ me. I determined
however to proceed from the natural
steadiness of my disposition, which would
never allow me to abandon any pursuit that
I had seriously undertaken.- I soon found
MEMOIR. XXXV
that I was altogether unequal to it; for
each night's labour fatigued me so much,
that I could not undertake a second for
several days after. In the mean time my
ancles began to swell in the evening, which
I regarded as a mark of general weakness.
At length, I became so infirm about the
end of 1813, that I was absolutely obliged
to give up any further visits to the country.
In the beginning of 1814, a considerable
snow having fallen, I could not resist the
temptation of going for several evenings to
Lincoln's-Inn-Fields, during a very severe
frost, in order to repeat and extend some
of Mr. Wilson's experiments upon snow.
I soon however was obliged to desist. I
became breathless on slight motion ; and
was frequently attacked with palpitation
of my heart. My friend, Dr. Lister, be-
came alarmed at my situation, and strongly
urged my remaining quiet, as he thought
it improbable I should survive more than
XXXV111 MEMOIR.
pened, I always found that I was lying
upon my right side, and when I placed
myself upon my back, the pains ceased.
This was the only situation I could as-
sume, for, ever since I had been affected
with palpitations and breathlessness, I
found it impossible to lie upon my left
side.
About the 10th or 12th of June, I was
seized at night with an attack of the
pains in my right side. As they did not
return, however, I went on the 14th on a
visit to Mr. Reid's, at Ewell. At dinner
on that day, I was as cheerful as usual,
and staid up as late as any of the family.
On the following day, I felt no disposition
to walk, but at dinner time it was re^
marked, that I took more than ordinary
pains to entertain Mr. Reid and his com-
pany, which was a large one. Early in
the evening, however, 1 became languid
and drowsy, went to bed several hours
MEMOIR. XXXIX
before the rest of the family, and slept
that night a much longer time than I had
been accustomed to do. In the morning I
was stupid and languid, but came to town
immediately after breakfast. On the same
day I informed Dr. Lister of my situation,
which he soon began to think required
the attendance of Dr. Baillie along with
his own. I shall not say any thing further
of my ailments, except, that at first I
never imagined that they would terminate
in hydro thorax.
I shall now attempt to give dates to
several events which occurred to me in
London.
I think it was in 1790, certainly not
later, probably twelve months earlier, that
I was appointed a physician to the Fins-
bury Dispensary ; I remained so till about
xl
MEMOIR.
the year 1798*. In November, 1795, I
was elected assistant physician to St. Tho-
mas's hospital, and in 1800, I became one
of the physicians to it, in which situation
I still remain. In November, 1793, 1 was
admitted into the Royal Society of Lon-
don; in 1814, into that of Edinburgh.
About four years ago Dr. Baillie asked
me, in the name of the President of the
Royal College of Physicians of London, if I
had any desire to become a Fellow of it ;
to which I answered that I had none.
In 1792, my Essay on Vision was pub-
lished. In the Philosophical Transactions
for 1795, appeared my paper on the " In-
fluence which incites the muscles of animals
to contract, in Mr. Galvani's Experiments;"
in those for 1797, my " Experiments on
the Colour of the Blood ;" and lastly, in
* Dr. Wells was elected Physician to the Finsbury
Dispensary on the 3d of September, 1789, and resigned
the office on the llth of December, 1799- E.
MEMOIR. Xli
*
those for 1811, some " Experiments and
Observations on Vision ." I have already
said that in 1814, my " Essay on Dew"
appeared. I formerly omitted to mention
that Dr. Darwin attacked in his Zoonomia,
what I had said upon giddiness*. I imme-
diately answered him in two letters sent
to the publisher of the Gentleman's Ma-
gazine. I have now referred to, every
thing which I have published on philoso-
phical subjects. In the second and third
volumes of the " Transactions of a Society
for the Promotion of Medical and Chi-
rurgical Knowledge/' almost every thing
that I have published upon medicine is
to be found ; but these are so numerous,
that I shall not particularize their dates.
In 1799, was printed by me, rather than
published, for it was never exposed to sale,
a " Letter to Lord Kenyon." These writ-
ings, as well as I can recollect, are the only
ones which ever were printed by me with
* In the Essay upon Single Vision with two Eyes.
xlii MEMOIR.
my name affixed to them; but several
others have been given by me to the
world without this attendant.
The first of these, and indeed the first
thing that ever I wrote for the public, was an
account of Mr. Henry Laurens, some time
president of the American Congress, which
appeared in the form of a letter, under
the signature of " Marcus," to the printer
of the Public Advertiser, in September,
October, or November, 1780. In 1800,
I published, in the Gentleman's Maga-
zine, some account of the life of Mr.
Anthony Lambert, formerly of Calcutta.
In the course of the same year, I pub-
lished also some account of Mr. Wilson,
of Bedford-street, Covent Garden. In
the course of the next twelve years, ap-
peared in the same magazine, " Biogra-
phical Sketches," also written by me, of
the following physicians; Dr. George
Fordyce, Dr. David Pitcairn, and Dr.
Andrew Marshall. In Carolina, during
" MEMOIR. xliii
the years 1780 and 1781, I published
many small political things, without at-
taching my name to them ; the principal
of which was written at the desire of the
commandant of the garrison of Charles-
town, the present General Nesbitt Balfour.
The cause was the following. Men of rank
in that country in the American service,
after having been taken prisoners, and sent
to their homes under their military paroles,
used to make no scruple whatsoever to
appear again in arms against the British
government. I therefore was desired to
show, by an appeal both to military usage,
and the nature of the thing itself, that such
conduct subjected them to the punishment
of death. This paper was held of that
importance by the commandant, that he
gave orders that its publication in the
public newspapers should be frequently
repeated ; and I think it highly probable,
that it was owing to this warning, that
General Balfour and Lord Moira thought
xliv MEMOIR.
themselves justified in putting to death a
Colonel Haynes, the propriety of whose
fate was afterwards a subject of debate in
the British House of Commons.
I think it right to say something more
particular than I have hitherto done, re-
specting the clear profits of my profession,
the only source of revenue that I have
ever enjoyed in London. In 1801, the
sixteenth year after I had become a phy-
sician in London, they amounted only to
£307, in which sum were included about
J60, which I had received in the form of
salary from St. Thomas's Hospital, and of
fees, for the attendance of medical pupils
there. The following year my total re-
ceipts diminished to £235. They remained
in this fluctuating state during the three
following years, that is, till 1806 included.
During the next six years, they fluctuated
between ^325 and ^455. In 1813, they
MEMOIR,
xlv
were <£457. In 1814, J441. In 1815,
^764. In 18165 £572 ; but in 1815, I
received at one time £%10 for giving me-
dical evidence at Exeter. The smallness
of these receipts will perhaps appear the
more extraordinary, when I say, that dur-
ing a great part of this time, Dr. Pitcairn,
and during the whole of it, Dr. Baillie,
often sent patients to me ; and made every
exertion to promote my interest. But I
lived at a considerable distance from them,
and was unable, from the want of a car-
riage, and from various other circum-
stances, to appear properly as their repre*
sentative. In spite, however, of this small-
ness of my income, (which, during almost
the whole of the time spoken of, that is,
from 1801 to 1816, was rendered still
smaller, by my paying most rigidly the
income and property tax, and allowing an
annuity, for a good many years, of £%Q to
a female relation,) so rigid was my eco-
nomy, that, during the few last years of
xlvi
MEMOIR.
my life, I paid off the whole of the money
which I had borrowed, amounting, as was
formerly mentioned, to about ^600; and
when I was taken ill, about three months
ago, I had in my desk, for I never kept a
banker, nor ever invested any money in
the funds, about ^350. This sum consti-
tuted the greater part of my property.
For all my books, my little plate and fur-
niture, probably, though much more va-
luable to myself, will not be supposed by
others worth more than <£200. In this
estimate, the value of my gold Rumford
medal is not included ; as the gold is quite
pure, it is held to be intrinsically worth
fifty guineas.
In the expectation that my life would
be prolonged, I had formed various literary
projects. One was, and this had often
passed rapidly through my mind during
the last forty years of my life, to show,
MEMOIR.
xlvii
that there is a material difference in the
manner in which we acquire our ideas of
the primary and secondary qualities of
matter*. If, after a closer examination
of this subject than I had formerly given
it, I should have found, that my notions
respecting it were just, I should have at-
tempted in treating of it, to imitate, in
some slight degree, the inimitable manner
employed by Berkeley in his Treatise on
Vision. I should then have presented to
the Royal Society several papers on vision,
the chief of which would have treated of
those phenomena of light, which have been
denominated by authors coloured shadows,
ocular spectra, and by various other titles.
In the last place, I should have brought
together into one volume, all my publica-
* He made out, in his own hand- writing, during his
last illness, a short statement of his opinion upon this
subject, which, by his desire, has been put, since his
death, into the hands of a philosopher, whose great learn-
ing and profound researches into the human mind pecu-
liarly fit him for estimating it justly. E.
xlviii MEMOIR.
tions upon vision ; which I would have
inscribed to the memory of Robert Wells
my father, in gratitude, for the great exer-
tions which he had made to give me the
education of a scholar and a gentleman,
when in narrow circumstances himself,
into which he had fallen, in consequence
of the American rebellion .
What I shall next say will no doubt be
held very ridiculous. I lived till I was
near eleven years old, close upon the har-
bour of a large sea-port in America, and
by this means associated much with black-
guard sailor boys. To this I attribute a
practice of swearing, of which I have from
the time of being a child, been frequently
guilty, when my feelings have been agi-
tated, and even sometimes when no excuse
of this kind has existed.
MEMOIR.
xlix
My last declaration will relate to the
obligations under which I lie to my friends.
I have already spoken of my rare good
fortune, in having acquired, in the course
of my life, five most intimate friends. All
of these are still in being, and from all of
them I have received, throughout my ill*
ness, the warmest -proofs of attachment.
Two of them, however, have most espe-
cially afforded such proofs, Dr. Lister and
Dr. Baillie, partly from their residing in
London, and partly from the nature of
their profession. My obligations to Dn
Lister are extreme. During the whole of
my disease, he has visited me constantly
twice, and sometimes thrice a day ; and
during each of these visits, he has con-
ducted himself towards me, with fully as
much kindness, as if I had been his
brother.
I have likewise to express my very great
obligations to two other of my friends, Mr*
d
1J MEMOIR,
James Dunsmure, merchant, in Lothbury,
and Mr. Samuel Patrick , of Bar tlett's Build-
ings, surgeon ; since, in the whole course
of my illness, their attentions to me have
been most unremitted, and they have
also most generously promised to burthen
themselves with the care of my concerns,
after my death.
It must not be regarded as an instance
6f the weakness of an old man's mind, my
desiring, that my body may be deposited
in Lady Jersey's vault in St. Bride's
Church; immediately above that of my
mother, and in contact with it, as her's is
now placed with respect to that of my fa-
ther; for it has been my wish, for many
years past, that this should be done. I
have, indeed, never been desirous to con-
quer any natural feelings, when their in-
dulgence led to no harm ; on the contrary,
MEMOIR.
ll
1 have always regarded such an indul-
gence, as highly conducive to the soften-
ing of the original hardness of my cha-
racter.
August 22d, 1817.
As I fancy that several parts of my cha-
racter, from various reasons, have been
a good deal misunderstood, even by my
most intimate friends ; I shall relate here,
with little regard to method or connection,
some circumstances which may tend to
illustrate it.
I began to show, even in my earliest
childhood, an invincible firmness of mind.
When my father, who was a passionate
man, beat me for a fault, which 1 was con-
scious 1 had committed, I used to entreat
mercy most piteously ; but if I believed,
that I was in the right, the utmost punish-
ment he could inflict would scarcely ever
a 2
Hi MEMOIR.
force a tear from me. When I was at
Dumfries school, I had a playfellow, the
present Mr. , of Edinburgh. He one
day called me by some improper name, in
consequence of which I beat him, being
the stronger of the two. He complained
to Mr. Chapman our master, who ordered
me to promise, that I would never do the
like again. I answered that I could not,
for I would certainly beat him if he re-
peated the offence. Mr. Chapman tried
first the effect of corporal punishment
upon me ; but finding this of no avail, he
ordered me to retire to my room, for I
was one of his boarders, and forbade the
other boys to hold society with me. This
happened upon a Saturday, which was at
our school a half holiday . On the Monday
following, I was summoned to appear in
the school, as I thought, for the purpose
of being [finally] expelled from it ; for, I
had determined to submit'to this disgrace,
MEMOIR. liii
rather than to swerve from my former de-
claration. To my astonishment, however,
I found that I was to receive from him the
highes^ commendation. On the Saturday
afternoon, my confinement not having been
strictly enforced, I was determined to break
through it, and to go into the neighbour-
ing country with some of my playfellows.
There was before the school a considera-
ble area, in which, while I was proceeding
to join my playfellows, I met a blind beg-
gar, who appeared to me to have lost his
way. The other toys had passed him
without attending to him. I went up to
him, and finding my conjecture to be
right, took him by the hand, and led him
to the house to which he was desirous of
going. My master was at his window,
and saw this. On the Monday he men^.
tioned it to the whole school, and received
me back into it, with great commendation
Hv
MEMOIR.
of my conduct, without making any refer-
ence to my former expulsion.
My father was a man of great sobriety
himself, and restricted me, while I was a
boy, from drinking any thing but water ;
and I never, in any posterior part of my
life, have had the least desire to taste any
stronger liquor, except in compliance with
the ordinary customs of society. In 1782,
I became president of a club in Florida,
and agreeably to the custom of the coun-
try, thought it necessary to make my sub-
jects intoxicated. In this attempt, I ne-
cessarily became somewhat intoxicated
myself, but still in a less degree than the
others, from proceeding more cautiously.
During the other six days of the week,
though living constantly in society, I drank
nothing but water, nor did I ever after-
MEMOIR. ly
wards, even before my health became in-
firm in 1800, desert this practice, except
I was in society.
My father, though naturally a passionate
man, in all the ordinary affairs of life con-
ducted himself with the greatest prudence,
except in the case already mentioned, when
he was induced, by too great ease of tem-
per, to swerve from it. My mother was
much his inferior in point of common sense,
and had a strong tendency to act a little
romantically.
I .resembled them both, not only in per-
son but disposition ; and, in consequence
of my resemblance to my mother in this
latter circumstance, began early to show
signs of a certain waywardness of dispo-
sition.
When I was a boy at Dumfries school,
1 used to wander on foot during the
v MEMOIR.
autumnal holidays through the country,
without any fixed object. In one of these
rovings, being then in the twelfth year of
my age, I went to call upon a friend of
my father's, without any other clothes than
those which I had upon me. The follow-
ing morning, I thought my shirt looked
dirty, and therefore determined to wash it
myself. I chose, as a place fit for this
purpose, a little meadow on the side of the
river* Milk, which was sheltered by a high
bank behind me. Having done the busi-
ness in the best manner I could, without
any assistance from soap, I placed my
shirt upon the grass for the purpose of
drying it, and laid myself in the mean
time in the sunshine, upon another piece
of dry grass in the neighbourhood. When
my shirt was dry, I put it on, and returned
to my friends. In the course of the night,
* This, I ain told, is a rivulet rather than a river. E.
MEMOIR.
Ivii
I was seized with a considerable degree of
fever, and in the morning my face, and
the parts of my body which had been ex-
posed to the sun, became considerably red
and swollen.
About a twelvemonth afterwards, I re-
ceived an invitation by letter from a school
chum, to visit him at his father's, who lived
in Galloway, about 31 miles from Dum-
fries. I showed this letter to Mr. Chap-
man, and requested money from him, to
enable me to make the visit. He most
properly refused to give any, upon the
ground, that the invitation had proceeded
only from a boy. I thought differently,
however, and, taking advantage of his ab-
sence, began my journey two days after,
without a halfpenny in my pocket, and
with no other clothes than I wore, as I had
determined to return to Dumfries the fol-
lowing day. My friend's father, whose
name was Macmurdo, had lived many years
Iviii
MEMOIR.
in Virginia as a merchant, and when he
returned to Scotland, brought with him a
wife, who was a native of the former coun-
try. They received me most kindly, no
doubt somewhat influenced by iny having
been born in America, and retained me
as their guest for upwards of a month ;
supplying, amongst my other deficien-
cies, that of raiment. At the expiration
of the holidays, they sent me in a post-
chaise to Dumfries, with a part of their
own family.
In my journey to Mr. Macmurdo's house,
which I accomplished in eleven hours, I
had no food but hips and blackberries,
and a little milk, which a cottager would
sometimes give me when I asked for a
little water to drink.
My temper was naturally irritable, and
in small differences which have occurred
MEMOIR. x
in society, particularly in my youth,
passionate and violent. But I must, in
justice to myself, say, in the first place, I
have not shown any considerable instance
of this kind for nearly twenty years ; and
in the second, that I did never show one,
even before that time, in any matter of
consequence, or when I had any respect
for the person with whom I differed. In
confirmation of both these remarks, I shall
mention, first, that I have never had the
smallest difference with any one of my five
most intimate friends ; and secondly, that
I have borne the grossest insult, when it
was unmanly to take immediate notice
of it.
From the time of the murder of the
princess of Lamballe, I foresaw the ruin
of all civilized society in France, and
dreaded a similar ruin of all civilized
x MEMOIR.
society in Europe. I have never, there-
fore, been able to hear, with the least
patience, any serious defence of the con-
duct of the French ; and have always at-
tributed such a defence to incurable folly,
self-interest, or madness. In all points of
domestic 'politics, I have kept myself free
from personal influence, by never seeking
the acquaintance of any person of the least
influence in the country. By principle I
am a constitutional Tory; but my man-
ners, I should think, would lead most per-
sons to regard me a republican.
August 28, 1817.
DR. WELLS, from a very early period in
his illness, looked forward to a fatal ter-
mination of it, and employed himself in
arranging his affairs with the utmost self-
possession and diligence, until he had
MEMOIR. x
settled, with great exactness, every thing
which he thought important. From the
8th of August, his physicians, as well as
himself, abandoned all hopes of his reco-
very. He died in the evening of the 18th
of September,
ADVERTISEMENT
THE Memoir, with the omission of an
anecdote, which might have given pain to
a family with which the author had been
on terms of great intimacy, and of a name
and designation, which it was believed the
very respectable person referred to might
wish to be suppressed, and with a very
slight alteration in a very few expres-
sions, is precisely as it was left by the
author. He dictated it to his friend Mr.
Patrick at intervals during his illness,
after he had lost all hope of recovery,
and while he was uncertain whether he
should live to finish it, and when he wras
too feeble to speak long, or to write
much. It must be considered a proof
of extraordinary composure and vigour of
mind in such circumstances.
ADVERTISEMENT.
The writings of the author, which have
been selected for publication with this
Memoir, either as the most interesting in
themselves, or as affording the best exhi-
bition of his character and talents, are, an
Essay upon Single Vision with two Eyes,
and an Essay upon Dew ; a Letter to the
Right Honourable Lloyd, Lord Kenyon,
relative to some conduct of the College
of Physicians of London, posterior to the
decision of the Court of King's Bench, in
the case of Dr. Stanger, and containing
observations on a principal ground of that
decision; and an Account of a Female of
the White Race of Mankind, part of whose
Skin resembles that of a Negro ; with some
observations on the causes of the differences
in colour and form between the white and
negro races of men. The last of these
writings was read before the Royal Society
in 1813, but was never printed until now.
It was put by the author into the hands
of the editor, with an express permission
Ixiv
ADVERTISEMENT.
to publish it, and no alteration has been
made in it, besides a very slight one of
expression, in a few places, which its being
presented to the public, instead of being
addressed to a philosophical society, ren-
dered necessary.
All his other works, whether philoso-
phical, literary, or medical, (excepting only
those of a political nature, which are men-
tioned in the Memoir, and to which no
more particular reference could be made
than what is made in it,) are enumerated
in the following list, in order that they
may be more generally known and more
easily referred to.
Two letters, in reply to Dr. Darwin's
remarks, in his " Zoonomia," upon what
Dr. Wells had written in his " Essay upon
single Vision with two Eyes/' on the ap-
parent rotation of bodies, which takes
place during the giddiness occasioned by
turning ourselves quickly and frequently
round. These were published in the
ADVERTISEMENT.
Gentleman's Magazine for September and
October 1794.
Observations on the influence which in-
cites the muscles to contract in Mr. Gal-
vani's experiments. These were published
in the Philosophical Transactions in 1795.
Observations and experiments on the
colour of blood.. These were published in
the Philosophical Transactions in 1797.
Some account of the life of Mr. Anthony
Lambert, formerly of Calcutta ; and some
account of Mr. George Wilson, apothecary,
of Bedford-street, Covent Garden. Both
these appeared in the Gentleman's Maga-
zine for 1800.
A biographical sketch of Dr. George
Fordyce. This appeared in the Gentle-
man's Magazine for 1802.
A short account of Mr. John Savage,
formerly of Charlestown. This appeared
in the Gentleman's Magazine in 1804.
A biographical memoir of Dr. David
ADVERTISEMENT.
Pitcairn. This appeared in the Gentle-
man's Magazine in 1809-
Observations and experiments on Vision.
These were published in the Philosophical
Transactions in 1811.
A biographical sketch of Dr. Andrew
Marshall. This was published in the Gen-
tleman's Magazine in 1813.
An answer to remarks in the Quarterly
Review, upon the Essay on Dew. An
answer to Mr. Prevost's queries respecting
the explanation of Mr. B. Prevost's expe-
riments on Dew. These appeared in Dr.
Thomson's Annals of Philosophy for 1815.
A short letter " on the Condensation of
Water upon Glass." This was published
in Dr. Thomson's Annals of Philosophy
for 1816.
The titles of his medical writings are,
1. Observations on Erysipelas.
2. An Instance of an entire want of Hair
in the Human Body.
ADVERTISEMENT.
Ixvii
3. Observations on the Dropsy which
succeeds Scarlet Fever.
4. A Case of Aneurism of the Aorta,
attended with ulceration of the (Esopha-
gus and Windpipe.
5. A Case of Epilepsy and Hemiplegia,
apparently produced by a sharp projection
from the inner Table of the Skull
6. A Case of Tetanus, with Observa-
tions on the Disease.
7. A Case of Aneurism of the Aorta,
communicating with the Pulmonary Artery.
8. A Case of considerable enlargement
of the Caecum and Colon.
9. A Case of an extensive Gangrene
of the Cellular Membrane, between the
Muscles and Skin of the Neck and Chest.
10. On Rheumatism of the Heart.
11. On the presence of the Red Matter
and Serum of the Blood in the Urine of
Dropsy, which has not originated in Scar-
let Fever.
Ixviii ADVERTISEMENT.
12. Observations on Pulmonary Con-
sumption and Intermittent Fever, chiefly
as Diseases opposed to each other ; with
an attempt to arrange several other dis-
eases, according to the alliance or opposi-
tion which exists between them and one
or other of the two former.
These were all published in the second
and third volumes of the " Transactions
of a Society for the promotion of Medical
and Chirurgical Knowledge/'
There is also a case of Aphonia Spasmo-
dica described by him, and communicated
by Dr. Carmichael Smith, in the second
volume of the " Medical Communications/'
CONTENTS.
ESSAY UPON SINGLE VISION WITH TWO EYES.
PART I.
Of the different opinions concerning single vision with two
eyes j and principally of those of Dr. Smith and Dr. Reid
pagel
PART II.
Of a new theory respecting visible direction, and of a solu-
tion hence derived of the question, why objects are seen
single with two eyes . . . .28
PART III.
Of some consequences from the foregoing theory of objects
being seen single with two eyes ; together with the ex-
planation of several other phenomena of vision . 51
EXPERIMENTS AND OBSERVATIONS ON SEVERAL
SUBJECTS IN OPTICS.
ARTICLE I.
On visible position and visible motion . . 69
Ixx CONTENTS.
ARTICLE II.
On a supposed consequence of the duration of impressions
upon the retina ; and the effects of accurate vision being
confined to a single point of that membrane . page 86
ARTICLE III.
On the connexion between the different refractive states of
the eyes, and the different inclinations of the optic axes to
each other . . . . , .94
ARTICLE IV.
On the limits of perfect and distinct vision . . 1O/
ESSAY ON DEW
AND
SEVERAL APPEARANCES CONNECTED WITH IT ug
Introduction . . . . . 123
PART I.
OF THE PHENOMENA OF DEW.
SECT, i.— Of circumstances which influence the production
of dew . . .. . . .127
SECT. ii. — Of the cold connected with the formation of
dew . . ; :. . . 152
PART II.
OF THE THEORY OF DEW.
Former theories . . . . . ] 77
A NEW THEORY PROPOSED.
Deto is the production of a preceding cold in the substances
upon which it appears . . . .181
CONTENTS. Ixxt
That cold precedes the formation of dew ascertained by ex-
periment '-1 %•* .... page 182
This fact applied to explain several natural appearances.
1. The variety in the quantities of dew on different bodies,
exposed to the air during the same time of the night,
but in different situations . . * 185
2. The cold connected with dew, not being always pro-
portional to the quantity of that fluid . . ib.
3. The production of heat by the formation of dew 186
4. The fact of more dew being acquired, in very calm
nights, by substances placed upon a raised board, than
by others of the same kind on the grass ; and that of
a slight agitation of the atmosphere, when very preg-
nant with moisture, increasing the quantity of dew
188
5. The fact of dew never being formed in temperate
climates upon the naked parts of a living and healthy
human body . . . L3lv;j «?'? 189
6. The fact of hygrometers, formed of animal and vegeta-
ble substances, when exposed to a clear sky at night,
marking a degree of moisture beyond what is actually
resident in the atmosphere . . .190
The cold which produces detv, is itself produced by the radia-
tion of heat, from those bodies, upon u>hich deiu is deposited
191
The cold produced by the radiation of heat from substances
upon the surface of the earth, is compensated or over-
balanced in the day-time by the heat from the sun, and
lessened at night by various causes . .196
The cold originating in the nightly radiation of heat from
bodies upon the surface of the earth, though lessened by
various causes, is often very considerable . .198
Some of the useful effects of the radiation of heat from the
earth at night . : ,'. . . . ^ ,, 201
Observations upon, or explanations of the under-mentioned
circumstances.
Ixxii CONTENTS.
1 . The prevention, wholly or in part, of cold from radia-
tion, in substances on the ground, by the interposition
of any solid body between them and the sky page 203
2. The prevention, wholly or in part, of cold from radia-
tion, in substances on the ground, by the interposition
of clouds ", '';•'- . . . . 205
3. The prevention, wholly or in part, of cold from radia-
tion, by fogs . ... 207
4. The prevention, wholly or in part, of cold from radia-
tion, by conduction from warmer substances in contact
with the radiating substance . . .211
5. The effect of wind in compensating the cold from ra-
diation, and sometimes in lessening, and sometimes in
increasing the production of dew . . 212
6. The cold from radiation, of a thermometer placed on a
board, being less diminished than that of one suspended
in the air ..... 213
7. The hurtful effects of cold occurring chiefly in hollow
places, according to a remark of Theophrastus 214
8. Frost being less severe upon hills, than in neighbouring
plains, in calm and serene nights . -<'.. 215
Reasons assigned for believing that air is actually heated by
the sunbeams which enter it, and that it not only absorbs,
but radiates heat .... >/;. 217
9. The leaves of trees often remaining dry throughout
the night, while those of grass are covered with dew
227
10. Bright metals exposed to a clear sky in a calm night
being less dewed on their upper surface, than other
solid bodies j and those metals which radiate heat most,
being most attractive of dew . . #?V 228
11. The difference between black and white bodies with
respect to radiation, when exposed to the sky at night
235
Whether dew is the product of vapour emitted during the
night by the earth and plants upon it . . 236
CONTENTS. Ixxiii
PART III.
OP SEVERAL APPEARANCES CONNECTED WITH DEW.
1. Of the greater moisture, sometimes observed in winter
mornings upon the insides of the panes of glass in win-
dows covered with inside shutters, than upon those not
covered by them .... page 247
2. Of the greater sensation of cold, which is sometimes ex-
perienced upon exposure to the sky in a clear night, than
is to be explained by the temperature of the atmosphere
249
3. Of the effect of those means, employed by gardeners to
protect tender plants from cold during the night, which
screen them from the sky . . .252
4. Of the effect of a covering of snow, or of other matters,
during still and serene nights, in protecting vegetables
from cold ..... 257
5. Of the putrefaction which has been supposed to take
place in animal substances exposed to moonshine 258
6. Of the formation of ice, during the night in Bengal,
when the temperature of the air is above '32° 200
Conclusion . .... 280
A LETTER
TO THE
RIGHT HON. LLOYD, LORD KENYON,
RELATIVE TO SOME CONDUCT OF THE COLLEGE OF PHYSICIANS
OF LONDON, POSTERIOR TO THE DECISION OF THE COURT
OF KING'S BENCH, IN THE CASE OF DR. STANGER;
AND CONTAINING
OBSERVATIONS ON A PRINCIPAL GROUND OF THAT DECISION 283
Ixxiv CONTENTS.
AN ACCOUNT
OF
A FEMALE OF THE WHITE RACE OF MANKIND,
PART OF WHOSE SKIN RESEMBLES THAT OF A NEGRO ;
WITH SOME
OBSERVATIONS ON THE CAUSES OP THE DIFFERENCES,
IN COLOUR AND FORM, BETWEEN THE WHITE AND
NEGRO RACES OF MEN . page 423
-
AN
ESSAY
UPON
SINGLE VISION WITH TWO EYES
TOGETHER WITH
EXPERIMENTS
AND
OBSERVATIONS
ON
SEVERAL SUBJECTS IN OPTICS.
AN
; .. ESSAY 't''
UPON
SINGLE VISION WITH TWO EYES.
PART I.
Of the different Opinions concerning single Vision with
two Eyes ; and principally of those of Dr. Smith and
Dr. Reid.
1 HE end I have chiefly in view, in this Essay,
being to offer a new solution of the question,
why objects are perceived single with two eyes,
I think it incumbent upon me, in the first
place, to show, that none of the opinions I have
met with upon this subject, can be admitted as
just.
These opinions, or such of them, at least, as
have gained any considerable reputation, may
be reduced into two classes. The first com-
prehends those of Galen, Alhazen, Rohault,
Dr. Briggs, and Sir Isaac Newton, all of whom
have regarded the question I have mentioned
^ AN ESSAY ON
as equivalent to the following one : Whence
comes it, that the mind should be affected with
only one perception from two impressions upon
the external organs of sight, since either of
those impressions is, of itself, sufficient to
produce a similar perception ? Their universal
answer has been : Because the two impressions
are united before they are communicated to
the mind. And the only difference among
these authors, has been with respect to the
manner in which such an union takes place.
To the second class are to be referred the
opinions of those, who hold it as certain, that
an object is seen single by both eyes, because
it is seen by each of them in the same external
place ; and who profess to point out some law,
or constant rule of vision, from which this
sameness of place is to be derived as a neces-
sary consequence. Aguilonius, I believe, first
gave this view of the question, which has since
been adopted by Dechales, Dr. Porterfield,
Dr. Smith of Cambridge, and Dr. Reid of
Glasgow.
In opposition to the opinions of the first
class, more especially as they have been re-
peatedly examined by others, I think I need
only say, that they must all be considered as
mere conjectures, founded upon certain sup-
posed changes in the brain and nerves, the
SINGLE VISION. *
existence of which it is impossible, from the
nature of the parts, either to demonstrate, or
to refute by experiments ; and that no one of
them, though admitted to be true, is yet suf-
ficient to explain the phenomena on account
of which it was framed.
The opinions of the second class being built,
as their authors think, upon experiments and
observations, both allow and demand a more
accurate investigation. I shall proceed, there-
fore, to examine such of them as I am acquainted
with, beginning with that of Aguilonius ; and
what I shall observe concerning it will apply
also to those of Dechales and Dr. Porterfield,
who have done little more than copy what he
has said.
If a line be drawn through the point of the
mutual intersection of the optic axes, parallel
to the interval between the eyes, Aguilonius
calls it, from its office, the horopter-, and if
through this line, a plane be made to pass at
right angles to that of the optic axes, he names
it the plane of the horopter. After defining
these terms, he asserts, that, by a law of our
constitution, all bodies which we see with one
glance or look, whatever are their real places,
appear to each eye to be situated in this plane.
And if this be granted to him, he easily and
satisfactorily shows, why some should be seen
B 2
AN ESSAY ON
single with two eyes, and others double. For
since, according to a second opinion maintained
by him, and not contradicted, I believe, by any
other writer upon vision, the two lines of direc-
tion, in which an object is seen when we em-
ploy-both eyes, can meet each other only in
one point, it follows, that all bodies which are
really situated in the plane of the horopter,
must necessarily appear single, as the lines of
direction in which any one of them is perceived
by the two eyes, coincide in that plane, and no
where else ; and that all bodies, which are not
situated in the plane of the horopter, must as
necessarily appear double, since, in this case,
the lines of their visible directions intersect
each other, either before or after they pass
through it*.
Against the truth of this explanation, only
one argument need be offered. Were the visi-
ble places of all bodies to be contained in the
plane of the horopter, these would appear of
magnitudes proportional to the angles which
they subtend at the eye. A finger, for instance,
held near to the face, would seem as large as
the part of a remote building it might conceal
from the sight. But as this is contrary to ex-
perience, the principle from which it is derived,
* Aguilonii Optica, p. 110, 148, 331, 344.
SINGLE VISION.
must be rejected, together with all its conse-
quences. To Aguilonius, however, the merit
is due, of being the first who so far generalised
the phenomena of single and double vision, as
to observe, that those objects alone are seen
single, which are really situated in the plane of
the horopter.
The opinion of Dr. Smith is the next in the
order of time. * " If it be asked (says that
author) why, in seeing with both eyes, we do
not always see double, because of a double sensa-
tion, I think it is sufficient to say, that in the
ordinary use of our eyes, in which the pictures
of an object are constantly painted upon | corre-
sponding places of the retinas, the predominant
sense of feeling has originally and constantly
informed us that the object is single. By this
* Complete System of Optics. Vol. I. p. 48.
f Dr. Smith gives the following definition of corresponding
points. " When the optic axes are parallel, or meet in a
point, the two middle points of the retinas, or any points
which are equally distant from them, and lie on the same
sides of them, either towards the right hand or left hand,
or upwards or downwards, or in any oblique direction, are
called corresponding points." Vol. I. p. 46. According to
this definition, points correspond which have a certain agree-
ment in situation. No contradiction is, therefore, implied
in this system, by saying, that an object may appear single,
though its pictures should fall upon points which do not
correspond. Dr. Reid's definition of the same term is very
different.
6 AN ESSAY ON
means our idea of its outward place is connected
with both those sensations, as is manifest by its
appearing in two places when its pictures are
not painted upon corresponding places of the
retinas; which is only a direct consequence
arising from our general habit of seeing."
Should any one now inquire whence it is, that,
to produce single vision, all men agree in di-
recting their eyes toward the object in such a
manner as to receive its pictures upon corre-
sponding points of the retinas, since custom
might have connected the sensations of any
other two points with the information of its
unity from feeling* : This answer maybe given
in the words of Dr. Smith t : " When we view
an object steadily, we have acquired a habit
of directing the optic axes to the point in
view; because its pictures falling upon the
middle points of the retinas, are then distincter
than if they fell upon any other places ; and
since the pictures of the whole object are equal
to one another, and are both inverted with re-
spect to the optic axes, it follows that the pic-
tures of any collateral point are painted upon
corresponding points of the retinas."
* This objection is made to Dr. Smith's theory by Dr.
Reid, who seems to have overlooked the answer. Reid's
Inquiry into the Human Mind, 8vo. p. 332.
t Vol. I. p. 46.
SINGLE VISION. '
Such is the solution which Dr. Smith has
given of this celebrated question, and such the
reply which his general account of vision fur-
nishes to one objection against it. But there
are others which, in my opinion, cannot be so
easily repelled. Before I offer these, however,
I beg leave to remark, that although it were
proved, as I think it maybe, that he is mistaken
in the fact of objects appearing single, when
their pictures fall upon the middle or other
corresponding points of the retinas, still the
truth of what is peculiar to him * of the solu-
tion he gives, might remain unshaken. Ob-
jects, it may be said, are constantly seen single
when we direct our eyes to them in a particular
manner. Their pictures must, consequently,
in every such case, fall upon the same places of
the retinas ; and whether these be correspond-
ing or not, the unity of the visible appearances
will be owing to the connexion, which has uni-
formly been observed between the sensations of
* Dr. Reid attributes to Bishop Berkeley the opinion,
that objects appear single to two eyes, from an experienced
connexion between particular sensations of sight, and the
informations of touch. But I no where find it mentioned in
the works of that author j and I even think it probable, that
he purposely avoided treating of the question, as he found
that the solution of it, which naturally flowed from his prin-
ciples of vision, was with difficulty to be reconciled to other
conclusions he had derived from the same source.
AN ESSAY ON
those places, and the information from feeling,
that the objects which cause them are single.
What I shall say, therefore, upon his opinion,
will tend to show, that, admitting the fact re-
specting corresponding points to be true, his
explanation of it ought, however, to be rejected.
For, first, it may be observed, that, if we are
taught by feeling to see objects single, notwith-
standing a sensation in each eye, the informa-
tions of the former sense ought to be uniform,
or else one set of visual appearances would be
associated with different reports from feeling,
and no certain mark afforded us which of them
we should trust. Now Dr. Smith himself is
obliged to confess, that we sometimes feel dou-
ble, " as in the dark, when a button is pressed
with two opposite sides of two contiguous fin-
gers laid across ; for this reason, that those op-
posite sides of the fingers have never been used
to feel one but always' two things at a time*."
He adds, " We have learned, therefore, by ex-
perience of both senses compared together, to
make their informations consistent with each
other." Here, then, we find him to allow, that
* Vol. I. p. 48. Dr. Smith, however, has, from the in-
fluence of system, I suppose, mistaken this fact; for the
button is felt double when pressed in the manner above
mentioned, though we should not be in the dark, and should
even see it to be single.
SINGLE VISION. 9
feeling is not always the predominant, but
sometimes the inferior sense ; that its informa-
tions are not constant and original, but change-
ful and derived ; positions directly contrary to
those he had immediately before maintained.
But, in the first instance of difference between
the informations of the two senses, what rule
had we for determining which .was the most
worthy of credit ? How does a blind man cor-
rect his errors of touch ? If the button be felt
double, because pressed by two parts not ac-
customed to feel the same thing at the same
time, there must have been a period in the life
of every person, when a body pressed by any
two parts would have been felt double, by three
parts triple, and so on. Nor could sight have
corrected those deceptions, if they can be called
such ; for every thing, by the same hypothesis,
must then have also been seen double. How
came we, therefore, both to feel and see things
single ? Surely not by comparing the informa-
tions of the two senses together.
But, secondly, were we to grant that the
sense of touch has originally and constantly
informed us that objects are single, it would
not follow, that we are thence taught to see
them also single. For, since the place which
an object seems to either eye to possess, mani-
festly depends both upon its apparent distance
10 AN ESSAY ON
and its apparent direction from that eye, if
visible place be, in the language of Dr. Smith,
only an idea of real or tangible place, visible
direction must bear the same relation to tangi-
ble direction ; a consequence of which is, that
\ve can never have a more accurate knowledge
of the direction, in which an object may lie
from any part. of our bodies, by sight than by
touch. Facts, however, prove the contrary.
Let any person, for instance, taking a pin in
his hand, endeavour, without looking, to bring
its head upon a level with either of his eyes ;
and there are many chances to one but he will
fail in the attempt, of which sight will inform
him, when he turns his eye to the object. This
to me is a convincing argument, that external
bodies are not seen in certain directions, be-
cause they have been previously felt in them ;
and, consequently, that visible place, of which
visible direction is a component part, is not
merely a representative of the place perceived
by touch. But if the place, in which an object
appears to each eye separately, does not entirely
depend upon any lesson from feeling, the in-
ference is, that when an object appears in one
and the same place to both eyes together, nei-
ther is this effect to be attributed solely to the
informations of that sense.
Thirdly, in whatever direction an object may
SINGLE VISION. 11
%
appear to either eye, it certainly cannot be seen
in the same place by both, except at some point
common to the two directions. Dr. Smith ac-
knowledges this, and says*, that when an ob-
ject is perceived single with both eyes, it is
seen at the mutual intersection of the two visual
rays ; the visible direction of any object coin-
ciding, according to him, with the visual ray,
or the principal ray of the pencil which flows
from it to the eye. Should we then even
allow, that all we know by sight of the places
of bodies has been borrowed from feeling, it
will still be easy to show, that the rule of vision
for each eye, which he has derived from such
experience, 'that of our seeing objects in the
directions of their visual rays, is inconsistent
with many of the phenomena of sight with two
eyes ; and, consequently, that he has left un-
removed the chief difficulty of his subject, which
was to explain the single appearance of objects
to both eyes, from those laws, or rules of vision,
which affect each of them singly. For it is a
well-known fact, that if two bodies of the same
shape, size, and colour, be placed, one in each
optic axis, they appear but as one body, pro-
vided they be at equal distances from the eyes.
Agreeably to the theory of our seeing objects
* Vol. II. Remarks, p. 86.
12 AN ESSAY ON
in the direction of their visual rays, this cannot
happen, except the united body appear at the
intersection of the optic axes. Dr. Smith, ac-
cordingly*, maintains that it does. Now, in
the first place, I appeal to experiment for a
direct proof that it does not ; and, in the se-
cond, I observe, that, as the two bodies in the
optic axes appear as one, whether they be
situated within or beyond the concurrence of
those lines, and as a right line joining the
bodies, and extended both ways, appears at the
same time to the sight as a right line, it follows,
upon admitting the fact which I have denied,
that all objects in the plane of the optic axes
which are seen in one position and" state of'the
eyes, however near to us, or however remote
they may in reality be, must appear to be
equally distant, or rather in a line drawn through
the concourse of the optic axes, parallel to the
interval between the eyes, and named by opti-
cians the horopter. Again, if a right line be
made to pass through any part of the plane of
the optic axes, at right angles to it, the por-
tions above and below this plane are perceived
to be in the same right line with the point which
is situated in it, and the whole appears perpen-
dicular to the plane. But the point in the plane
* Vol. II. Remarks, p. 86.
SINGLE VISION. 13
is seen, by the last article or proposition, in the
horopter ; the whole, therefore, of the perpen-
dicular line must be seen in a plane passing
through the horopter at right angles to that of
the optic axes ; or in other words, in the plane of
the horopter, in which consequently all bodies
will have their visible places. But this was the
very opinion of Aguilonius, to which he was
probably led by a similar train of reasoning ;
though, as a teacher, he might choose rather
to ground it immediately upon an original law
of our constitution.
It is probable, however, that Dr. Smith did
not perceive the conclusions which might be
drawn from' his doctrine of objects being seen
in the directions of their visual rays, since he
has no where spoken of them. At any rate, it
is manifest he did not admit them, as he has
mentioned the following circumstance as a
fact*, to which they cannot be reconciled;
that, when an object is seen double, both its
apparent places are situated between its real
place, and the mark at which we look. For if
this were just, together with what he has else-
where advanced, phenomena ought in many
cases to be observed, very different from those
which are in truth found to exist. Thus, for
* Vol. I. p. 48.
14 AN ESSAY ON
example, if a right line be any where placed in
the plane of the optic axes, it follows, from
what he has said in one part of his book, that
those points of it, through which the axes pass,
must be seen united at the mark we look at,
the axes crossing each other there ; and from
what I have just quoted, that every other point
must be seen by each eye between its real place
and that mark. The appearances, therefore, of
all the points, if they do not lie disjoined, but
are connected together in some orderly manner,
will be arranged in the forms, either of two
curves, both passing through the intersection of
the optic axes, or of four right lines meeting
one another at that point. If the right line be
placed nearer to the face than the mark we
look at, the apparent lines, whether curved or
straight, will approach toward us from their
common point, but recede from us, if the real
line be situated beyond the mark. Such are
the phenomena which ought to follow upon the
admission of these two parts of Dr. Smith's
theory of vision with two eyes, but which are
not found to exist in nature. Aguilonius was
at least consistent when he maintained, that all
objects are seen in the plane of the horopter ;
while Dr. Smith, by deserting that opinion in
part, seems only to have involved himself the
more deeply in error.
SINGLE VISION. 15
Having now said what, I hope, will be
thought sufficient to show, that the reason given
by Dr. Smith, for our seeing objects single with
both eyes, is neither grounded on well-attested
facts, nor adequate to the explanation of the
phenomena observed, I pass to the examination
of the opinion of Dr. Reid.
As this neither rests upon nor includes any
new fact in vision, I need only mention, in
order to give an account of it*, that its author
maintains with Dr. Smith, that an object is seen
in the same place with both eyes, and conse-
quently single, when its pictures fall upon the
centres of the retinas, or upon points in them,
which are similarly situated with respect to the
centres ; but differs from him in this, that he
makes the property to be original, by which
any two places in those membranes exhibit only
one object, while Dr. Smith derives it altoge-
ther from custom f.
In my examination of the opinion of Dr.
* Inquiry into the Human Mind, c. vi. sect. 13.
f They differ also with respect to the meaning of a term -,
Dr. Smith calling corresponding points, such as have the posi-
tion just mentioned, whether they represent objects single or
not j whereas Dr. Reid says, that those points correspond,
whatever their position may be, which represent objects
single 5 and he appears to me not always to attend to the
double use of the same term, when he speaks of the opinions
of Dr. Smith.
AN ESSAY ON
Smith, I took occasion to remark, that the truth
of what distinguished it from all others might re-
main unshaken, though it were proved, that ob-
jects do not appear single, when their pictures
occupy any of the corresponding points of the
two retinas, since custom might have associated
the perceptions of touch, with the sensations of
any other parts whatsoever of those membranes.
The same observation will not apply with equal
justice to the opinion of Dr. Reid. On the
contrary, could it be shown, that the places of
the two retinas, which represent an object single
when each receives its picture, are not the cen-
tres, or such others as are similarly situated, an
obvious inference would be, that the single ap-
pearance of the object is not occasioned by a
property in those places, bestowed upon them
for this special purpose by nature ; it being
reasonable to expect, that such a property
should be found, if any where, in those parts of
the retinas which are the most like to each
other. I have, therefore, reserved till now, the
observations which have occurred to me upon
this subject, and which, when stated, must at
least, raise some doubt concerning what has
been regarded as true by Dr. Smith and Dr.
Reid, and by almost every other writer on
vision since the time of Kepler.
Anatomists have commonly taught, that the
SINGLE VISION. 17
centres of the spheres, to which the cornea, the
ball of the eye, and the two portions of the
crystalline belong, are all placed in the same
right line, hence called the optic axis, and that
this being produced both ways, passes through
the centres of the cornea and retina, considered
as surfaces. Opticians, on their part, observe,
that an object appears single to both eyes, when
the axis of each is accurately directed to it;
from which they infer, that the centres of the
retinas agree in suggesting but one object,
though each receives its picture. — Again ; since
it is known by experience, that, while any object
is seen single, to which the optic axes are turned,
others at the same distance from the eyes like-
wise appear so ; and since the pictures of these
lateral objects fall upon points in the two retinas,
equidistant from their centres, and both upon
the same side, that is, both to the right or left
of the centres, or both above or below them,
opticians conclude, that every two places of the
retinas, which are similarly situated with respect
to the centres, must also agree in exhibiting
but one object, though pictures are received by
both.
But the whole of this reasoning is built upon
a circumstance in the fabric of the eye, which
has been shown by some of the most eminent
c
IS AN ESSAY ON
anatomists not to have place. For Varolius*
long ago observed, that the crystalline is not
situated in the middle of the eye, but more in-
wardly ; and the accurate Zinnt has more lately
mentioned, that if the eye be divided into a
right and left half, the centre of the crystalline
will be found in the inner portion. HallerJ
confirms this fact ; and Winslow's || observation,
that the centres of the pupil and iris do not
coincide, but that the former is 'nearer to the
nose than the latter, is connected with it ; since
both Zinn and Haller agree, that the centre of
the pupil is placed in the axis of the crystalline,
while that of the iris is evidently in the common
axis of the cornea and globe. Now, a conse-
quence of this position of the crystalline is, that,
contrary to what I believe is universally main-
tained, no ray of light whatsoever can pass un-
bent to the retina from the atmosphere, or any
other medium differing in refractive power from
the aqueous humour. If, then, the line joining
the centres of the cornea and globe of the eye
be what is called the optic axis, and if it be
true, that objects appear single when we direct
* Varolii Anatomia, 12mo. p. 16.
f De Oculo, 4to. p. 127.
+ Elementa Physiologiae, tozn. v. p. 403.
J| Winslow's Anatomy, vol. ii. p. 379, English edition* 8vo.
SINGLE VISION. 19
both these axes to them, it must be evident, to
such as are acquainted with the common rules
of optics, that the pictures of those objects do
not fall upon the centres of the retinas, but
more internally ; and, therefore, that the centres
and all the other points of those membranes,
which by the present system are supposed to
represent objects single, do in fact exhibit them
double.
It will be said here, perhaps, that the line*
passing from each eye, which we turn to objects
when we see them single, is not a production of
the common axis of the cornea and globe, but
some other, disposed in such a manner, that the
pictures of those objects are received by the
centres of the retinas. I answer; I readily
grant the possibility of the thing, but I assert,
at the same time, that we have no proof of it,
-
* I am of opinion, that this line, or at least the line which
we turn to objects when we see them most distinctly with
one eye, is not the common axis of the globe and cornea.
For I find, that, when I plac j the flame of a candle between
either of my eyes, and a plane mirror, in such a manner that
it may conceal its own image in the mirror from the sight of
that eye, or rather that it may be a little below this image,
but in the same vertical plane with it, the image of the flame,
seen by reflection from the cornea, does not appear upon the
middle point of this coat, but upon that point of it which is
opposite to the centre of the pupil.
C 2
20 AN ESSAY ON
which is a sufficient reason for rejecting every
conclusion that depends upon its truth.
Admitting, however, that objects are repre-
sented single, when their pictures fall upon the
centres of the retinas, or upon any other two
points which are equally distant from the cen-
tres, and both upon the same side, it appears to
me, notwithstanding, to be in violation of all
analogy, to ascribe this effect, with respect to
the points, at least, on the right and left sides
of the centres, to any peculiar property which
they possess from nature. For when anatomists
find, in a new species of animals, organs similar
in structure to those of others they are already
acquainted with, they immediately conclude,
that they are also similar in regard to their use.
In animals of the same species, they believe
with certainty, that the organs they see in one
have the same properties, as the corresponding
organs of another ; and, if it be possible, they
attribute with greater certainty the same pro-
perties to two organs of the like kind, which
are found in the same individual. Such is the
influence of the rule, that resemblance of pro-
perty is implied by resemblance of structure.
Now it is an universal fact, that if an animal
be divided into a right and left half, the cor-
responding parts of those organs, which exist in
pairs, are found at equal distances from the plaae
SINGLE VISION. 21
of partition. Thus, for instance, in respect to
the eyes, the two optic nerves penetrate their
outward coat at the same distance from this
plane. Their muscles, blood-vessels, and every
other of their component parts and appendages,
are arranged in the like manner ; those nearest
to the dividing plane, or the innermost, in the
one, being similar in structure to the innermost
in the other, the outermost to the outermost,
and the intermediate to the intermediate. It
is surely, therefore, natural to expect, that such
parts should also be similar in their properties ;
and we in fact find this similarity to exist, where-
ever it can be clearly ascertained what the pro-
perties are. Every person, for example, admits,
that the internal straight muscle of the right
eye performs the same office, with respect to
that eye, as the other internal straight muscle
does with respect to the left eye. What judg-
ment are we then to form of the opinion of Dr.
Reid, which attributes the same original pro-
perties, or rather the joint possession of one ori-
ginal property, to places in the retinas situated
at unequal distances from the general plane of
partition ; which makes an external point in one
to correspond, in use, with an internal point in
the other, and this too by a principle implanted
by nature ? If such things exist, they may,, at
22 AN ESSAY ON
least, be said to stand opposed to a most ex-
tensive analogy.
To these arguments, a priori, against the
opinion of Dr. Reid, I shall now add others,
which are derived from a consideration of its
consequences.
First ; Since visible place, as was formerly
observed, includes in it visible distance, it is
evident that, if both eyes, by virtue of an ori-
ginal property, see an object in the same place,
distance must also be originally perceivable by
sight. Dr. Reid*, however, has himself so
ably shown, that we should never have ac-
quired, by means of our eyes, any knowledge of
distance, unless they had been assisted by the
sense of feeling, that I forbear to say any thing
more upon this head, than that the existence of
no property can be admitted, which leads to the
conclusion I have stated.
Secondly ; If distance be not immediately
perceivable by sight, the only manner, in which
an original property of the eyes can affect the
visible places of bodies, is by occasioning them
to appear in certain directions. Now Dr. Reid
maintains t, that every external point is seen in
* Inquiry into the Human Mind, chap. vi. sect. 3 & 20.
f Ibid. chap. vi. sect. 12.
SINGLE VISION. 23
the direction of a line passing from its picture
on the retina, through the centre of the eye.
If, therefore, this direction be the same as that
suggested by the original property so often men-
tioned, the latter law is merely another ex-
pression for the former, and ought to be rejected
as superfluous. If it be different, and should
the two laws exist together, objects seen with
both eyes might sometimes appear quadruple,
sometimes triple, but never single. Were they
to exist successively, one when we employ one
eye, the other when both, an object, though at
rest, should always appear to move when viewed
alternately by one and by both eyes ; neither of
which conclusions is agreeable to experience.
Thirdly ; To show in a different way, and one
perhaps more easily understood, that the opinion
of Dr. Reid is not consistent with the pheno-
mena of vision it ought to explain, I shall sup-
pose an experiment to be made upon a person
who squints. But I must premise, that it ap-
pears, from the observations of Dr. Jurin* and
himselff, that all such persons have one eye of
a weaker sight than the other ; that when both
eyes are open, the weaker is turned away from
objects, which are attentively viewed \ but that
* Smith's Optics, Vol. II. Remarks, p. 3O»
f Inquiry, chap. vi. sect. 10.
24 AN ESSAY ON
when the strong eye is closed, the weaker is
pointed to objects,, exactly as the former would
be in the same situation ; and that it likewise
perceives them in similar directions. Let now
the ordinary position of the person's eyes, upon
whom the experiment is made, be such, that
the optic axes intersect each other about an
inch or two from the face ; and while the other
is closed, let the flame of a candle be placed in
the axis of the weak eye, which I shall call the
left, at the distance of some feet from it, and on
the right side of the body. The flame will con-
sequently appear in the same direction, as if his
eye had no fault, and will be seen on his right,
where it is in reality situated. Both eyes re-
taining the same position with respect to his
head and each other, let the weak eye be after-
ward shut, and the right opened, and let another
object be placed in the axis of the latter, an
opake body being at the same time so disposed,
as to hide from it the candle which is in the
axis of the left eye. This object in the right
axis will consequently appear on the left side*
Now, since the two objects, which have been
thus viewed separately, are situated, one to the
right, and one to the left ; and since they have
been also seen in those positions, their visible
places must be two, as well as their tangible,
and must be remote from each other. How
SINGLE VISION. 25
then should these objects appear, if, instead of
being viewed alternately, each by the eye in
the axis of which it is placed, they were seen
by the two together ; the positions and internal
states of the eyes being in both cases the same ?
Dr. Reid must answer ; They will possess but
one visible place, since their pictures fall upon
the centres of the two retinas, points endowed
with the original property of representing ob-
jects single. But where is this one place to be
found ? In the axis of the right eye, or in that
of the left, or between the two ? In any of these
cases, or in any other that can be imagined, the
law of visible direction, so much insisted upon
by Dr. Reid, that objects appear in the per-
pendiculars to their pictures upon the retina,
and in truth every other law of visible direction
hitherto published, must be suspended with re-
spect to one or both eyes ; unless, indeed, the
united object be referred to the intersection of
the optic axes, about an inch or two from the
face. This, I believe, Dr. Reid would not
readily admit ; but if he should, another case
of squinting may be imagined, in which the
optic axes recede from each other, and where
the same reasoning will apply without the pos-
sibility of its force being thus eluded. It now
remains for me to mention, that the experiment
here stated by the way of supposition, in which
26 AN ESSAY ON-
the optic axes cross each other near to the fare,
was actually made by Dr.. Reid, with this result,
that the two objects appeared in different places,
when seen by both eyes together ; and that the
other experiment, in which the optic axes are
supposed to diverge, was made by myself, with
a similar event. Dr. Reid, however, instead of
being led, by the termination of his experiment,
to impute a fault to the principle from which
he had expected a different one, concluded from
it, that there was something unnatural, beside
the squinting, in the person's eyes, upon whom
it was made ; though it had been previously
ascertained, that objects appeared in the ordi-
nary manner to each of them, when separately
employed.
My examination of the second class of opi-
nions, respecting the cause of the single appear-
ance of objects to two eyes, being finished, some
person, perhaps, will now say ; Granting that no
error can, at first sight, be shown in your argu-
ments against those of Dr. Smith and Dr. Reid,
is it not a sufficient reason for believing them
fallacious, that they prove too much ? If objects
appear single neither from custom, nor an ori-
ginal property of the eyes, have we not an effect
without a cause, and must there not be some-
thing wrong in the facts or reasoning which lead
to such a conclusion ? The answer I make is
SINGLE VISION. 27
as follows : Since visible place contains in it
both visible distance and visible direction, it is
not necessary that the single appearance of an
object, to both eyes, should depend altogether
either upon custom, or an original principle of
our constitution ; for its visible distance to each
eye may be learned from feeling, and its visible
direction be given by nature ; in which case,
the unity of its place to the two eyes, will be
owing to neither of those causes singly, but to
a combination of both ; and this I regard as a
sufficient reply.
PART II.
Of a new Theory respecting Visible Direction, and of' a
Solution hence derived of the Question, why Objects are
seen single with two Eyes.
1 NOW proceed to offer a new opinion, why
objects are seen single with two eyes ; or in
other words, why they appear in the same place
to both, this being the light in which I view the
fact to be explained.
In every part of natural philosophy, accidents
often lead to discoveries, which reason alone
might not easily have reached. Under this
cover I hope to shelter myself from the charge
of presumption, in venturing to give the solu-
tion of a problem, upon which the talents of
many persons of great learning and "genius have
been unsuccessfully employed ; for should I
prove more fortunate than such men have been,
this must be attributed to the knowledge of a
circumstance I observed by chance, in repeating
some very common experiments.
The visible place of an object being com-
posed, as I have already several times remarked,
of its visible distance and visible direction, to
show how it may appear the same to both eyes,
it will be necessary to explain, in what manner
AN ESSAY O1ST SINGLE VISION. 29
th£ distance and direction, which are perceived
by one eye, may coincide with those which are
perceived by the other : and first with respect
to the distance.
In judging of distance by sight, we frequently
make considerable mistakes, even when the ob-
jects are not very remote ; but no person, I
believe, has ever observed, that while an object
seemed to one of his eyes at a certain distance,
it has appeared to the other to be at a different
distance, and from this circumstance alone has
been seen double ; or, to express the same thing
in another way, that while the visible appear-
ance of an object to one eye, covered the visible
appearance of the same object to the other eye,
the two appearances did not seem entirely to
coincide, and make one, but were seen separate
by the two eyes. I do not stop to give the rea-
son of this fact, which must be plain to those who
are acquainted with Bishop Berkeley's theory of
visible distance ; but proceed to mention, that
the difficulty in finding a true and sufficient
cause for the union of the two visible places of
one or two objects to two eyes, must therefore
consist altogether in showing, in what manner
the two apparent directions may coincide, con-
sistently with the attending phenomena.
Since Kepler's great discovery of the seat and
manner of vision, there have been, as far as I
30 AN ESSAY ON
know, only two theories offered respecting the
apparent directions of objects. One is, that
they are perceived in the direction of lines pass-
ing from their pictures on the retina, through
the centre of the eye ; the other, that their
apparent directions coincide with their visual
rays *. But both of these theories are inconsist-
ent with the phenomena of single vision with
two eyes. For according to neither of them can
an object, placed at the concourse of the optic
axes, be seen single, unless we have a most ac-
curate knowledge of its distance ; nor will either
admit two objects to be seen as one, which are
situated in the optic axes, whether on this side,
or beyond where they meet, unless the united
object be referred by sight to their very point
of intersection ; both of which conclusions are
contradicted by experience. It is evident,
therefore, that some other theory of visible di-
rection is required, which shall not be liable to
* Mr. D'Alembert has said (Opuscules Mathematiques,
Tom. I. p. 265) that all optical writers before him had re-
garded it as an axiom, that every visual point is seen in the
direction of its visual ray. But the assertion is not well
founded. For Kepler long ago taught (Paralipomena in
Vitellionem, p. 173), that objects are perceived in lines pass-
ing from their pictures upon the retina, through the centre of
the eye j in which he was followed by Dechales and Doctor
Porterfield ; to the latter of whom Dr. Reid improperly at-
tributes the discovery of the same supposed law.
SINGLE VISION. 31
"these objections ; and such a theory, I hope, I
shall bring forward in the following proposi-
tions, after mentioning the meanings which I
&ffix to several terms I shall frequently employ.
EXPLANATION OF TERMS.
I. When a small object is so placed with re-
spect to either eye, as to be seen more distinctly
than in any other situation, I say it is then in
the optic axis, or the axis of that eye ; and if
another small body be interposed between the
former and the eye, so as to conceal it, and if
a line joining the two be produced till it falls
upon the cornea, I call this line the optic axis>
or the axis of the eye ; leaving for future de-
termination the precise point of the cornea it
falls upon, or what part of the retina receives
the picture of an object which is placed in it.
II. When the two optic axes are directed to
a small object not very distant, they may be
conceived to form two sides of a triangle, the
base of which is the interval between the points
of the corneas, where the axes enter the eyes ;
but if the object be very distant, then they may
be supposed to be two sides of a parallelogram,
whose base is the same interval. To avoid cir-
cumlocution, I shall call this interval the visual
base.
III. If there be drawn a line from the middle
32 AN ESSAY ON
of the visual base, through the point of inter-
section of the optic axes, or parallel to them, if
they be parallel to each other, I name it the
common axis*. This term, I believe, was in-
vented by Alhazen ; but with him it signified a
line drawn from the centre of the junction of
the optic nerves, through the middle of the in-
terval between the centres of the retinas. Such
a line was consequently immoveable. As the
term, however, is not in modern use, no mis-
take can arise from confounding the two mean-
ings, and the reason will soon be seen, why I
employ it in the sense I have mentioned. Those
who are acquainted with the writings of the
older opticians, will perceive, that I give it
nearly the same signification as they did to their
common radius.
* It may be said, perhaps, that as I do not define the
points of the corneas, upon which the optic axes fall, I can-
not, with propriety, desire the line which connects them to be
divided. To this I answer, that it is not necessary for the
purpose I have mentioned, that they should be defined ; if it
be granted to me, and I think it cannot be refused, that upon
whatever point of the right cornea the right axis falls, the
left axis will fall upon a similarly situated point of the left
cornea ; that is, if this point of the right cornea be at any
given distance from its middle, and upon the inside of it, the
corresponding point of the left cornea will be at the same
distance from the middle of this, and also upon its inside.
Whatever extent, therefore, the line connecting these places
of the corneas may have, its middle point will be the same.
SINGLE VISION.
PROPOSITION I.
Objects situated in the Optic Axis, do not appear to be in
that Line, but in the Common Axis.
EVERY person knows, that, if an object be
viewed through two small holes, one applied to
each eye, the two holes appear but as one. The
theories hitherto invented afford two explana-
tions of this fact. According to Aguilonius,
Dechales, Dr. Porterfield and Dr. Smith, the
two holes, or rather their borders, will be seen
in the same place as the object viewed through
them, and will consequently appear united, for
the same reason, that the object itself is seen
single. But whoever makes the experiment
will distinctly perceive, that the united hole is
much nearer to him than the object ; not to
mention, that any fallacy on this head might be
corrected by the information from the sense of
touch, that the card, or other substance, in
which the holes have been made, is within an
inch or less of our face. The other explana-
tion is that furnished by the theory of Dr. Reid.
According to it, the centres of the retinas, which
in this experiment receive the pictures of the
holes, will, by an original property, represent
but one. This theory, however, though it
D
34 AN ESSAY ON
makes the two holes to appear one, does not
determine where this one is to be seen. It
cannot be seen in only one of the perpendicu-
lars to the images upon the retinas, for no reason
can be given why this law of visible direction,
which Dr. Reid thinks established beyond dis-
pute, if it operates at all, should not operate
upon both eyes at the same time ; and if it be
seen by both eyes in such lines, it must appear
where those lines cross each other, that is, in
the same place with the object viewed through
the holes, which, as I have already mentioned,
is contrary to experience. Nor is it seen in any
direction, the consequence of a law affecting
both eyes considered as one organ, but sus-
pended when each eye is used separately. For
when the two holes appear one, if we pay at-
tention to its situation, and then close one eye,
the truly single hole will be seen by the eye re-
maining open, in exactly the same direction as
the apparently single hole was by both eyes.
Hitherto I have supposed the holes almost
touching the face. But they have the same
unity of appearance, in whatever parts of the
optic axes they are placed ; whether both be
at the same distance from the eyes, or one be
close to the eye in the axis of which it is, and
the other almost contiguous to the object seen
through them. If a line, therefore, be drawn
SINGLE VISION. 35
from the object to one of the eyes, it will re-
present all the real or tangible positions of the
hole, which allow the object to be seen by that
eye, and the whole of it will coincide with the
optic axis. Let a similar line be drawn to the
other eye, and the two must appear but as one
line ; for if they do not, the two holes in the
optic axes will not, at every distance, appear
one, whereas experiments prove that they do.
This united line will, thereforer represent the
visible direction of every object situated in
either of the optic axes. But the end of it,
which is toward the face, is seen by the right
eye to the left, and by the left eye as much to
the right. It must be seen then in the middle
between the two, and, consequently, in the
common axis. And as its other extremity coin-
cides with the point where the optic axes inter-
sect each other, the whole of it must lie in the
common axis. Hence the truth of the proposi-
tion is evident, that objects, situated in the optic
axis, do not appear to be in that line, but in the
common axis.
Many other experiments might be mentioned
which demonstrate the same thing. If, for ex-
ample, the head of a pin, or of a needle, be
interposed between each eye, and any small
object to which both the optic axes are directed,
D 2
36 AN ESSAY ON
the heads of the two pins or needles will con~
stantly appear as one in the common axis.
When the heads, however, are near to the eyes,
this experiment is not so satisfactory as the
former, since, in these positions, they seem as
broad transparent shadows, for reasons known
to every person a little conversant in optics;
whereas the holes appear well defined, though
almost touching us. Again ; if we hold two
thin rulers in such a manner, that their sharp
edges shall be in the optic axes, one in each, or
rather a little below them, the two edges will
be seen united in the common axis, and this
apparent edge will seem of the same length with
that of either of the real edges, when seen alone
by the eye in the axis of which it is placed. If
instead of two rulers we employ two strings of
different colours, as red and green, the like unity
of appearance will be observed. But in this
experiment it frequently happens, that, contrary
to what we might naturally expect, only one of
the strings is seen at a time. When, however,
only one is seen, its apparent situation is exactly
the same as that of the string, compounded, if
I may so express myself, of the two when seen
together; and hence we have a convincing
proof, if any were wanted, that the single ap-
pearances of objects must depend upon some
SINGLE VISION. 37
law of visible direction affecting each eye, when
employed by itself, in the same manner as when
it is used conjointly with the other*.
* Du Tour expected, that if two objects of different colours
were seen in the same place by both eyes, which however he
says, he was never able to observe, the colour of the ap-
parently united object would be compounded of those of the
two really single objects. Memoires des Savans Etrangers,
torn. iv. p. 500. And Dr. Reid mentions expressly that it is
so compounded. Inquiry, p. 2Q3. But in all my experi-
ments upon this subject I have remarked, that, when the two
objects appeared united, each was seen, notwithstanding, in
its proper colour ; the red, for example, appearing as it were
through a transparent green, and the green, in the same ex-
periment, as through a transparent red. Nor is there any
thing in this inconsistent with the received doctrine of the
composition of colours. For in every instance of the pro-
duction of a new colour, from rays of different colours being
at the same time sent to the eye, these rays fall upon the
same sentient extremities of the same nerve. But, in the
case before us, the differently-coloured rays fall upon the
sentient extremities of two different nerves, which have no
communication with each other, except through the medium
of the brain. We have greater reason, therefore, for expect-
ing, that the colours impressed upon the two eyes, should be
perceived uncompounded, than there is for two colours being
perceived separately, which are impressed upon two different
parts of the same eye.
From the fact of the two colours being thus perceived di-
stinct from each other, I would infer, by analogy, a mode of
argument indeed often fallacious, that if it were possible for
<as to hear any one sound with one ear only, and another
sound with the other ear only, such sounds would in no case
38 AN ESSAY ON
PROPOSITION II.
Objects, situated in the Common Axis, do not appear to
be in that Line, but in the Axis of the Eye, by which
they are not seen.
THE facts which demonstrate the truth of
this proposition, are both numerous and com-
mon. If a piece of wire, or any other sub-
stance, representing a physical line, be placed
in the common axis, with one of its extremities
near to the visual base, and if both the optic
axes be directed to its farther or distant ex-
tremity, instead of one, two wires will be seen,
meeting each other at their farther ends, and
gradually diverging as they approach the face,
till they apparently terminate at the eyes. If
the right eye be closed, the wire which seemed
coalesce either wholly or in part, as two sounds frequently do,
when heard at the same time by one ear 5 that consequently,
if the sounds of one musical instrument were to be heard by
one ear only, and those of another, by the other ear only, we
could have little or no perception of harmony from such
sounds -j and that, if in any succession of sounds emitted by
one instrument, we were to hear the 1st, 3d, 5th, and so on,
by one ear only, and the 2d, 4th, 6th, and so on, by the other
ear only, we should be deprived, in a considerable degree, of
the melody of such sounds, as this seems to depend in a great
measure upon a new impression being made upon the audi-
tory nerve by one sound, before the impression of the sound
immediately preceding has passed away.
SINGLE VISION. 39
to terminate at the left eye, disappears ; and if
the left eye be closed, then the other wire dis-
appears, whose termination was at the right
eye. The real wire, therefore, in the common
axis, appears to the right eye to be situated in
the axis of the left, and to the left eye to be
situated in the axis of the right, agreeably to
what the proposition asserts.
The following experiments will illustrate and
confirm both this arid the preceding proposition.
Through a piece of card, or pasteboard, let two
small holes be made, the interval between which
is such, that while a very remote object is seen
through one of them by the right eye, the same
object may be seen through the other by the
left eye. Make afterward another hole in the
card, or pasteboard, exactly in the middle be-
tween the two former ; and let the object be
viewed through them as before. These, or
the outer holes, will now appear one, precisely
where the sense of feeling indicates the middle
hole to be ; while the middle hole will appear
as two, which seemingly occupy the places of
the real outer ones. The two appearances of
the middle hole, which is placed by construction
in the common axis, are therefore seen in the
optic axes ; and as the left is not seen when the
right eye is shut, nor the right when the left
eye is shut, each appearance is observed in the
40 AN ESSAY ON
axis of the eye, by which it is not seen. As I
have supposed the distance between the outer
holes to be adapted to the interval of the eyes
when they are directed to a very remote object,
the optic axes may, in this case, be regarded as
parallel to each other. The object, therefore,
will still be seen through those holes, though
the distance of the card from the eyes be con-
siderably varied ; and at all the different di-
stances, the same appearances will be observed,
as those which have been mentioned.
Again ; take three strings of different colours,
as red, yellow, and'green, and fasten, by means
of a pin, one end of each to the same point of a
table. Place now their loose ends in such a
manner, that when you look at the pin with
both eyes, the visual base being parallel to the
edge of the table, the red string may lie in the
axis of the right eye, the green in that of the
left, and the yellow in the common axis. When
things are thus disposed, and both eyes are di-
rected to the pin, the red and green strings, in-
stead of appearing separate, each in one of the
optic axes, and inclined to the visual base or
edge of the table, will now be seen occupying
but one place, either together or successively,
as was formerly mentioned, and at right angles
to the visual base, or edge of the table ; in short,
exactly in the situation, which the yellow string
SINGLE VISION. 41
in reality possesses ; and the yellow string, in-
stead of appearing single in the common axis,
and perpendicular to the visual base, will now
be seen as two, each inclined to the base ; that
seen by the right eye, apparently occupying the
place in reality possessed by the green string,
and that seen by the left eye, the place of the
red string.
PROPOSITION III.
Objects, situated in any Line drawn through the mutual
Intersection of the Optic Axes to the Visual Base, do
not appear to be in that Line, but in another, drawn
through the same Intersection, to a Point in the Visual
Base distant half this Base from the similar Extremity
of the former Line, towards the left, if the Objects be
seen by the Right Eye, but towards the right, if seen
by the Left Eye.
Two cases of this proposition have already
been proved. For it has been shown by the
first proposition, that objects, placed in the axis
of either eye, appear to it to be situated in the
common axis. But the common axis is a line
drawn through the mutual intersection of the
optic axes to the visual base, and its termina-
tion there is distant, by construction, half that
base, from the similar terminations of the axes
42 AN ESSAY ON
of both eyes, to the left of the right axis, and to
the right of the left. Again, it has been shown
by the second proposition, that objects, placed
in the common axis, appear to each eye to be
situated in the axis of the other ; and the ter-
minations of both optic axes, at the visual base,
are distant half this base, from the similar ter-
mination of the common axis, the left being to
its right, and the right to its left.
Let it now be supposed that two objects, one
placed in the axis of either eye, the right for
instance, and the other in the common axis, be
viewed at the same lime by that eye, it is evi-
dent that the visible directions of both will be
equally removed to the left, from their real posi-
tions. But such an alteration of visible direc-
tion, from real position, cannot be imagined to
happen, with respect to objects placed in the
optic and common axes, unless a similar effect
be, at the same time, produced upon such as are
situated any where between those lines, or in
their vicinity. Facts confirm this : If a line, for
example, be drawn through the intersection of
the optic axes to a point in the visual base,
exactly in the middle between the terminations
there of the right and common axes, its ap-
parent situation, to the right eye, will be found
to have the same relation to the apparent situa-
tions of lines placed in the right and common
SINGLE VISION. 43
axes, as its real situation has to the real situa-
tions of such lines. And the like will be found,
by observation, to be true of every other line,
which may be drawn through the point of inter-
section of the optic axes to the visual base.
The whole of what has here been said may be
illustrated and confirmed, by having again re-
course to the experiments with strings of dif-
ferent colours. In formerly describing those
experiments, I did not mention all the appear-
ances which occurred upon making them, hut
only such, as had immediate reference to the
points then under consideration. When, for
instance, a red string was placed in the axis of
the right eye, and a green one in that of the
left, I said that they both appeared in the com-
rnon axis. But this is not the only phenomenon
to be observed with respect to their apparent
number in this experiment. For as the red
string is also seen by the left eye, and the green
by the right, two other strings become visible,
beside that in the common axis, the apparent
positions of both of which will be found to be
the same with those, which ought to follow from
the present proposition. iShould now a yellow
string be placed between the two former, as in
the proof of the second proposition, its appear-
ance to the right eye will bisect the space be-
tween the appearances of the red and green
44 AN ESSAY ON
strings to that eye ; and the like will be true
with respect to the appearances of the three
strings to the left eye, agreeably to what the
same proposition teaches us to expect.
I believe I need scarcely remark, that, al-
though in most of the proofs and illustrations
of these propositions, I have confined myself to
the visible appearances of lines between the in-
tersection of the optic axes and the visual base,
the same things, however, must be equally true
of those lines, when they are produced beyond
the intersection, with this difference only, that,
while the portions within, seem, to the right eye,
to be farther situated to the left than they really
are, but to the left eye farther to the right, the
portions beyond the intersection will seem to
the right eye to the right of their real positions,
but to the left eye to the left of them. For it is
manifest, that, if a line be seen by one eye in a
certain direction, a prolongation of it must be
seen in the same direction ; and that, if a line
be made to turn upon any point in itself, the
two extremities must move contrary ways.
Should the optic axes be parallel to each
other, the same proofs and illustrations will still
apply, since we may here suppose them to meet
at an infinite distance from the visual base. In
this case, the visible appearances of lines, drawn
from this supposed point of intersection to the
SINGLE VISION. 45
visual base, will be parallel to the real lines, and
distant half this base from them, through their
whole extent.
As I have thus, I think, sufficiently proved,
that the apparent directions of objects are go-
verned by a law, different from any which has
hitherto been thought to exist, I shall now pro-
ceed to state, in a few words, in what manner
the phenomena of single and double vision with
two eyes are dependant upon it.
I formerly mentioned, that, since an object is
never seen double, merely from its being seen
at different distances by the two eyes, the only
difficulty in explaining its single appearance
consists in showing how its two visible direc-
tions may coincide, consistently with the attend-
ing phenomena. But we are enabled to do this,
with the utmost ease, by the theory 1 have en-
deavoured to establish. For, if the question be
concerning an object at the concourse of the
optic axes, I say it is seen single, because its two
similar appearances, in regard to size, shape,
and colour, are seen by both eyes in one and
the same direction, or, if you will, in two direc-
tions, which coincide with each other through
the whole of their extent. It therefore matters
46 AN ESSAY ON
not, whether the distance be truly or falsely
estimated ; whether the object be thought to
touch our eyes, or to be infinitely remote. And
hence we have a reason, which no other theory
of visible direction affords, why objects appeared
single to the young gentleman mentioned by
Mr. Cheselden, immediately after his being
couched, and before he could have learned to
judge of distance by sight.
When two similar objects are placed in the
optic axes, one in each, at equal distances from
the eyes, they will appear in the same place,
and therefore one, for the same reason that a
truly single object, in the concourse of the optic
axes, is seen single. Here again, as the two
visible directions coincide in every point, it is
not necessary that the united appearance should
be judged to be at any particular distance ; that
it should be referred, for instance, to the con-
course of the optic axes, where the two other
theories of visible direction are obliged to place
it, in opposition to the plainest observations.
Objects, any where in the horopter, will be
seen single, because their apparent directions to
the two eyes will then completely coincide.
And for a contrary reason, those placed in any
other part of the plane of the optic axes will
appear double. To make these things evident,
let a line pass through the point of intersection
SINGLE VISION. 47
of the optic axes and any given object, to the
visual base, which is to be produced, if neces-
sary ; and let it be called the line of the ob-
ject's real position. Take afterward, in the
visual base, or its production, two points, one
on each side of the line of real position, and
both distant from its termination there, half
the visual base. Lines drawn from these points,
through the point of intersection of the optic
axes, must consequently contain the two visible
positions of the object. But when this is
situated in the horopter, the line of real posi-
tion will coincide with the horopter, and will
not therefore reach the visual base, unless at
an infinite distance from the eyes. For which
reason, the two lines, containing the visible po-
sitions of the object, must fall upon the visual
base at a like distance, and must consequently
be regarded as coinciding with each other.
When the object is not in the horopter, the two
lines of visible direction will be found, by the
same means, not to coincide.
That I might simplify a matter, which under
my management, must, I fear, still be of dif-
ficult apprehension, I have, in expressing the
law of vision, so frequently mentioned, pur-
posely confined it to objects situated in the
plane of the optic axes. But in persons who
do not squint, or whose eyes are not distorted
48 AN ESSAY ON
by external violence, the two appearances of
an object, seen double, are always, either in
that plane, or in some one parallel to it; so
that, if the visual base be parallel to the ho-
rizon, a line joining the two appearances will,
in every case, be also parallel to the horizon.
Whoever then is able to explain, why objects
in the plane of the optic axes appear either
single or double, may readily give a reason for
the like appearances of such as are placed any
where else. Not to spend much time, there-
fore, upon this part of the subject, I shall
shortly observe, that if planes be supposed to
pass through the two optic and common axes,
perpendicular to that in which they all lie, and
if two lines be drawn from any point of the
common intersection of the former planes to
the visual base, one along each of the perpen-
dicular planes which pass through the optic
axes, these two lines will appear as one, in the
perpendicular plane of the common axis ; the
single visible line, however, possessing the same
elevation, in regard to the horizon, as the two
real lines : And again, that, if a line be drawn
from any point of the same intersection to the
visual base, along the perpendicular plane of
the common axis, it will appear as two, one in
each of the planes which pass through the optic
axes ; the two visible lines having the same in-
SINGLE VISION. 49
clination to the horizon in their progress to the
visual base, as the real single one. In this man-
ner, every thing may be shown to be true, with
respect to the single and double appearances of
objects without the plane of the optic axes,
which has already been done with regard to
those placed in it. But farther ; since any
point, taken at pleasure, in the common inter-
section of the three perpendicular planes, ap-
pears single, the whole of the line of intersec-
tion must appear so, and likewise every point
of a plane made to pass through it, parallel to
the visual base. Such a plane necessarily in-
cludes the horopter, and is the same as that,
which is called by Aguilonius the plane of the
horopter.
To exemplify the principal property of this
plane, I shall mention an experiment, which at
first I did not understand, though the result
was a direct consequence of my own principles.
I suspended a fine chord at right angles to the
horizon, and retreating a step or two, I looked
steadily at a point in it, which was upon a level
with my eyes. The chord, in these circum-
vstances, appeared single ; but whenever I di-
rected my eyes to any other point of it, either
above or below the former, two chords would
appear, crossing each other at the part, to which
the eyes were directed. In the first case, the
50 AN ESSAY ON SINGLE VISION.
whole chord was in the plane of the horopter,
but in every other, only that point of it to which
both eyes happened to be turned. A conclusion
from this experiment is, that no object, which
is truly perpendicular to the horizon, will ap-
pear to be so, while our bodies are erect, unless
we direct our eyes to a point in it exactly upon
a level with themselves.
It was once my intention to subjoin here
several instances, from the most approved
authors, of inaccurate descriptions of the single
and double appearances of objects ; in order to
show, that the theory of visible direction, which
I have advanced, is not only consistent with the
universally received facts, but that it also dis-
covers to us, some minute errors, which un-
guided sense has committed upon this subject ;
it being, perhaps, one of the surest tests of the
soundness, as well as one of the chief uses, of
theories in philosophy, that they lead to the
knowledge of what, otherwise, might have re-
mained for ever hidden. But fearing I have
already proved tiresome, I give up this design,
and hasten to the consideration of some conse-
quences from my theory, which seem to me
both curious and important, and which, when
first mentioned, may appear to carry with them
their own refutation.
PART III.
Of some Consequences from tfie foregoing Theory of
Objects being1 seen single with two Eyes, together with
the Explanation of several other Phenomena of Vision,
IT has hitherto, I believe, been thought by
opticians, that, if the position of the eye be un-
changed, the visible direction of an object will be
the same, as long as its picture occupies any one
point of the retina ; and that, in every different
position of the eye, a picture, which continues to
occupy the same point of the retina, will repre-
sent its object in a different direction. But if
the theory be just, which I have advanced in
the preceding part of this Essay, neither of
those opinions can be universally true. For it
follows, from what was there mentioned, that
if one of the optic axes be kept fixed, and the
other be at different time's variously bent to-
ward it, objects, though situated in the fixed
axis, will nevertheless change their visible di-
rections, with every variation of the moveable
axis j since they must always appear in the
common axis, which alters its position with
every change of the moveable axis : And again,
that>, if the two optic axes should vary their
E 2
52 AN ESSAY ON
inclinations to each other in such a manner, that
the common axis, may, notwithstanding, remain
fixed, an object placed in either optic axis, and
following it in every motion, will possess but
one visible direction,, in all this variety of real
positions. That these conclusions from my
theory, or rather parts of it, are true in fact,
I can assert upon the authority of observations,
and I shall now attempt to trace them both to
a common principle, by means of some experi-
ments, which were instituted with a very dif-
ferent view.
When we have looked steadily for some time
at the flame of a candle, or any other luminous
body, a coloured spot will appear upon every
object, to which we shortly after direct our
eyes, accompanying them in all their motions,
and exactly covering the point, which we desire
to see the most accurately. Whatever there-
fore can be proved concerning the apparent
direction of such a spot, in any given position
of the eyes, must likewise be true in the same
position of the eyes, with regard to the appa-
rent direction of an object, situated at the con-
currence of the optic axes ; as its pictures must
occupy, in this case, the very -parts of the re-
tinas, upon the affections of which the illusion of
the spot depends. This being premised, I shall
now relate one or two observations, respecting
SINGLE VISION.
the apparent directions of the spot, and conse-
quently upon those of external objects, which,
as far as 1 know, have not been mentioned by
any other person.
1. The spot is always seen single, whether
the surface, upon which it is projected, be
touching the face, or at the greatest distance
from us j and the reason is plain. For the
parts of the retinas, by whose affections from
the luminous body it is occasioned, are those
likewise which receive the pictures of objects,
placed at the intersection of the optic axes;
and as such objects always appear single, so
must also the spot. The fact indeed is so open
to observation, and its cause so easily shown,
that I should scarcely have thought of men-
tioning it, had not Dr. Darwin* lately told us,
that the spot is seen double, as often as the
eyes are directed to an object more or less
distant than the luminous body which gave rise
to it. With respect to our different assertions
upon this point, I shall only say, that I have
made the experiment, I believe, upward of an
hundred times, uniformly with the same result;
and that, if the spot ever appears double, this
must be from some cause very wide of a change
* Philosoph. Transact, for 1786, p. 318. Dr. Darwin
indeed, says, p. 341, that Buffon had observed the same
fact ; but it is evident he has mistaken that author's meaning,
AN ESSAY ON
in the mutual inclination of the optic axes, to
which he attributes it*.
2. The spot not only appears single in every
ordinary position of the optic axes, but cannot
even be made to appear double, by any means
whatsoever. If it be projected, for example,
upon a piece of white paper, whoever makes
the trial will find, that, although, on pressing
one eye upward or downward, or to either side,
the paper will be seen double, yet the spot will
always appear single, and to possess its former
place on the paper, as se6n by the eye, which
is not disturbed. Before I knew the result of
this experiment, I had imagined, that, the po-
sition of one eye being forcibly altered, the
* The only way, in which I think it possible for the spot
to appear double, consistently with the universally acknow-
ledged fact, that an object at the intersection of the optic
axes is always seen single,, is this, that, when the intersec-
tion is near to the face, an object placed in it shall not send
its pictures to the same points of the two retinas, as it does,
when the intersection is more remote. And such I once
hoped to find to be the case; for I had formed, upon the
supposition of its truth, a more plausible account of the
manner in which the eyes are fitted to receive, successively,
pictures equally distinct from objects at different distances,
than any I had met with. But, after many experiments to
ascertain the matter, I was obliged to return to the common
opinion, that the picture of an object in the optic axis, what-
ever be its distance from the eye, is always received upon
the same point of the retina.
SINGLE VISION. 55
external situation of the spot, which was sug-
gested by the affection of that eye, would like-
wise be altered, and the spot by consequence
be seen double. As the event, however, was
contrary to my expectation, I began to suspect
some cause of fallacy had been overlooked,
which at length I thought might be this, that
the spot had been seen by that eye only whose
position was not disturbed, the violence, suf-
fered by the other, interrupting the due exer-
cise of its functions. To determine, therefore,
whether my conjecture was well founded or
not, I made another experiment, which is men-
tioned in the following article :
3. Having looked steadily for some time at
the flame of a candle, with one eye only, I
directed afterward, with both eyes open, my
attention to the middle of a sheet of paper, a
few feet distant; the consequence of which
was, that a spot appeared upon it in the same
manner, as if I had viewed the flame with both
eyes, though somewhat fainter. My attention
remaining fixed upon the sheet, I now pushed
the eye, by which the spot was seen, succes-
sively upward and downward, to the right and
to the left, and in every oblique direction ; the
spot however never altered its position, but
kept constantly upon the middle of the appear-
ance of the paper, perceived by the undistorted
AN ESSAY ON
eye, though the appearance of the paper to the
distorted eye, was always separate from the
former, and the sheet consequently seen double.
My conjecture, therefore, was proved to be ill
grounded, and all suspicion of fallacy in the
former experiment ceased.
Now it is evident, from these two last experi-
ments, that the situation of the spot does not
depend upon the bare position of the eyes, or
else, in the former of them, it would have ap-
peared double, and in the latter, it would have
been moved from the middle of the paper,
when the only eye by which it was seen was
pushed from its place. Neither can it depend
upon the bare position of the muscles of the
eye, as these were also moved in the same ex-
periments ; nor upon any affection whatever of
the optic nerve. For since this last substance
is altogether passive, even in those motions of
the eyes which do occasion a change of the
spot's situation, every alteration, induced upon
the nerve by those motions, must be ultimately
ascribed to a change of its position ; and we
have seen, that similar changes of its position
have been produced by external violence, with-
out any alteration of the spot's situation. The
apparent situation of the spot being, therefore,
dependant upon none of these circumstances,
and being at the same time affected by the
SINGLE VISION.
voluntary motions of the eye, it must, I think,
be necessarily owing to the action of the mus-
cles, by which these motions are performed.
Assuming then as true, that the apparent di-
rection of an object, which sends its picture to
any given point of the retina, depends upon the
state of action existing at the same time in the
muscles of the eye, and consequently that it
cannot be altered, except by a change in the
state of that action, I shall proceed to trace
to this principle, several phenomena of vision,
particularly the uniform singleness of the spot
already described, and the two facts respecting
the visible directions of objects in the optic
axis, which were mentioned in the beginning
of this part of my Essay.
The thing itself is universally acknowledged,
though a dispute has arisen whether custom or
an original property be the cause, that every
voluntary motion of one eye, in persons who
do not squint, is attended with a corresponding
motion in the other. Now as all voluntary
motions are produced by muscular action, it
follows, that every state of action, in the
muscles of one eye, has its corresponding state
in those of the other, and that the two are
constantly conjoined. When, therefore, the
spot appears single to both eyes in their free
positions, the states of action in the muscles
58 AN ESSAY ON
must be such, that the direction, in which it is
seen by one eye, coincides with that in which
it is seen by the other. But, if we push one
eye from its place, no change is hereby made
in the action of its muscles ; for the state of
action in those of the free eye is confessedly
the same as it was ; and it will be attended
with a corresponding state in those of the dis-
torted eye ; in proof of which it may be ob-
served, that, whenever the pressure is removed,
the distorted eye immediately returns to its
former position, without the aid of any new
muscular effort. The conclusion then is, that,
since there has been no alteration in the action
of its muscles, neither ought there to be any in
the direction of the spot seen by it, which is
the fact to be explained.
Hence also is to be derived the true reason,
why objects appear double, when one eye is
pushed from its place. For as their pictures
must fall upon points of the retina in this eye,
different from what they formerly possessed ;
and as no change is made, by the distortion,
upon the visible direction, suggested by any
part of the retina, the objects will be seen by
the pressed eye, exactly in the same directions
as they would have been, before it was pressed,
had the pictures then fallen upon the points of
the retina, which they now occupy. They must
SINGLE VISION. 59
therefore be now seen in different directions by
the two eyes, and consequently double. An
experiment with a contrary event will confirm
this explanation, and likewise show more
clearly, in what I differ from those who have
endeavoured to account for the same fact.
Both eyes being open, let one of them be
pushed from its situation, and let two similar
objects, such as two pieces of money of the
same metal and stamp, be afterward so placed,
that one shall He in each optic axis ; these two
objects will now appear to be one, and the
object so compounded will be seen in the place,
to which the undisturbed eye refers the truly
single object lying in its axis.
Another inference from this doctrine is, that,
if the eyes are in any very unusual position
with respect to each other from the action of
their own muscles, as in persons who squint,
two objects placed in the optic axes, one in
each, will not appear as one object ; for each
will be seen in the direction, which is de-
termined by the state of action in the muscles
of the eye, upon whose retina its picture falls ;
and as this state, in one eye, does not corre-
spond with that in the other, the directions
cannot coincide. This conclusion is verified
by the result of an experiment of Dr. Reid
upon a person, affected with strabismus, and
60 AN ESSAY ON
by that of another, made by myself, both of
which have been already related.
To explain, therefore, why an object in the
optic axis appears at different times in different
directions, though the axis be kept fixed, it
is only necessary to show, that, whenever this
happens, a change, notwithstanding, occurs in
the actions of the muscles which move the eye.
With this view, I observe, that the motions of
that organ maybe divided into two sets; the
first, consisting of those, by which one eye
is carried along with the other, upward and
downward, to the right and to the left, and in
every oblique direction, the interval between
the pupils remaining constantly the same ; the
second, of the motions of the pupils, or the an-
terior parts of the eyes, to and from each other.
Supposing now, that both the optic axes are
perpendicular to the visual base ; should the
left axis be afterward inclined to the right side,
the natural tendency of the right axis is to in-
cline equally to the same side, so as to preserve
its former parallelism to the left. This ten-
dency, however, in the right axis to follow the
left, may be counteracted by an effort of the
muscles, which regulate the interval of the
pupils, until the two axes intersect each other
within two or three inches of the face. But it
is evident, that the same degree of muscular
SINGLE VISION. 61
force will be required to retain the right eye in
its original position, as is necessary to give to
the left eye its motion toward the right ; and
hence, that, in every different inclination of
the left axis to the right, an object placed in
the latter, though its real position be un-
changed, will, nevertheless, appear in a dif-
ferent direction, in consequence of the dif-
ferent state of action in the muscles of the right
eye, which accompanies every new degree of
inclination of the axes to each other. As the
object must always appear in the common axis,
the alteration, in this example, of its visible di-
rection, from an increase of the mutual inclina-
tions of the optic axes, will be from left to
right ; but when the inclination decreases,
from right to left. If the right axis be the one
which is moved, and the left fixed, the altera-
tions of visible direction in an object placed
in the latter, from similar changes in their in-
clinations, will be contrary to those which have
just been mentioned.
The reason also can now be made to appear,
why an object, preserving constantly its place
in the optic axis, may, in a considerable variety
of its real positions, possess but one visible
direction. For, in such cases, the change of
its visible direction, which might be expected
to accompany the motion of the eye in the axis
62 AN ESSAY ON
of which it is situated, is prevented from oc-
curring, by a tendency to a change of its visible
direction the contrary way, produced by the
muscular actions which regulate the mutual
distance of the pupils. To know how this
happens, suppose the two optic axes to be pa-
rallel to each other, and perpendicular to the
visual base ; and let a physical line be placed
in either of them, so as entirely to coincide
with it. This line will, therefore, not only be
in reality perpendicular to the visual base, but
will, in the present state of things, likewise
appear so. — Incline afterward both the axes
equally to the left side, and it is manifest that
the line coinciding, say, with the right axis,
must appear equally inclined. Let now the
right axis be kept fixed, and the left be carried
back again, and its motion continued, until it
be as much inclined toward the right side, as
itself was just before, and as the right axis is
still to the left side ; the consequence will be,
that the line in the right axis must again be
seen perpendicular to the visual base; for such
is the present position of the common axis.
Here then we have had two opposite causes of
change of apparent direction acting in succes-
sion. The muscular actions, producing the joint
motions of the eyes, first bent the visible posi-
tion of a line, in the right optic axis, from a
SINGLE VISION. 63
perpendicular to the visual base toward the left;
and the muscular actions, which regulate the
mutual distances of the pupils, by increasing
the inclinations of the axes to each other,
moved it afterward, from the left to the right,
back again to a perpendicular to the visual base.
Let these two causes act together, and it is
plain, that no observable effect will be pro-
duced by either, as long as they are thus pro-
portioned. When they are not so, only the
difference of their forces will be exhibited by
the phenomena.
But farther ; to show the extent of this theory
of visible direction being dependant upon the
actions of the muscles of the eyes, I shall now
apply it to the explanation of an instance of
apparent motion, which at first may be thought
to furnish an argument against it. Look with
one eye, the other being closed, at any remote
object through a small hole in a card. If you
should afterward suddenly attempt to view the
hole itself accurately, with the same eye, you
will observe both it and the distant object, par-
ticularly the latter, to move from left to right,
if the right eye be used ; but; if the left eye be
the one employed, then from right to left.
Shift now your attention as suddenly back from
the hole to the object seen through it, and
both will return to the places they formerly
AN ESSAY ON
occupied. In this experiment, no real change
can be supposed to have occurred in the posi-
tion of the distant object ; and had any hap-
pened with respect to either the eye or the hole,
the object would not have been seen through
the latter. No other fallacy, therefore, exists
here, than that things, which are truly at rest,
appear, notwithstanding, to be in motion.
The argument, which I have mentioned may
hence be derived against my theory, is this :
The visible directions of objects, in the optic-
axis which remained fixed, were formerly said
to be altered, because a new state of muscular
exertion was required to keep it so, in every
different degree of the inclination to it of the
moveable axis. But in the last experiment,
there seems no good reason for supposing any
change in the inclination of the moveable axis
to the other ; for, as one eye is closed, the
obvious intention of directing the two axes to
the same object, which is, that we may see it
single, no longer exists. If then an apparent
lateral motion be, in one instance, observed in
objects truly at rest, without any change of the
interval of the pupils, may not every other
motion of the like kind be also independent
of the muscular actions, which regulate that
interval ?
Itisevident, that this argument rests altogether
SINGLE VISION. 5
upon the supposition, that in the experiment
just mentioned, no alteration occurs in the in-
terval of the pupils. Now, we iriay be easily
convinced, that some alteration does occur, by
applying a finger to the closed eye, which will,
by this means, be felt to move toward the nose,
when we endeavour to view the hole accurately,
and from the nose, when we carry our attention
back again to the remote object. Were, in-
deed, the opinion of Aguilonius * just, that the
mind perceives only those objects distinctly,
which are situated at the concourse of the optic
axes, whether they are seen with one or with
two eyes, both the necessity and the degree of
the alteration would be clearly ascertained.
But as this opinion is not just, which I mean
to prove from experiments in a succeeding part
of this work, I shall proceed to give another
reason, and I think the true one, why the in-
terval of the pupils should be as much altered,
when we look with one eye at objects succes-
sively, which are placed at different distances,
as if we were to view them with both.
It is a fact, for which I have the authority of
experiments almost without number, though I
do not recollect to have seen it mentioned by
any author beside Dr. Porterfield, that every
* Optica, p. 84.
F
66 AN ESSAY ON
change of the mutual positions of the optic axes
is conjoined, in persons who do not squint,
with a change of the power, in both eyes, to
refract the rays of light which fall upon them.
When the axes are parallel to each other, the
eyes are in their lowest refracting state ; but
in their highest, when the axes are mutually
intersected within two or three inches of the
face ; every intermediate inclination being also
conjoined with an intermediate degree of re-
fracting power. Now, since those objects are
seen most distinctly, the radious pencils from
which are accurately brought to points in the
retina, it follows, that, although we employ one
eye only, the same reason exists for adjusting
its refractive power to their distances, as if we
saw with both. When, therefore* we view a
remote object with one eye, we use it in its
lowest refracting state, which, I have observed,
is conjoined with the widest interval of the
pupils. Should we afterward attempt to see
accurately a very near object, the eye will
assume its highest refractive state, and the in-
terval of the pupils be lessened ; the conse-
quence of which must be, that both the objects
lying in the optic axis will appear to move in
the manner already related.
To finish this part of my subject, it seems
SINGLE VISION. 67
only necessary to determine, whether the de-
pendance of visible direction upon the actions
of the muscles of the eyes be established by
nature, or by custom. But facts are here
wanting. As far as they go, however, they
serve to prove, that it arises from an original
principle of our constitution. For Mr. Chesel-
den's patient saw objects single, and conse-
quently in the same directions with both eyes,
immediately after he was couched; and persons
affected with squinting from their earliest in-
fancy, see objects in the same directions with
the eye they have never been accustomed to em-
ploy, as they do with the other they have con-
stantly used.
Having thus shown in what directions ex-
ternal bodies are seen, when their situation
with respect to the eye is given, and upon what
circumstance the various directions depend, in
which a picture upon any one place of the
retina can exhibit the object producing it ; I
should render the theory of visible direction
complete, were I now to point out the relative
positions of the two lines of direction, in which
any two different parts of the retina represent
their objects. To ascertain this, the first step
must be, to find the place of the retina which
receives the picture 'of an object, whose situa-
tion with respect to the external eye is known;
F2
68 AN ESSAY ON SINGLE VISION.
and if two such points of the retina were deter-
mined, I think the chief difficulty in this matter
would then be overcome. But as it appears to
me, that the structure of the eye has not yet
been sufficiently explained, to enable any person
to take this first step, I forbear saying any thing
more upon the subject*
.EXPERIMENTS AND OBSERVATIONS
ON
SEVERAL SUBJECTS IN OPTICS.
ARTICLE I.
On Visible Position, and Visible Motion.
IN the estimates we make by sight of the
situation of external objects, we have always
some secret reference to the position of our
own bodies, with respect to the plane of the
horizon ; and from this cause, we often judge
such to be at rest, whose relative places to us
are continually changing ; and others to be in
motion, though they may constantly preserve,
in regard to us, the same distance and direc-
tion. To give an instance, let us suppose our
eyes first directed to a star near to the horizon ;
should we afterward, by a mere motion of the
head, point them to another, some degrees
above the former, this second star will appear
higher than the first did. Were we now, while
the eyes are kept fixed in relation to the head,
70 EXPERIMENTS, &c.
and the head in relation to the shoulders, to in^
cline the trunk of the body backward, until we
bring the optic axes to a third star, this will ap-
pear still higher than the second was perceived
to be. If instead of directing the eyes succes-
^sively to different objects, the same object be
suffered to remain at the concurrence of the
optic axes in all these different positions of the
body, it is evident, that it must be seen to move,
during the change from one position to another.
The facts I have mentioned are so obvious,
that I should not have spoken of them, had I
not intended they should introduce the follow-
ing question : What is there within us, to in-
dicate these positions of the body ? To me it
appears evident, that since they are occasioned
and preserved by combinations of the actions of
various voluntary muscles, some feeling must
attend every such combination, which suggests,
from experience perhaps, the particular posi-
tion produced by it. But in almost all the posi-
tions of the body, the chief part of our muscular
efforts is directed toward sustaining it against
the influence of its own gravity. Each posi-
tion, therefore, in which this takes place, must
be attended with a feeling, which serves to in-
dicate its relation to the horizontal plane of the
earth ; and consequently, if aur bodies pos-
sessed no gravity, or, if the thing were possible,
IN OPTICS. 71
had we been created unembodied spirits, but
with the same faculties of perception as we
enjoy at present, we could no more have judged
one line to be perpendicular, and another to
be parallel to the horizon, than we can at pre-
sent determine, without some external aid,
which is the eastern, and which the western
point of the heavens. I shall now draw from
these principles, the explanation of a fact,
which was first mentioned by one of the most
ingenious authors that have written upon vision,
but left by him still to be justly accounted for.
" I have frequently" (says Mr. Melvill)*
<c observed, when at sea, that, though I pressed
my body and head firmly to a corner of the
cabin, so as to be at rest in respect to every
object about me, the different irregular mo-
tions of the ship, in rolling and pitching, were
still discernible by sight. How is this fact
to be reconciled to optical principles? Shall
we conclude that the eye, by the sudden
motions of the vessel, is rolled out of its due
position? Ov, if it retains a fixed situation
in the head, is the perception of the ship's
motion, owing to a vertigo in the brain, a de-
ception of the imagination, or to what other
cause ?"
* Edinburgh Physical Essays, vol. ii. p. 8O.
72 EXPERIMENTS, &c.
I need not, I believe, offer to show, that the
fact here spoken of, is not owing to any of the
causes Mr. Melvill has specified. I shall there-
fore, in a few words, point out its dependance
upon the principles which have just been men-
tioned.
It is generally known, I suppose, that when
a vessel at sea, in the language of sailors, is
said to pitch, its two extremities turn upon its
shorter axis, and that the term of rolling is con-
fined by them to its motions upon the longer
axis. In both pitching and rolling then, the
relative position of a vessel to a horizontal
plane is necessarily changed. Consequently,
though, in the abovementioned experiment,
Mr. Melvill's body and head were at rest with
respect to every object about him, still a dif-
ferent degree of muscular effort was required
to ]keep them so, in every such different posi-
tion of the vessel. But each degree of mus-
cular effort, to sustain his body against the
operation of its gravity, would suggest to him
its concomitant position with regard to the
plane of the horizon ; each deviation, there-
fere, of the vessel from its former situation, re-
latively to the same plane, would be perceived,
and the vessel itself be seen to move. In short,
nothing more takes place in this, than in the
following experiment: Let a pole be placed
IN OPTICS. 73
upon firm ground, at right angles to the horizon.
If, while we are standing erect, it be inclined
upon its lower extremity, successively backward
and forward, to the right and to the left, these
motions must, without contradiction, be per-
ceived. Suppose now, our bodies to be simi-
larly inclined with the pole, during its different
positions, so as to be constantly parallel to it ;
it is evident, that its motions will be as readily
perceived in this case, as they were, when our
bodies were erect ; and this is all that happens
in the experiment of Mr. Melvill.
Should the necessity of supporting the body
against its gravity, by the actions of our volun-
tary muscles, be suspended in whole, or in part,
our judgments of the situation of objects, with
respect to the horizon, must become irregular
and uncertain, notwithstanding any general
habit we may have acquired from experience.
An instance of this, I think, I have observed;
for I have frequently remarked during a sea
voyage, that, when the wind blew so strongly,
and in such a direction, as to occasion the vessel
to heel, or lean much to one side, chords freely
suspended from the roof of the cabin, and kept
Stretched by heavy bodies attached to them,
have appeared to me, as long as I lay in bed,
though they were necessarily perpendicular to
the horizon, to decline considerably from that
74
EXPERIMENTS, &c.
position ; while the sides of the cabin seemed,
if not perpendicular, at least much less inclined
to the horizon than they were in reality. My
body being here supported by the bed, I was
consequently without those feelings, which in-
dicate its position with respect to the horizon.
Objects therefore appeared to me in those situa-
tions, in which I had been accustomed to see
them. In confirmation of which I may men-
tion, that, when I got up, and stood upon the
floor of the cabin, the chords seemed perpendi-
cular, or nearly so, and the sides of the cabin
inclined ; for I was now obliged to exert a pro-
per degree of muscular force, to keep myself
upright. What I here say, however, is from
the recollection of things observed some years
ago, when I had no thought of making the use
of them I now do ; for which reason, I may
possibly have committed some trifling error in
stating them ; but none, I believe, sufficient to
affect the theory they are brought to support.
It. being my intention to treat, in the present
article, of several facts relative to visible posi-
tion and motion, which seem to me to need
explanation, without regarding whether or not
they depend5 upon any common cause; I pass
to the consideration of the apparent rotation of
objects, when we have become giddy, by turn-
ing ourselves quickly and frequently round.
IN OPTICS. 75
Some of the older writers upon optics ima-
gined the visive spirits to be contained in the
head, as water is in a vessel, which therefore,
when once put in motion by the rotation of our
bodies, must continue in it for some time after
this has ceased ; and to this real circular move-
ment of the visive spirits, while the body is at
rest, they attributed the apparent motions of
objects in giddiness. Dechales* saw the weak-
ness of this hypothesis, and conjectured, that
the phenomenon might be owing to a real
movement of the eyes, but produced no fact in
proof of his opinion. Dr. Porterfieldf, on the
contrary, supposed the difficulty of explaining
it to consist in showing, why objects at rest
appear in motion to an eye which is also at
rest. The solution he offered of this repre-
sentation of the phenomenon, is not only ex-
tremely ingenious, but is, I believe, the only
probable one which can be given. It does not
apply, however, to the fact which truly exists ;
for I shall immediately show, that the eye is
not at rest, as he imagined. The last author,
I know of, who has touched upon this subject,
is Dr. Darwin t. His words are, " When any
one turns round rapidly on one foot till he
* Cursus Mathemat. Tom. ii. p. 422.
t Treatise on the Eye, Vol. ii. p. 426.
I Philosoph, Transact. Vol. Ixxvi. p. 315.
76 EXPERIMENTS, &c.
becomes dizzy, and falls upon the ground, the
spectra of the ambient objects continue to pre-
sent themselves in rotation, or appear to librate,
and he seems to behold them for some time in
motion." I do not indeed pretend to under-
stand his opinion fully; but this much seems
clear, that, if such an apparent motion of the
surrounding objects depends, in any way, upon
their spectra, or the illusive representations of
those objects, occasioned by their former im-
pressions upon the retinas, no similar motion
would be observed, were we to turn ourselves
round with our eyes shut, and not to open them
till we became giddy ; for in this case, as the
surrounding objects could not send their pic-
tures to the retinas, there would, consequently,
be no spectra to present themselves afterward
in notation. But whoever will make the expe-
riment, will find, that objects about him appear
to be equally in motion, when he has become
giddy by turning 'himself round, whether this
has been done with his eyes open or shut. I
shall now venture to propose my own opinion
upon this subject.
If the eye be at rest, we judge an object to
be in motion when its picture falls in succeed-
ing times upon different parts of the retina ;
and if the eye be in motion, we judge an object
to be at rest, as long as the change in the place
IN OPTICS. 77
of its picture upon the retina, holds a certain
correspondence with the change of the eye's
position. Let us now suppose the eye to be in
motion, while, from some disorder in the system
of sensation, we are either without those feel-
ings, which indicate the various positions of the
eye, or are not able to attend to them. It is
evident, that, in such a state of things, an
object at rest must appear to be in motion,
since it sends in succeeding times its picture to
different parts of the retina. And this seems
to be what happens in giddiness. I was first
led to think so from observing, that, during a
slight fit of giddiness 1 was accidentally seized
with, a coloured spot, occasioned by looking
steadily at a luminous body, and upon which I
happened at that moment to be making an ex-
periment, was moved in a manner altogether
independent of the positions I conceived my
eyes to possess. To determine this point, I
again produced the spot, by looking some time
at the ftame of a candle ; then turning myself
round till I became giddy, I suddenly discon-
tinued this mption, and directed my eyes to the
middle of a sheet of paper, fixed upon the wall
of my chamber. The spot now appeared upon
the paper, but only for a moment 5 for it im-
mediately after seemed to move to one side,
and the paper to the other, notwithstanding I
7H EXPERIMENTS, &c.
conceived the position of my eyes to be in the
mean while unchanged. To go on with the
experiment, when the paper and spot had pro-
ceeded to a certain distance from each other,
they suddenly came together again ; and this
separation and conjunction were alternately re-
peated a number of times ; the limits of the
separation gradually becoming less, till, at
length, the paper and spot both appeared to
be at rest, and the latter to be projected upon
the middle of the former. I found also, upon
repeating and varying the experiment a little,
that when I had turned myself from left to
right, the paper moved from right to left,
and the spot consequently the contrary way :
but that when I had turned from right to
left, the paper would then move from left to
right. These were the appearances observed
while I stood erect. When I inclined, how-
ever, my head in such a manner, as to bring
the side of my face parallel to the horizon, the
spot and paper would then move from each
other, one upward and the other downward.
But all these phenomena demonstrate, that
there was a real motion in my eyes at the time
I imagined them to be at rest ; for the apparent
situation of the spot, with respect to the paper,
could not possibly have been altered, without
a real change of the position of those organs.
IN OPTICS. 79
To have the same thing proved in another way,
I desired a person to turn quickly round, till he
became very giddy ; then to stop himself and
look stedfastly at me. He did so, and I could
plainly see, that, although he thought his eyes
were fixed, they were in reality moving in their
sockets, first toward one side, and then toward
the other.
The last instance of visible motion I shall
notice, is one which has been mentioned by
Mr. Le Cat, in the following words * : " Place
a lighted candle at a moderate distance from a
polished body of considerable convexity, so that
the image of the flame, which is seen by reflec-
tion from it, may appear as a small luminous
point. The experiment will succeed better, if
the direct rays of the flame be intercepted from?
the sight. Close, after this, one eye, and view
the luminous point in a careless way, (en revanf)
that is to say, with the eye in a relaxed or dilated
state. The point will then be seen enlarged and
radiated. If you bring now your finger to the
right of the eye which i& open, and gradually
move it toward the left, in order to conceal the
luminous point from this eye, you will distinctly
perceive the shadow of your finger to proceed
from left to right, and to pass over the point in
* Trait6 des Sens. p. 419-
80 EXPERIMENTS, &c.
a direction, contrary to that which you gave it-
Should you, afterward, move your finger back
from right to left, and in like manner, if your
finger be moved from above downward, or from
below upward,- the shadow will always proceed
the contrary way. It is therefore manifest,
that the soul must here see objects inverted, as
their images in the eye truly are ; and that it-
refers impressions to those parts of the eye
where it feels them, and not to the places from
which the rays are emitted, as it does when it
possesses the means of rectifying its judgment.
Whence does this happen ? Doubtless, because
the luminous point has neither a high nor a
low, neither a right nor left side, nor any well-
enlightened object in its vicinity, to awaken
the attention of the soul ; in short, nothing
which can determine its judgment."
I should scarcely have mentioned this experi-
ment, from any respect for the authority of its
author in optics ; but as-Haller* seems to assent
to the conclusion he draws from it, that the
soul sometimes sees objects inverted; and as
the Abbot Derochonf, a member of that learned
body, the Academy of Sciences of Paris, has
lately, but in my opinion unsuccessfully, at-
tempted to reconcile it to the commonly received
* Eleraenta Physiologiae, Tom. v. p. 479-
f Memoires de Physique, p. 66.
IN OPTICS. 81
principles of vision, I think it worth while to
show, in a few words, that it is a direct conse-
quence of the very doctrine Mr. Le Cat means
to overthrow by its means.
It would be proper, indeed, to mention before-
hand, the opinion of the Abbot Derochon ; but
this I must, notwithstanding, omit doing, as it
could not be understood without the figure by
which he has illustrated it. I shall observe,
however, respecting it, first, that it requires the
side of the ringer next to the eye, to be without
the least illumination ; whereas the experiment
will succeed, whether it be illuminated or not :
secondly, that, according to it, the experiment
ought to succeed equally well, whether the
image of the flame in the mirror be seen as a
point, or as a surface; though, in truth, it
never does succeed, except in the latter cases :
thirdly, that the apparent shadow of the finger
is always much larger than it ought to be, were
it seen by reflection, as the Abbot thinks :
fourthly, that, while the eye, mirror, flame, and
finger, remain in the same positions, the shadow
seems at one time larger than at another, owing
to the different degrees of relaxation in the eye ;
but that this, for the reason just mentioned,
ought never to happen, according to his theory :
fifthly, that agreeably to his own reasoning, the
82 EXPERIMENTS, &c.
shadow ought to move in the same direction
with the finger, which is the very reverse of
the fact to be explained. But as arguments
against error may be infinitely extended, and
as only one solution of a phenomenon can be
true, the readiest way of exposing the insuf-
ficiency of others, is to exhibit that which is
just.
This, in the present case, seems to lie upon
the very surface of optical knowledge, and has
already been given by others, of various forms
of the same fact. When the image of the flame
is seen in the mirror as a point, its rays must be
accurately collected to a focus in the retina ;
but when seen as a surface, this must necessarily
be attributed to their focus being either before
or behind it ; in either of which cases, they will
occupy a place upon that membrane of some
assignable dimensions. In the present instance,
their diffusion over a part of the retina, depends
on the focus being behind it; for the eye is
now, from a condition of the experiment, in a
more relaxed state than it was just before, when
the rays of the same object were brought there
accurately to a point. The rays, therefore,
which go to the right side of the enlightened
surface of the retina, or picture as I shall call
it, are those which enter the eye at the right
IN OPTICS. 83
side of the pupil, and its left side is formed of
the rays entering at the left side of the pupil j
and the like must be true of its upper and lower
parts. Should we then begin to move a finger
from right to left across the eye, the rays form-
ing the right side of the picture must be first
intercepted. But from the known fact, that
the points of an external object are always in
an inverted position, with respect to the parts
of the retina, by the affections of which they
are suggested, when the right side of the picture
there is effaced, the left side of the external
object it suggests must disappear. And for
the same reason, if the motion of the finger be
continued from right to left across the eye, the
other parts of the luminous surface in the mirror
will successively vanish from left to right, and
thereby furnish the appearance of a shadow
passing over it in that direction. — In like man-
ner, it maybe shown, that if the finger proceeds
from left to right, from above downward, or
from below upward, the shadow must move the
opposite way.
That this is the true explanation of Mr. Le
Cat's experiment, is, I think, plain, both from
its intrinsic evidence, and the following consi-
derations : — If the mirror be brought within
four or five inches of the eye, and the candle
84
EXPERIMENTS, &c.
be so placed, that the image of the flame must,
from the laws of reflection, be regarded as a
mere point; though we should now view it
with the utmost care, and though there should
be in its neighbourhood some well-enlightened
object to awaken the attention of the soul, as
Mr. Le Cat expresses it, still the seeming sha-
dow will move in a direction contrary to the
finger. For the image is now so near to the
eye, that no exertion we can make is sufficient
to bring its rays to a point upon the retina f the
picture, therefore, upon that membrane will
be formed of rays passing to a focus behind it,
which is the only condition necessary for the
success of the experiment. Again, if a short-
sighted person should place the mirror at the
distance of some feet from him, complying in
other respects with Mr. Le Cat's instructions,
he will constantly observe the shadow to move
in the same direction with the finger. For, in
his eye, the rays of the image, when at such a
distance, must meet before they fall upon the
retina. The right side, therefore, of the pic-
ture upon that membrane, must be composed,
in this case, of rays which enter the eye at the
left side of the pupil. Consequently, when these
are cut off, the left side of the apparent lu-
minous surface must disappear, and the shadow
IN OPTICS. 85
be seen to move the same way as the finger,
when this successively intercepts the rays pro-
ceeding from the image to the eye*.
* Scheiner observed a fact of the like kind (Fundamentum
Opticum, p. 33) namely, that, if a small hole, made in any
substance, be held near to the eye, and an opaque body be
passed between them, from right to left, the left side of the
hole will first disappear. Mr. Grey afterward took notice
(Philosoph. Transact. Vol. xix. p. 286) that a needle he em-
ployed in this experiment was seen inverted ; from which he
supposed that the hole, or something in it, produced the
effect of a concave speculum. Mr. Harris, however, says
(Treatise of Optics, p. 141 ) that it is not the needle, but its
shadow on the other side, which is seen, and is the cause of
the inverted appearance. But the truth is, that the hole is
to be regarded as a luminous point, the rays of which fall
upon the retina before they are collected to a focus ; and
hence that the same appearances must be here observed as
in the experiment of Mr. Le Cat. In proof of this it may
be mentioned, that if the hole be placed at such a distance,
that the eye may refract its rays accurately to a point on the
retina, ;:no shadow or image of the needle will be seen j that
if the hole be still farther removed, and the eye be adapted
to a less distance, the shadow or image will again appear,
but its position will now be upright, and its motion the
same way as that of the needle itself $ and lastly, that, at
one given distance of the hole, either no shadow will appear,
or it will be seen upright, or it will be seen inverted, accord-
ing as the eye may be made to assume different states with
respect to its power of refraction,
EXPERIMENTS, &c.
ARTICLE II.
On a supposed Consequence of the Duration of Impres-
sions upon the Retina; and the Effects of accurate
Vision being confined to a single Point ofiliat Mem-
brane,
FEW things, at first, appear more incredible
to a person, not conversant in optics, than that
he does not, at any one time, see distinctly a
surface larger than the head of a pin. After
he is convinced, by proper trials, of the truth
of this, he naturally asks, Whence comes it
then, that, in ordinary vision, I seem to view
distinctly so many objects at once ? I go into
a crowded street, and I fancy I have an accu-
rate perception by sight, of men, houses, car-
riages, and many other things, all at the same
time 5 whence proceeds this illusion ?
Only one answer, as far as I know, has been
given to this question. The impressions made
upon the retina by external objects, do not, it
is said, immediately cease, along with the re-
ception of the rays which flow from them ; and,
as in the ordinary mode of vision, the eye is
continually passing from object to object, the
impression left by a former one may be still
vivid, though the eye be directed to another j
IN OPTICS. 87
and hence we may imagine we see both of them
distinctly, though the picture of only one occu-
pies that place of the retina, which alone fur-
nishes us with accurate vision.
There are, however, objections to this answer,
which seem to me insurmountable. For, in the
Jirst place, as the duration of impressions on the
retina must be greater or less, according to the
vivacity of the pictures which occasion them,
it follows, that, were this answer just, the ap-
parent field of our distinct vision ought to be
in proportion to the quantity of light admitted
by the eye ; that it should be contracted, there-
fore, by every cloud which passes over us, and
be enlarged by every burst of sunshine ; that,
at mid-day, it should possess its greatest extent,
and ought from that time gradually to decrease
till the evening, when its limits should be nearly
tjie same with those of the real field of accu-
rate vision. Secondly, since the coloured spot,
which is produced by looking steadily for some
time at aluminous body, appears projected upon
every object to which we direct our eyes, dur-
ing its continuance, and as such a spot is neces*
sarily the sign and effect of the duration of an
impression upon the retina ; every other visible
appearance from the same cause ought, in like
manner, to have its situation determined by
the position of the eye, as far as this may be
88 EXPERIMENTS, &c.
occasioned by the action of its muscles. No
object, therefore, ought to appear separate and
distinct from others, if the answer were true
which I am combating ; but, on the contrary,
all those to which we successively direct our
eyes during the limits of the duration of an
impression upon the retina, should seem
crowded into one place; and, consequently,
none of them should be perceived with any
tolerable accuracy. — Such are the conclusions
from the truth of this answer. I need scarcely
mention, that they are contradicted by expe-
rience.
There is another form of the same fact, to
which, it may be thought, an explanation taken
from the duration of impressions on the retina
will better apply ; I mean the appearance of a
fiery circle, when any red-hot body is moved
quickly round. But it seems, to me, that such
an explanation cannot even here be admitted.
For, if the circle depended upon the cause I
have mentioned, it could only be observed as
long as the impressions upon the retina were
also disposed in the form of a circle. Were this
broken upon, which it must be by every move-
ment of the eye, the appearance suggested by
the last impression would no longer be so ar-
ranged, with respect to the appearance suggested
by the present impression, as to lie with it in the
IN OPTICS. oy
circumference of a circle; and hence some
very different figure would be observed. Every
person, however, may easily convince himself,
that the circular form of the fiery appearance
is equally perceived, whether the eye be at rest,
or be moved in the most irregular manner.
If these arguments be thought sufficient for
the purpose I had in view, it must also follow
from them, since the fact still remains to be ex-
plained, why we apparently see so many objects
with equal distinctness at once, that past im-
pressions upon the retina are perceived as pre-
sent, by means of some higher faculty than that
of sight. This faculty cannot, with propriety,
be named memory y as it is essential to a thing's
being remembered, that it be perceived #s past.
Nor can it be called imagination, since we be-
lieve in the present existence of what it per-
ceives. In one point of view it may seem
rather a defect in our natures, that we should
not be able to distinguish between things past
and present. However this may be, I am in-
clined to be of opinion, that many other pheno-
mena, both of thought and external sense, are
partly to be resolved into the same general fact.
From the present instance of it, we learn, that
several muscular actions may be performed, in
succession, during the least perceptible portion
of time.
EXPERIMENTS, &c.
The question I have just treated, naturally
gives rise to another : Would it have been
more to our advantage, if accurate vision, in-
stead of being confined to one point of the
retina, had been possessed by every part of that
membrane? I answer, I think not, for the fol-
lowing reasons.
First ; The diffusion of such a property over
the whole retina would be of little use, unless
our power of attention was also increased. For
we should otherwise be still unable to perceive
more than one visible object at once, with di-
stinctness, since, by our present constitution,
we are capable of attending accurately to only
one thing at a time. The only benefit, indeed,
I can see to arise from such a condition of the
retina, is this; That our attention might be
shifted more quickly from picture to picture on
that membrane, than our eyes can be turned
from one external object to another. This ad-
vantage, however, would be far out-weighed by
an inconvenience accompanying it. For it is
a well-known fact, with respect to perception,
that we are capable of attending, more or less
accurately, to any particular impression upon
the senses, in proportion to the inferior force of
other impressions, which are at the same time
received. But in the supposed state of the
retina, there would be, almost always, several
IN OPTICS. 91
impressions of the same strength as the one to
which we might desire particularly to attend ;
whereas, in its present state, the vivacity of the
impression from the object, to which we turn
the optic axis, most commonly surpasses, con-
siderably, that of every other upon the same
membrane ; by which means our attention is
rendered less liable to interruption.
Secondly ; The extension of accurate vision,
to every part of the retina, would deprive us, in
great measure, of the help, which we obtain, at
present, from the eye, in learning the thoughts
of other men. As far as I have been able to
observe, the changes produced by our internal
feelings, upon the state of the eye itself* are
very few, and relate only to the quantity of
moisture, which is diffused over its surface, and
the degree of fulness in the blood-vessels, which
are spread upon its white and glistening part.
Both of these circumstances, however, are
similarly altered by opposite passions, and, con-
sequently, neither of them can be regarded as
the appropriate expression of any. The whole
variety, then, of the expressions of feeling which
are justly attributed to the eye, must, I think,
depend upon its motions. Some of these are
the immediate effects of certain passions ; the
eye, for instance, being moved differently in
anger and in grief 5 and such may be esteemed
EXPERIMENTS, &c.
as directly expressive of the passions by which
they are produced. But the far greater number
of them do little more, than merely point out
the external cause, or object of the sentiment,
which the changes of other parts of the counte-
nance declare to exist within us ; or distin-
guish certain external appearances depend-
ing upon a mental cause, from similar appear-
ances arising from a different source. Thus,
blushing is often distinguished from an ac-
cidental flush of the cheek, by the eye being
turned away from the person who occasions it.
That many of the expressions, which we at-
tribute to the eye, do in fact depend on changes
in other ^parts of the countenance, is evident
from the alterations we think induced upon it,
by the eyelashes falling off from disease, by a
slight inflammation of the edges of the eye-lids,
without its being communicated to the eye
itself, by artificially colouring the eye-brows,
and by many other similar circumstances. And
how essential to the right understanding of the
expressions of the other features, are the mo-
tions of the eyes, when conducted with design,
and properly directed, must be known to every
one, who has attended in discourse to the
countenances of very short-sighted people, and
more especially to those of persons afflicted with
blindness from a gutta serena, in which the eye,
IN OPTICS. &3
•: • --'^\ ' .-.- *•
with respect to its external condition, seems
without fault. But whatever is the assistance
the motions of the eye afford, in expressing our
internal feelings, the whole of it must ulti-
mately be referred to the circumstance of ac-
curate vision being confined to one point of the
retina ; since the intent of those motions is, to
bring the pictures of external objects upon the
most sensible part of that membrane. Their
necessity, therefore, would no longer exist, if
the same property were extended, and the ad-
vantages we at present enjoy from them, would,
consequently, cease.
94 EXPERIMENTS, &c.
ARTICLE III.
On the Connexion between the different refractive States
of the Eyes, and, the different Inclinations of the Optic
Axes to each other.
I HAVE mentioned, in my Essay upon Single
Vision with Two Eyes *, that I had been con-
vinced, by experiments almost without number,
that every different degree of the mutual in-
clination of the optic axes, is attended by a dif-
ferent state of the refracting power of each eye.
The experiments I there alluded to were chiefly
of this sort. I placed a luminous point, most
commonly the reflected image of the flame of a
candle from the bulb of a small thermometer,
at such a distance, that when both my eyes
were accurately directed to it, its visible ap-
pearance to one of them was likewise that of a
point. Keeping then the axis of this eye fixed,
and making the other to cross it, sometimes
before and sometimes behind the luminous
point, I found that in both cases it appeared as
a surface to the eye, in the axis of which it was
* Page 66.
IN OPTICS. 95
situated j and that the more remote from it was
the concurrence of the axes, the larger was the
luminous surface. Now when the axes met be-
fore the point, the apparent surface must have
been occasioned by the rays coming to a focus,
previously to their incidence upon the retina ;
because, when I passed my finger across the
eye by which it was seen, its parts disappeared,
in an order corresponding to the direction in
which the finger moved. The disappearance
of the parts was in an order, contrary to the
motion of the finger, when my optic axes inter-
sected each other beyond the point; which is
an equal proof, that the rays, in that case,
tended to a focus behind the retina.
One application of this fact has already been
shown*, and I shall now proceed to mention
several other phenomena in vision, which it
may serve either in whole, or in part, to ex-
plain.
1. It accounts for the following beautiful
observation made by Aguiloniusf, that if we
close one eye, and look with the other at an
object placed in its own axis, we shall not be
able to see this object distinctly, unless we also
direct to it the axis of the closed eye. For in
persons, who are neither presbytic nor myopic,
* Essay upon Single Vision, p. 66.
f Aguilonii Optica, page 84.
96
EXPERIMENTS, &c.
the refractive states of the eyes are so adapted
to the mutual inclinations of the optic axes,
that pencils of rays flowing from bodies at mo-
derate distances are more accurately collected
upon the retina, when they are situated at the
intersection of those lines, than if their position
was, in any considerable degree, either nearer
or more remote. The reason given by Aguilonius
himself, is, that the mind perceives only those
objects distinctly, which are placed at the con-
course of the optic axes. But the following
experiment proves that the solution is true no
farther, than as it coincides with the one I have
advanced. Hold, in the axis of either eye, a
concave lens, at such a distance, that the letters
of a book, placed a little farther off, may appear
through it very indistinct to that eye, when
both axes are directed to any particular word.
View afterward the lens itself with both eyes,
and the letters will immediately become more
distinct. In this experiment then, an object is
more accurately perceived when distant from
the concourse of the optic axes, than when
situated exactly in it.
It may be said, perhaps, that the distinctness
of the letters is here to be attributed to the con-
traction of the pupil, which is occasioned by
the eyes being directed to a nearer object than
they were formerly. But that this is not the case*
IN OPTICS. 97
maybe made evident by another experiment:
Place a convex lens in such a manner before
one eye, that the flame of a candle, at the
distance of two or three feet from the face,
may appear indistinctly terminated to that eye,
when both axes are pointed to it. The same
eye being kept fixed, let the two axes after-
ward meet beyond the flame, and it will now
be seen much better defined, though the pupil
is at the same time become larger. The insuf-
ficiency of the explanation of Aguilonius, is also
proved, by a circumstance frequently noticed
in persons who are very short-sighted ; for such
are observed, when they desire to view an ob-
ject with much attention, to hold it close to
one eye, and to turn the other aside ; in this
way occasioning the two axes to meet very re-
motely from the object.
2. The reason commonly given, why short-
sighted people view an object with one eye
only in the manner above-mentioned, is, that
by this means they avoid the uneasy straining
of the muscles, which must be employed to
direct both axes to the same point. But it is
evident they must derive from the practice this
farther advantage, that, as their optic axes are
now parallel to each other, or nearly so, they,
consequently, see the object in the least refrac-
tive state of their eyes. Pencils, therefore, will
H
98 EXPERIMENTS, &c.
now have their focuses in the retina, the rays
of which would have crossed each other, before
they fell upon it, had both the axes been
directed to the object.
3. Spectacles were long employed, before the
manner in which they assisted sight was known.
About the year 1601, this was proposed as the
subject of a question to Kepler,* by his prin-
cipal patron at that time, Ludovie L. B. a Die-
trickstein, a learned nobleman of Austria. The
first answer he gave was, that convex glasses
were of use, by occasioning objects to appear
larger. But his patron observed, that if ob-
jects were rendered by them more distinct, be-
cause larger, no person would be benefited by
concave glasses, since these diminish objects. It
was not till three years after, that, in conse-
quence of finding out in what manner vision is
performed, he was able to give a just solution
of this problem, though his attention had been
directed to it during the whole of that interval.
According to the discovery he then made, con-
vex glasses were said by him to assist the sight
of presbytic persons, by so altering the directions
of rays diverging from a near object, that they
shall afterward fall upon the eye, as if they
had proceeded from a more remote one 5 and
* Paralipomena in Vitellionem, p. 200.
IN OPTICS. 99
concave glasses to benefit the myopic, by
producing a contrary effect upon rays which
diverge from a distant object. Now it is ma-
nifest, that by this theory, to which I believe
no addition has been made by any succeeding
writer, precisely the same effects are attributed
to lenses, whether they be employed singly, or
in the form of spectacles. I am inclined, how-
ever, to think, that a difference, sometimes at
least, exists here, which has hitherto escaped
notice. For in regard to such spectacles as I
have tried upon myself, I have always found,
that, when I looked with them at objects placed
at moderate distances directly before me, my
optic axes passed through the glasses, more in-
wardly than their centres. With respect, there-
fore, to spectacles for long-sighted people, as
the inner halves of their glasses may be re-
garded as two prisms, whose refracting angles
face each other, to have allowed both my eyes
to receive through them pencils of rays from
the same point of an object, the intervals of my
pupils must have been less than was necessary
for that purpose in naked vision. The conse-
quence of which would be, an increase of the
refractive power of my eyes. Again ; as the
like parts of glasses in spectacles for short-
sighted persons, may be esteemed to be two
prisms, the refracting angles of which are
100 EXPERIMENTS, &c.
turned from each other, the interval of the
pupils must have been increased, and the re-
fracting power of my eyes by this means di-
minished, when I looked at an object through
them, which was directly before rne. And
effects similar to what I have mentioned, must
have followed my viewing objects placed ob-
liquely, through glasses of both kinds. Here
then is one advantage, which persons, who see
with both eyes, either do or may enjoy from
spectacles, but which they cannot derive from
using single glasses. For if they are presbytic,
they can see an object by the means of them
with a higher refractive state of the eyes, than
if the optic axes met there, as in naked vision ;
and if myopic, with a less. It is also worthy
of remark, that this advantage does not ulti-
mately tend to increase the evil, which first
gives occasion for spectacles. On the con-
trary, if what every writer upon vision asserts
be true, that we are apt to become short or
long-sighted, according as we are much accus-
tomed to view near or distant objects, it must
serve to diminish that evil. In support of this
opinion, I shall mention a fact, with which I
have been made acquainted by Mr. George
Adams*, of this place, who is not only well
* Mathematical Instrument Maker to the King.
IN OPTICS. 101
skilled in the theory of vision, but, from his
situation, as an artist, has better opportunities,
than most persons, of learning such matters.
The fact is this, that lie does not know a short-
sighted person, who has had occasion to in-
crease the depth of his glasses, if he began to
use them in the form of spectacles ; whereas
he can recollect several instances, where those
have been obliged to change their concave
glasses repeatedly, for others of higher powers,
who had been accustomed to apply them to one
eye only. This indeed may have happened by
accident ; but at any rate, the fact is worthy of
farther attention and inquiry.
It would seem, however, that the long-sighted
derive more benefit from the alteration in the
mutual inclinations of the optic axes, which, is
produced by spectacles, than the short-sighted.
For, as .the inner halves of the convex glasses
are to ,be regarded as prisms, with their re-
fracting angles continually increasing as we
approach their edges, if two objects, situated
at different distances, be viewed successively
through them, the inclination of the optic axes
to each other, when the nearer object is seen,
must bear a higher proportion to their inclina-
tion, when we look at the one more remote,
than the different inclinations of the optic axes
do to each other, when they are successively
102 EXPERIMENTS, &c. ,
directed to the same objects, without the inter-
vention of such glasses. Hence the nearer the
object is, the greater will be the effect of the
variation in the inclination of the axes produced
by spectacles with convex glasses ; which is
the order of things, the best adapted to the
wants of those who use them. But with respect
to short-sighted persons, since the refracting
angles of their glasses, considered as prisms,
decrease, in proportion as the objects seen
through them become more remote ; they must,
consequently, derive the least benefit from an
alteration in the mutual inclinations of the
optic axes occasioned by their spectacles, at
the time they most require it.
If it were asked, then, what is the real founda-
tion of the common reproach against spectacles
for long-sighted people ? I should answer, a
very different one from that, which is, for the
most part, assigned. — For the change, in the
conformation of the eyes, which renders them
useful, seems to be one of those which nature
has destined to take place at a particular age,
and to which there is no gradual approach
through the preceding course of life. A per-
son, for instance, at forty, sees an object di-
stinctly, at the same distance that he did at
twenty. When he draws near to fifty, the
change I have spoken of commonly comes on,
IN OPTICS.
and obliges him in a short time to wear specta-
cles. As it proceeds, he is under the necessity
of using others with a higher power. But, in-
stead of supposing that his sight is thus gra-
dually becoming worse, from a natural process,
he attributes the increase of the defect in it to
his too early and frequent use of glasses. Upon
the whole, I should draw this inference from
what has been said, that no person, whose sight
begins to grow long, ought to be, in the least,
prevented from enjoying the immediate advan^
tage which, spectacles will afford him, by the
fear that they will ultimately injure his eyes j
not that I think the convexity of each glass,
considered by itself, can do 110 harm, but that
I believe the benefit, arising from the combina-
tion of the two, to be at least sufficient to com-
pensate it. Whether those, who have a ten-
dency to short-sight, should be also early in
their employment of spectacles, I shall not pre-
tend to say ; as there is not the same ground,
from theory, for supposing, that the benefit
arising from the combination of the two glasses
is able to over-balance the injury, produced by
the concavity of each considered separately.
All that I have said, however, upon the sub-
ject of spectacles, proceeds upon the supposi-
tion, that, when objects, placed directly before
us, at moderate distances, are viewed through
104 EXPERIMENTS, &c.
them, the optic axes penetrate the glasses more
inwardly than their centres. But I can be by
no means sure, that the interval of the pupils
of other persons, bears the same proportion to
the interval of the centres of the lenses in spec-
tacles, as that of mine does. It concerns those,
therefore, who are choosing them, to have at-
tention to this circumstance. To me it appears
proper, that the glasses in spectacles, both for
long and short-sighted people, should be so far
asunder, that, when we look at a very remote
object directly before us, our optic axes may
pass exactly through their centres. For if the
centres of convex glasses be nearer to each
other, very remote objects will appear double ;
and if they are more distant, though the object
viewed be infinitely far from us, the optic axes
will, however, be inclined to one another, and
the refractive power of the eyes increased,
when this may be of disservice ; since there
are few eyes which are not able, even without
the aid of the convexity of a glass, to bring
parallel rays to a focus upon the retina. If the
centres of lenses in spectacles, for the short-
sighted, be less distant than what I have men-
tioned, the optic axes must be bent toward
each other, when very remote objects are seen,
and the refractive state of the eye, therefore,
heightened, which is the very reverse of whaj:
IN OPTICS. 106
is here to be desired. Should the interval of
the centres of those lenses be greater, objects
at very considerable distances will be seen
double.
There are two other observations relative to
glasses for the sight, which I wish to add to
what I have already said upon this subject.
Thejfirst is, that the single convex glasses with
which some persons read, must be very inju-
rious, if they be sufficiently large, to admit the
same object to be seen with both eyes. For as
both axes will then pass through them, one on
each side of the centre, the interval of the pu-
pils will be widened, and the refracting power
of the eyes be diminished ; so that here a dis-
advantage is to be added to the prejudice of the
convexity of the glass, not a benefit to be placed
against it, as in the case of common spectacles
for the long-sighted. If, indeed, the defect in
sight does not arise from the conformation of
the eye, but from a want of transparency in its
cornea or humours, then such glasses, by mag-
nifying objects, will be useful, for the same
reason, that, in a very faint light, we can read
a book of a large print, with more ease than
one of a smaller. The second observation is,
that if flat-sided prisms were fixed in spectacle-
frames, with their refracting angles toward each
other, they would assist the long-sighted some-
106 EXPERIMENTS, &c.
what, without producing the evil which is said
to arise from the convexity of lenses ; and spec-
tacles of this kind might, with more propriety,
I think, than any others, be called presemers.
A like combination of such prisms, but with
their angles turned the other way, might, when
the object was moderately distant, be of service
to the short-sighted. But objects, very remote,
would be made by them to appear double.
IN OPTICS. 107
ARTICLE IV.
On the Limits of perfect or distinct Vision.
DR. Jurin*, I believe, was the first who di-
stinguished between perfect and distinct vision ;
confining the former term to those cases, where
the rays of a single pencil are collected to a sin-
gle point of the retina ; and marking, by the
latter, the perception we have of visible objects,
when the rays of the pencils, diverging from
them, though not collected to single points of
the retina, yet occupy so small portions of it, as
to allow the objects to be distinctly seen. But
as few authors have adopted this division,' I
shall, in the present article, use both terms in
the sense, which he has appropriated to the
first. Neither of them is indeed free from ob-
jection, since bodies to be distinctly or perfectly
seen, not only require, that their pictures should
be accurately formed upon the retina, but that
they should fall upon a particular part of it.
Although it has long been a subject of in-
quiry, within what limits of distance objects
are distinctly perceived by sight, yet the only
* Essay on distinct and indistinct Vision.
108 EXPERIMENTS, &c.
experiments I have met with in books, which
have been made, with any tolerable show of ac-
curacy, to determine this matter, are those of
Dr. Porterfield. I shall not here say what
they are, as his Treatise is in every body's
hands, but shall only mention the principal
conclusions which he drew from them ; .first,
that objects could be distinctly seen by him,
that is, the pencils of rays which came from
them could be accurately collected to points
upon the retina, when their distances from his
eye did not exceed twenty-seven inches, and
were not less than seven ; and secondly, that,
as often as the axes of both eyes were directed
to any one point, situated within those distances,
the rays proceeding from it had their focus in
each retina.
As the results of some experiments, which I
have made upon the same subject, differ from
these conclusions of Dr. Porterfield, I have read
over what he has written upon the matter with
more than ordinary attention, and I think I can
thence show reason, why they should not be
received without caution. For, in the first
place, his experiments are related so circum-
stantially, and with such an appearance of ac-
curacy in the making of them, that you would
scarcely suppose he left the least possible room
for error. And yet, after finishing his account
IN OPTICS. 109
of them, he tells us, that he would have repeated
them with more care and exactness*, had he not
been interrupted. Secondly, his experiments
were made upon one eye only, though his con-
clusions apply to both eyes ; an inaccuracy
which gives occasion to suspect others. Lastly,
he says, that he could not see an object distinctly
at the distance of seven inches, unless both axes
were pointed to another object, at only half that
distance. Had he then directed both axes to
an object seven inches distant, which he does
not mention he ever did, it must consequently
have been seen indistinctly ; and yet one of his
conclusions states, that objects, distant from
about seven, to about twenty-seven inches, were
always distinctly seen, when the axes of both
eyes were directed to them. Such are the rea-
sons which lead me to think, that the whole of
the difference, between the results of the expe-
riments of Dr. Porterfield and myself, is not to
be attributed to a difference in the structure of
our eyes.
The experiments, which I made upon this
subject, were with luminous points. They
proved to me,Jlrst, that, when both optic axes
are directed to any object, placed at a less
distance from my eyes than about seventeen
• Treatise on the Eye, Vol. I. p. 423.'
110 EXPERIMENTS, &c.
inches, my vision of it by the left eye is in-
distinct, from the rays of light tending to fo-
cuses behind the retina ; secondly, that my vision
by the same eye is perfect, if the object seen,
and to which both axes are turned, be from
about seventeen to about nineteen inches di-
stant ; thirdly, that the vision of my left eye
becomes again imperfect, if the object be moved
to a greater distance than that of nineteen
inches, the rays being now collected to focuses,
previously to their falling upon the retina ; and
fourthly, that I have, by my right eye, imperfect
vision of all objects, to which I direct both axes,
unless their distances be so great, ' that the rays
of each pencil, proceeding from them, may be
regarded as parallel. ,
A conclusion is furnished by these experi-
ments, similar to one, which was drawn by Mr.
Delahire *, from some made by himself; namely,
that each eye sees objects distinctly only at one
distance ; as I take for granted, that, in every
case of ordinary vision, both axes are directed
to the object which is viewed. But Mr. Dela-
hire drew a second conclusion from his experi-
ments, which he seems to have regarded only as
another expression of the first, but which, in
truth, includes a very different fact. It was,
* Memoires de Mathematiquc et de Physique, 4to. p. 2Q8.
IN OPTICS. Ill
that the refractive state of the eye is always
the same, whether we look at a very near or a
very distant object. The following observations,
however, will prove the contrary, at the same
time that they show, in what I farther differ
from Dr. Porterfield.
1. Though an object, to which both axes are
pointed, does not appear distinct to my left eye,
unless it be from about seventeen to about
nineteen inches distant ; nor to my right eye,
unless it be at a very considerable distance ;
yet 1 find, that when the axes are made to meet
at a point, about two inches distant from a line
connecting the two pupils, which however can-
not be effected without much straining, my left
eye will now see an object distinctly, which is
only about seven inches from it, and my right
eye will at the same time see an object distinctly,
the distance of which is about ten inches. I
find also, that my left eye is made to see an
object distinctly, though placed more than nine-
teen inches from it, if I direct both axes to a
point still more remote.
2. I formerly mentioned, that every degree
of the mutual inclination of the optic axes is
attended, by a particular state of the refracting
power of each eye. But I must now remark,
that these states are sometimes subject to slight
variations, while the inclinations of the optic
112 EXPERIMENTS, &c.
axes to each other remain the same. For
I find, that, when a luminous point, to which
both axes are turned, is distinctly seen by my
left eye, I can, by certain efforts not easily to
be described, but without changing the position
of either axis, make it afterward appear as a
surface, and this too, at one time, from the rays
coming to a focus too soon, and at another, too
late, for perfect vision*. One instance of these
variations deserves to be minutely described, as
it proves, that the refractive power of the eyes
is subject to greater changes, than what are
shown by any experiments I have met with in
authors. When I look attentively at a bright
star, with the optic axes parallel to each other,
it appears to my left eye a surface of some ex-
tent, and to my right eye, though not a point,
yet a surface of very small extent, as small as-
* The variations, however, seem produced in such a man-
ner, that the middle of the set belonging to one degree of
the mutual inclination of the optic axes, is always differen t
from the middle of the set belonging to another degree of
their inclination ; and that, when no other effort is made,
than to direct both axes to the same object, the eyes always
assume the middle state of the refractive power, which ac-
companies that particular inclination of the axes. No argu-
ment, therefore, can hence be derived against the applications
I formerly made of the general fact, respecting the connexion
of the refractive states of the eyes with the mutual inclina-
tions of the optic axes.
IN OPTICS. 113
the sphericity of the cornea and crystalline, the
various refrangibility of the different kinds of
light, and the width of the pupil at night, can
be supposed to allow; for I find, that, if I now
pass a needle across the axis of the right eye,
its shadow will not be seen. But should I, after
this, withdraw my accurate attention from the
star, and view it in the state of sight we have,
when we are said to be in a reverie, in which,
though our eyes are open, we are yet scarcely
conscious of seeing surrounding objects, the
appearance to the right eye expands itself, and
if a needle be again passed before this eye, its
shadow will be observed to move over the star,
in a direction contrary to that of the needle
itself; a sure indication that the rays of light
now tend to a focus behind the retina. In the
same state of things, the appearance -of the star
to the left eye contracts, and if a needle be
held before the eye, no shadow is seen ; a sign
that the rays are collected to a focus on the
retina ; whereas they had formerly crossed one
another before they reached that membrane.
Upon the whole then it is manifest, from the
experiments I have related, that my left eye
can collect to focuses in the retina, rays which
proceed from objects at every distance what-
soever, not less than seven inches; that my
right eye can collect to focuses in the retina,
i
114 EXPERIMENTS, &c.
rays which proceed from objects at every di-
stance whatsoever, not less than ten inches, and
even such as are somewhat convergent, since it
can make those, which are parallel, to meet be-
fore they fall upon the retina ; and lastly, that,
while both the optic axes are directed to a
point within the limits of distinct vision, the
rays proceeding from it are never accurately
collected to focuses in both retinas, and scarcely
ever to a focus in either retina. These are like-
wise the > principal circumstances, in which my
experiments differ in their results from those of
Dr. Porterfield.
In making such experiments with luminous
points, one or other of two appearances very
constantly occurs, neither of which, as far as I
know, has been spoken of by any preceding
author. The most proper way of mentioning
what they are, is, perhaps, to show what ought
to happen in those situations, in which they
are observed.
When a beam of white light passes, obliquely,
from one medium into another of different re-
fractive power, its variously coloured rays must
begin to diverge from each other, at the point
of the beam's incidence upon the latter me-
dium. In achromatic telescopes, the mutual
separation of these rays is checked, and its
farther increase prevented, before it becomes
IN OPTICS. 115
perceptible to sense, by the contrary refractions
which they undergo, from passing, successively,
through the different parts of the object-glass.
Hence, some have imagined, that, since objects,
in ordinary vision, are seen without colour, as
far as this depends on the refractions of the
eye, nature has furnished us with an instru-
ment, constituted upon principles similar to
those of the object-glass of an achromatic tele-
scope. But every one, the least acquainted
with the structure of the eye, must know, that
this cannot be the case, as the refractions in it
are all made one way*. And there are experi-
mental proofs, that compounded light is always
separated into its parts, by passing through the
eye. For if we interpose any opake substance
between us and a luminous body, so that only
a very small portion of this may remain visible,
it will appear to consist of three differently
coloured parts, red, yellow, and blue* The
reason, therefore, of objects being, for the most
part, seen colourless, must be elsewhere sought f.
Now let us suppose, that a luminous point is
the only object which is seen at any one time ;
should the focus of its mean refrangible rays be
* There are indeed some exceptions to this, but not of
sufficient consequence to affect the present argument.
f Dr. Maskelyne has very learnedly treated this subject ii*
the Philosophical Transactions, vol. Ixxix. part 2.
116 EXPERIMENTS, &c.
anterior to the retina, the middle of its picture
upon that membrane must be chiefly composed
of the less refrangible rays ; and this must be
the reason, that, when I look attentively at a
bright star with my left eye, the centre of it
always appears of a light orange colour. As
the beams, however, from the luminous point,
which enter the eye near to its axis, suffer but
little refraction, the brightness of their white
light, will, in great measure, overpower the
colour given to the middle of the picture upon
the retina, by the less refrangible rays of those,
which enter the eye at a distance from its axis.
Were you then to intercept the former beams,
the effect I have mentioned of the latter, must
be more observable ; and hence it is, that when
I place a pin or needle between my eye and
a luminous point, the rays of which come to
a focus before they fall upon the retina, the
shadow, instead of appearing black, is always
of a red or deep orange colour ; which is one
of the phenomena respecting luminous points,
to which I have alluded.
On the other hand, should the focus of the
mean refrangible rays of a luminous point lie
behind the retina, the middle of the picture
there will be principally formed of the more re-
frangible rays ; and if the beams, which enter
the eye near to its axis, be also in the present
IN OPTICS. 117
case intercepted, the effect of the latter rays, in
giving colour to the middle of the picture, will
consequently be rendered more evident. Hence
it is, that, when a luminous point is not suf-
ficiently remote for distinct vision, the seeming
shadow upon it, occasioned by any small opake
object held before my eye, is always blue; and
this is the second of the appearances, which I
said are frequently to be observed, in experi-
ments upon luminous points.
AN
ESSAY
ON
DEW,
AND
SEVERAL APPEARANCES
CONNECTED WITH IT.
TO
JAMES DUNSMURE, ESQUIRE,
MERCHANT IN LONDON.
MY DEAR SIR,
Without your aid, I should, in all
probability, never have acquired the knowledge, upon
which the following Essay is principally grounded ; since
I could not, I believe, have found any other place, con-
sidering that I was obliged to be daily in London, so well
fitted for my experiments, as that which you permitted
me to use during a very long time, though manifestly to
the great inconvenience of yourself and your family. I
beg leave to assure you, that I feel this kindness most
strongly, and that my gratitude for it will never cease to
exist.
I am,
My dear Sir,
Your most obedient Servant,
and faithful Friend,
WILLIAM CHARLES WELLS.
Augu$tt5, 1814.
The following notice was prefixed by the Author to the
second edition, published in 1815.
The infirm state of the Author's health having prevented
him, since the publication of the former edition of his
Essay on Dew, from making experiments in the open air
during the night, and his reading having in the meanwhile
been directed to other objects, the present edition of that
Essay will be found to contain almost nothing more than
. the other. The chief difference between the two arises
from a change in the form of several of his expressions.
He has, for instance, altered the expression of ' saturation
with moisture' to that of ' repletion with moisture', in
order to avoid the appearance of maintaining, that com-
mon air is capable of dissolving water; a tenet uncon-
nected with his theory. Sometimes he has subjoined to
the phrase, which he now employs, on that subject, the
words ' in a pellucid state ;' when this addition has not
been made, he wishes it to be understood.
ESSAY ON DEW, &c.
INTRODUCTION.
I WAS led, in the autumn of 1784, by the event
of a rude experiment, to think it probable, that
the formation of dew is attended with the pro-
duction of cold. In 1788, a paper on hoarfrost,
by Mr. Patrick Wilson of Glasgow, was pub-
lished in the first volume of the Transactions
of the Royal Society of Edinburgh, by which
it appeared, that this opinion had been enter-
tained by that gentleman, before it had oc-
curred to myself. In the course of the same
year, Mr. Six of Canterbury mentioned in a
paper communicated to the Royal Society, that,
on clear and dewy nights, he always found the
mercury lower in a thermometer laid upon the
ground, in a meadow in his neighbourhood,
than it was in a similar thermometer suspended
in the air, six feet above the former ; and that,
upon one night, the difference amounted to 5°
of Fahrenheit's scale. Mr. Six, however, did
124 ESSAY
not suppose, agreeably to the opinion of Mr.
Wilson and myself, that the cold was occasioned
by the formation of dew ; but imagined, that it
proceeded, partly from the low temperature of
the air, through which the dew, already formed
in the atmosphere, had descended, and partly
from the evaporation of moisture from the
ground, on which his thermometer had been
placed. The conjecture of Mr. Wilson, and
the observations of Mr. Six, together with many
facts, which I afterwards learned in the course
of reading, strengthened my opinion ; but I
made no attempt, before the autumn of 1811,
to ascertain by experiment if it were just, though
it had, in the mean time, almost daily occurred
to my thoughts. Happening, in that season,
to be in the country on a clear and calm night,
I laid a thermometer upon grass wet with dew,
and suspended a second, in the air, two feet
above the other. An hour afterwards, the
thermometer on the grass was found to be 8°
lower, by Fahrenheit's division, than the one
in the air. Similar results having been obtained
from several similar experiments, made during
the same autumn, I determined, in the next
spring, to prosecute the subject with some de-
gree of steadiness, and with this view went
frequently to the house of one of my friends,
who lives in Surrey. At the end of two months,
ON DEW, &c. 125
I fancied that I had collected information
worthy of being published; but fortunately,
while preparing an account of it, I met, by
accident, with a small posthumous work of Mr.
Six, printed at Canterbury in 1794, in which are
related differences observed on dewy nights,
between thermometers placed upon grass and
others in the air, that are much greater than
those mentioned in the paper presented by him
to the Royal Society in 1788. In this work,
too, the cold of the grass is attributed, in agree-
ment with the opinion of Mr. Wilson, altogether
to the dew deposited upon it. The value of
my own observations appearing to me now
much diminished, though they embraced many
points left untouched by Mr. Six, I gave up
my intention of making them known. Shortly
after, however, upon considering the subject
more closely, I began to suspect, that Mr. Wil-
son, Mr. Six, and myself, had all committed an
error, in regarding the cold, which accompanies
dew, as an effect of the formation of that fluid.
I, therefore, resumed my experiments, and
having, by means of them, I think, not only
established the justness of my suspicion, but
ascertained the real cause both of dew, and of
several other natural appearances, which have
hitherto received no sufficient explanation, I
venture now to submit, to the consideration of
126 ESSAY ON DEW, &c.
the learned, an account of some of my labours,
without regard to the order of time, in which
they were performed, and of various conclusions
which may be drawn from them, mixed with
facts and opinions already published by others,
PART I.
OF THE PHENOMENA OF DEW.
SECTION I.
Of Circumstances which influence the Production of Dew.
ARISTOTLE* and many other writers have
remarked, that dew appears only on calm and
serene nights. The justness of this observa-
tion, however, has not been universally ad^
mitted. For M usschenbroek f says, that dew
forms in Holland, while the surface of the
country is covered with a low mist ; but, asf
he mentions at the same time, that it is de-
posited upon all bodies indiscriminately, the
moisture, of which he speaks, cannot properly
be called dew, as will be more distinctly seen
hereafter. Other writers of considerable reputa-
tion have also regarded clearness of the atmo-
sphere, as not being requisite for the production
of dew, misled, I believe, partly by theory, and
* Meteor. Lib. I. c. x. et De Mundo. c. iii.
t Nat. Phil. T. ii. De Rore.
128 ESSAY
partly by observing on misty mornings copious
dews, which had been produced during preced-
ing clear nights. Respecting this point I can
aver, after much experience, that I never knew
dew to be abundant, except in serene weather.
In regard to the necessity of the air being still,
I know of no person who rejects it, except Mr.
Prieur*, a late French author of little con-
sideration, and he affirms, in opposition to the
most common observation, that a fresh wind is
requisite for the production of dew.
The remark of Aristotle, however, is not to
be received in its strictest sense, as I have fre-
quently found a small quantity of dew on grass,
both on windy nights, if the sky was clear, or
nearly so, and on cloudy nights, if there was no
wind. If, indeed, the clouds were high, and
the weather calm, I have sometimes seen on
grass, though the sky was entirely hidden, no
very inconsiderable quantity of dew. Again ;
according to my observation, entire stillness of
the atmosphere is so far from being necessary
for the formation of this fluid, that its quantity
has seemed to me to be increased, by a very
gentle motion in the air. Dew, however, has
never been seen by me, on nights both cloudy
and windy.
* Journal de 1'Ecole Poly technique, Tom. ii. 409.
ON DEW, &c. 129
If, in the course of the night, the weather,
from being calm and serene, should become
windy and cloudy, not only will dew cease to
form, but that, which has formed, will either
disappear, or diminish considerably.
In calm weather, if the sky be partially co-
vered with clouds, more dew will appear, than
if it were entirely covered, but less than if it
were entirely clear.
Dew probably begins, in this country, to ap-
pear upon grass, in places shaded from the sun,
during clear and calm weather, soon after the
heat of the atmosphere has declined. My op-
portunities, however, for making such observa-
tions have not been numerous, since, while
pursuing this subject, I seldom went into the
country, till late in the afternoon ; but I have
frequently felt grass moist, in dry weather,
several hours before sunset. On the other
hand, I have scarcely ever known dew to be
present in such quantity upon grass, as to ex-
hibit visible drops, before the sun was very near
the horizon, or to be very copious, till some time
after sunset. It also continues to form, in
shaded places, after sunrise; but the interval
between sunrise, and its ceasing to form, is,
according to my observation, which, upon this
point, has not been extensive, considerably
shorter, than that between its first appearance
130 ESSAY
in the afternoon, and sunset. Contrary, how-
ever, to what happens at sunset, if the weather
be favourable, more dew forms a little before,
and, in shaded places, a little after sunrise, than
at any other time. Musschenbroek, therefore,
errs greatly when he says, that dew does not
form after the sun has risen. The preceding
observations, on the early appearance of dew in
the afternoon, are to be restricted to what hap-
pens to grass, or other substances highly attrac-
tive of dew placed on the ground ; for it occurs
much later on similar substances, which are
elevated a few feet above the ground, though
upon these it continues to form, as long after
the rising of the sun, as upon the others, if they
be equally sheltered from the rays of that body.
The formation of dew, after it has once com-
menced, continues during the whole night, if
the weather remain still and serene. Mr.
Prieur, indeed, of whom I have already spoken,
asserts, that dew forms only in the evening and
morning, and that any which occurs in the
former season always disappears in the course
of the night. I can affirm, however, from long
experience, that grass, after having been dewed
in the evening, is never found dry until after
sunrise, unless the weather has, in the mean
time, changed. Upon one serene and still
night, I placed fresh parcels of wool upon grass
ON DEW, &c. Ml
every hour, and by weighing each of them, after
exposure for an hour, found, that they had all
attracted dew.
When dew forms upon a smooth dense body
as glass, and it is only by means of such a body,
that the process can be accurately observed, the
appearances are altogether similar to those,
which occur on a like body, when exposed to
the steam of water, a little warmer than itself.
The exposed surface has first its lustre dimi-
nished, by a slight damp uniformly spread over
it. As the moisture increases, it gathers into
irregularly shaped flat drops, which are, at first,
very small, but afterwards enlarge and run into
one another, forming streamlets, by means of
which a great part escapes from the body which
had received it.
During nights, that are equally clear and
calmy dew often appears in very unequal quan-
tities, even after allowance has been made, for
any difference in their lengths. One great
source of these differences is very obvious.
For, it being manifest, whatever theory be
adopted concerning the immediate cause of
dew, that the more replete the atmosphere is
with moisture, previously to the operation of
that cause, the more copious will the precipita-
tion of water be, after this operation has com-
menced, all the circumstances, which tend to
K 2
132 ESSAY
increase the quantity of moisture in the atmo-
sphere, must likewise tend to increase the pro-
duction of dew. Thus dew, in equally calm
and clear nights, is more abundant shortly after
rain, than during a long tract of dry weather.
It is more abundant, also, throughout Europe,
with perhaps a few exceptions, and in some
parts of Asia and Africa, during southerly and
westerly winds, than during those, which blow
from the north and the east. Aristotle* says,
that Pontus is the only country, in which dew
is more copious during a northerly, than during
a southerly wind. But a similar fact occurs in
Egypt ; for dew is scarcely ever observed there,
except while the Etesian winds prevail. Both
cases, however, though contrary to the letter,
are consonant with the spirit of the rule ; since
the north wind, in one country, proceeds from
the Euxine sea, and, in the other, from the Me-
diterranean. Another circumstance, of the same
kind with the blowing of wind from the south
and the west, as shewing that the air contains
much moisture, is the lessening of the weight of
the atmosphere. My experience on this point
has not, indeed, been great, as the falling of the
mercury in the barometer is very commonly
attended with wind or clouds, both unfavourable
* Meteor. Lib. 1 . c. x.
ON DEW, &c. 133
to the production of dew ; but still the greatest
dew, I have ever witnessed, occurred while the
barometer was sinking. A corresponding ob-
servation is made by Mr. de Luc, who says,
that rain may be foretold, when dew is uncom-
monly abundant, in relation to the climate and
season*.
To the greater or less quantity of moisture in
the atmosphere, at the time of the action of the
immediate cause of dew, are likewise to be re-
ferred several other facts respecting its copious-
ness, the explanation of which is, perhaps, not
so apparent, as in the preceding examples.
In the first place ; dew is commonly more
plentiful in spring and autumn, than in summer;
the reason is, that a greater difference is ge-
nerally found between the temperatures of the
day and the night, in the former seasons of the
year, than in the latter. In spring, this cir-
cumstance is prevented often from having a
considerable effect, by the opposite influence of
northerly and easterly winds ; but, during still
and serene nights in autumn, dew is almost
always highly abundant.
In the second place ; dew is always very co-
pious, on those clear and calm nights, which
are followed by misty or foggy mornings j the
* Rech. sur les Mod. de 1' Atm. $ 725.
134 ESSAY
turbidness of the air in the morning shewing,
that it must have contained, during the preced-
ing night, a considerable quantity of moisture.
Thirdly ; I have observed dew to be unusually
plentiful on a clear morning, which had suc-
ceeded a cloudy night. For the air, having in
the course of the night lost little or no moisture,
was in the morning more charged with watery
vapour, than it would have been, if the night
had also been clear.
Fourthly ; heat of the atmosphere, if other
circumstances are favourable, which, according
to my experience, they seldom are in this
country, occasions a great formation of dew.
For, as the power of the air, to retain watery
vapour in a pellucid state, increases considerably
faster, while its temperature is rising, than in
proportion to the heat acquired, a decrease of
its heat, in any small given quantity, during the
night, must bring it, if the temperature be high,
much nearer to the point of repletion, before it
be acted upon by the immediate cause of dew,
than if the temperature were low. We read,
accordingly, in the writings of those, who have
travelled into hot climates, of a copiousness of
dew frequently observed by them there, which
very much exceeds what occurs, at any time, in
this country. But even here, dew, though for
the most part scanty in our hottest season, is
ON DEW, &c. 135
sometimes very abundant during it, an example
of which occurred to me on the night, common
to the 29th and 30th of July 1813 ; for on that
night, notwithstanding its shortness, more dew
appeared, than has ever been observed by me
on any other.
In the last place ; I always found, when the
clearness and stillness of the atmosphere were
the same, that more dew was formed between
midnight and sunrise, than between sunset and
midnight, though the positive quantity of mois-
ture in the air, must have been less in the
former, than in the latter time, in consequence
of a previous precipitation of part of it. The
reason, no doubt, is the cold of the atmosphere
being greater in the latter, than in the prior
part of the night.
But there are many circumstances, influ-
encing the quantity of dew, which, though
much more open to accurate observation, than
those tiitherto mentioned, are yet much less
easy to be understood.
In my first attempts to compare the quantities
of dew formed during different times, or in dif-
ferent situations, I attended only to the appear-
ance, which it made on bodies having smooth
surfaces. But quickly seeing this method to
be very imperfect, I next employed wool to
collect dew from the atmosphere, and found it
136 ESSAY
well adapted for my purpose, as it readily admits
amongst its fibres the moisture, which forms on
its outer parts, and retains what it receives so
firmly, that I never but once had occasion to
suspect, that it suffered any portion of what it
had thus acquired to pass entirely through it.
The wool, which I used, was white, moderately
fine, and already imbued with a little moisture,
from having been long exposed to the air of a
room, in which no fire was kept. I divided it
into parcels of 10 grains each, and, immediately
before exposure, pulled the fibres of every
parcel somewhat asunder, so as to give it the
form of a flattened sphere, the greatest dia-
meter of which was about 2 inches. As in
doing this, I went by the judgment of my sight
alone, some little inequality, in point of size,
must have existed among different parcels, but
none, I think, sufficient to affect the accuraqy
of my conclusions from the experiments, in
which they were employed, more especially as
my conclusions scarcely ever rested upon single
trials.
Previously to mentioning the results of any
of my experiments with these parcels of wool,
I think it right to describe the place, where by
far the greater part of my observations on dew
were made. This was a garden in Surrey,
distant, by the public road, about three miles
ON DEW, &c. 137
from the bridge over the Thames at Blackfriars,
but not more than a mile and a quarter, from a
densely built part of the suburbs on the south
side of that river. The form of the garden was
oblong, its extent nearly half an acre, and its
surface level. At one end was a dwelling-
house of moderate size, at the other a range of
low buildings ; on one side a row of high trees,
on the other a low fence, dividing it from
another garden. If this fence had been absent,
the garden would have been on the latter side
entirely open. Within it were some fruit trees,
but, as it had not been long made, their size
was small. Towards one end, there was a grass-
plat, in length 62 feet, and nearly 1.6 broad, the
herbage of which was kept short by frequent
mowing. The rest of the garden was employed
for the production of culinary vegetables. All
of these circumstances, however trifling they
may appear, had influence on my experiments,
and most of them, as will hereafter be seen,
must have rendered the results less remarkable,
than they would have been, if they had occurred
on a wide open plain, considerably distant from
a large city.
I now proceed to relate the influence, which
several differences in the situation, mechanical
state, and real nature of bodies, have upon the
production of dew.
138 ESSAY
I. One general fact relative to situation is,
that whatever diminishes the view of the sky,
as seen from the exposed body, occasions the
quantity of dew, which is formed upon it, to be
less than would have occurred, if the exposure
to the sky had been complete.
I placed, on several clear and still nights, 10
grains of wool upon the middle of a painted
board, 4^ feet long, 2 feet wide, and 1 inch
thick, elevated 4 feet above the grassplat, by
means of 4 slender wooden props of equal
height; and, at the same time, attached, loosely,
10 grains of wool to the middle of its under-
side. The two parcels were consequently only
an inch asunder, and were equally exposed to
the action of the air. Upon one night, how-
ever, I found, that the upper parcel had gained
14 grains in weight, but the lower only 4. On
a second night, the quantities of moisture, ac-
quired by like parcels of wool, in the same
situations as in the first experiment, were 19
and 6 grains ; on a third, 1 1 and 2 ; on a
fourth, 20 and 4 ; the smaller quantity being
always that, which was gained by the wool at-
tached to the lower side of the board.
I bent a sheet of pasteboard into the shape of
a house-roof, making the angle of flexure 90
degrees, and leaving both ends open, This was
placed one evening, with its ridge uppermost,
ON DEW, £c. 139
upon the same grassplat, in the direction of the
wind, as well as this could be ascertained. I
then laid 10 grains of wool on the middle of
that part of the grass, which was sheltered by
the roof, and the same quantity on another part
of the grassplat fully exposed to the sky. In
the morning, the sheltered wool was found to
have increased in weight only 2 grains, but
that, which had been exposed to the sky, 16
grains.
In these experiments, the view of the sky
was almost entirely cut off from the situations,
in which little dew was formed. In others,
where it was less so, the quantity gained, was
greater. Thus, 10 grains of wool, placed upon
the spot of the grassplat, which was directly
under the middle of the raised board, and which
enjoyed, therefore, a considerable oblique view
of the sky, acquired during one night 7, during
a second 9, and during a third 12 grains of
moisture, while the quantities gained, during
the same times, by equal parcels of wool, laid
upon another part of the grassplat, which was
entirely exposed to the heavens, were 10, 16,
and 20 grains.
As no moisture, falling like rain from the at-
mosphere, could, on a calm night, have reached
the wool in any of the situations, where little
dew was formed, it may be thought, that the
140 ESSAY
substances, under which the wool was placed,
prevented, mechanically, the access of that fluid.
But on this supposition it cannot be explained,
why some dew was always found in the most
sheltered places, and why a considerable quan*
tity occurred upon the grass under the middle
of the raised board. A still stronger proof of the
want of justness in this supposition is afforded
by the following experiment. I placed, up-
right, on the grassplat a hollow cylinder of
baked clay, the height of which was 2^ feet,
and diameter 1 foot. On the grass, surrounded
by the cylinder, were laid 10 grains of wool,
which, in this situation, as there was not the
least wind, would have received as much rain,
as a like quantity of wool fully exposed to the
sky. But the quantity of moisture, obtained
by the wool surrounded by the cylinder, was
only a little more than 2 grains, while that ac-
quired by 10 grains of fully exposed wool was
16. This occurred on the night, during which
the wool under the bent pasteboard gained only
2 grains of moisture.
Dew, however, will, in consequence of other
varieties of situation, form in very different
quantities, upon substances of the same kind,
although these should be similarly exposed to
the sky.
In the first place ; it is requisite, for the most
ON DEW, &c. 141
abundant formation of dew, that the substance
attracting it should rest on a stable horizontal
body of some extent. Thus, upon one night,
while 10 grains of wool, laid upon the raised
board, increased 20 grains in weight, an equal
quantity, suspended in the open air, 5j feet
above the ground, increased only 1 1 grains,
notwithstanding that it presented a greater sur-
face to the air than the other parcel. On an-
other night, 10 grains of wool gained on the
raised board 19 grains, but the same quantity
suspended in the air, on a level with the board,
only 13; and on a third, 10 grains of wool ac-
quired, on the same board, 2 J grains of weight,
during the time in which other 10 grains, hung
in the air, at the same height, acquired only \
a grain.
In the second place ; the quantities of dew
attracted by equal masses of wool, similarly ex-
posed to the sky, and resting on equally stable
and extended bodies, oftentimes vary consider-
ably, in consequence of some difference in the
other circumstances of these bodies. 10 grains
of wool, for instance, having been placed upon
the grassplat, on a dewy evening, 10 grains
upon a gravel walk which bounded the grass-
plat, and 10 grains upon a bed of bare garden
mould, immediately adjoining the gravel walk;
in the morning, the wool on the grass was
142 ESSAY
found to have increased 16 grains in weight,
but that on the gravel walk only 9, and that on
the garden mould only 8» On another night y
during the time that 10 grains of wool, laid
upon grass, acquired 2^ grains of moisture, the
same quantity gained only 3- a grain upon the
bed of garden mould, and a like quantity,
placed upon the gravel walk, received no acces-
sion of weight whatever.
Two objections will probably be made against
the accuracy of these, as well as my other ex-
periments with wool. One is, that wool placed
on grass may, by a kind of capillary attraction
receive dew previously formed on the grass, in
addition to its own. To this I answer, that
wool in a china saucer, placed on the grass, ac-
quired very nearly as much weight, as an equal
parcel immediately touching the grass. The
second objection is, that a part of the increased
weight in the wool might arise from its im-
bibing moisture, as a hygroscopic substance. I
do not deny, that some weight was given to the
wool in this way; but it may be safely affirmed,
that this quantity must have been very small.
For, on very cloudy nights, apparently the best
fitted to increase the weight of hygroscopic
substances, wool upon the raised board would,
in the course of many hours, acquire little or
no weight j and in London, 1 have never found
ON DEW, &c. 143
10 grains of wool, exposed to the air on the
outside of one of my chamber windows, to in-
crease, during a whole night, more than ^ a
grain in weight. When this weight was gained,
the weather was clear and still ; if the weather
was cloudy and windy, the wool received either
less or no weight. This window is so situated,
as to be, in great measure, deprived of the
aspect of the sky.
It being shewn, that wool, though highly at-
tractive of dew, was prevented, by the mere
vicinity of a gravel walk, or a bed of garden
mould, for only a small part of it actually
touched those bodies, from acquiring nearly as
much dew, as an equal parcel laid upon grass,
it may be readily inferred, that little was formed
upon themselves. In confirmation of this con-
clusion, I shall mention, that I never saw dew
upon either of them. Another fact of the same
kind is, that, while returning to London from
the scene of my experiments about sunrise, I
never observed, if the atmosphere was clear,
the public road, or any stone pavement on the
side of it, to be moistened with dew, though
grass within a few feet of it, and painted doors
and windows of houses not far from it, were
frequently very wet. If, indeed, there was a
foggy morning, after a clear and calm night,
even the streets of London would sometimes be
144 ' ESSAY
moist, though they had been dry the day before,
and no rain had in the meanwhile fallen. This
entire, or almost entire, freedom of certain si-
tuations from dew depends, however, much
more upon extraneous circumstances, than upon
the nature of the substances found there ; for
river sand, though of the same nature with
gravel, when placed upon the raised board, or
upon grass, attracted dew copiously.
A third difference, from situation, in the
quantity of dew collected by similar bodies,
similarly exposed to the sky, depends upon
their position with respect to the ground. Thus,
a substance placed several feet above the
ground, though in this situation later dewed,
than if it touched the earth, would, notwith-
standing, if it lay upon a stable body of some
extent, such as the raised board lately men-
tioned, acquire more dew during a very still
night, than a similar substance lying on grass.
A fourth difference of this kind occurred
among bodies placed on different parts of the
raised board. For one, that was placed at the
leeward end of it, generally acquired more dew
than a similar body at the windward extremity.
II. Difference in the mechanical state of
bodies, though all other circumstances be similar,
has likewise an effect on the quantity of dew,
which they attract. Thus, more dew is formed
ON DEW, &c. 145
upon fine shavings of wood, than upon a thick
piece of the same substance. It is chiefly for a
similar reason, I believe, that fine raw silk, fine
unwrought cotton, and flax, were found by me
to attract somewhat more dew, than the wool
I employed, the fibres of which were thicker,
than those of the other substances just men-
tioned.
III. Bright metals, in consequence of some
circumstance in their constitution, attract dew
much less powerfully than other bodies ; all of
which, after allowance has been made for any
difference, which may exist in their mechanical
states, seem to attract dew in quantities not
very unequal, if they be similarly situated.
Musschenbroek was the first, who distinctly
remarked this peculiarity of metals ; but Dufay *,
I believe, published it before him, referring,
at the same time, the discovery to its proper
author. Both Musschenbroek and Dufay, how-
ever, made too large an inference from their
experiments ; for they asserted, that dew never
appears on the upper surface of bright metals,
whereas the contrary has since been observed
by many persons, and I have myself known dew
to form on gold, silver, copper, tin, platina,
iron, steel, zinc, and lead. Dew, however, *
* Mem. de 1'Acad. Fran. 1736.
L
146 ESSAY
when it does form upon metals, commonly
sullies only the lustre of their surface ; and
even when it is sufficiently abundant to gather
into drops, these are almost always small and
distinct. Two other facts of the same kind
are ; fast, that the dew, which has formed
upon a metal, will often disappear, while other
substances in their neighbourhood remain wet ;
and secondly, that a metal, which has been pur-
posely moistened, will often become dry, though
similarly exposed with bodies which are attract-
ing dew. This inaptitude to attract dew, in
metals, is communicated to bodies of a very
different nature, which touch or are near to
them. For I have found, that wool laid upon
a metal will acquire much less dew, than an
equal quantity laid upon grass in the immediate
vicinity.
A large metallic plate, lying on grass, resists
the formation of dew more powerfully than a
very small one similarly situated. I conclude
from various collateral facts, that a considerable
difference in the thickness of two pieces of
metal, exposing equal surfaces to the sky, will
be attended with a similar consequence, where-
ever they be placed, though I have no observa-
tion, which proves this directly. If, however,
a large and a very small plate be suspended
horizontally, at the same height, in the air, the
ON DEW, &c. 147
small plate will resist the formation of dew
more powerfully than the large.
If a metal be closely attached to a substance
of some thickness, which attracts dew power-
fully, the attraction of the metal itself for dew,
instead of being increased from this circum-
stance, becomes diminished, provided the metal
cover the whole of the upper surface of the
other body. If only a part of this body be co-
vered, the production of dew on the metal is
forwarded by the conjunction, and this some-
what in proportion, to the quantity of surface
in the lower body left uncovered. The justness
of the first of these observations is proved by
the following experiment. I joined, in the
form of a cross, two pieces of very light wood,
each 4 inches long, a third of an inch in
breadth, and 1 line in thickness. To one side
of this cross I fastened, by means of mucilage,
a square piece of gilt paper, and then exposed
the instrument to the sky, with its metallic side
uppermost, on a dewy night, by suspending it,
in a horizontal position, about 6 inches above
the ground. A few hours after, the unattached
parts of the metalled paper were found covered
with minute drops of dew, while those, which
adhered to the cross, were dry.
A large metallic plate, laid upon grass, was
dewed with more difficulty on its upper surface,
*L 2
148 ESSAY
than a similar plate elevated a few inches above
the grass, by means of slender props, which
allowed the air to pass freely under the metal.
But the case with respect to small pieces was
the reverse ; for I have often seen, covered
with dew, the metallic sheath of a small ther-
mometer lying upon grass, while the similar
sheath of another thermometer, suspended in
the air, remained dry.
Removing a metal several times, in the course
of the night, from one part of the grassplat to
another, facilitated its being dewed. The same
effect was produced on gilt and silvered paper,
by first exposing them to the sky, for some
time, with the bare side uppermost, and then
turning them.
If a piece of glass, covered on one side with
a metal, be placed upon the ground, with this
side downwards, the upper surface will attract
dew, precisely as if no metal were attached to
the lower surface.
The upper surfaces of metals are most readily,
and most copiously dewed, on those nights, and
in those parts of the night, during which other
substances are the most readily, and the most
copiously dewed.
If a metallic plate had been laid upon grass,
before dew began to form anywhere, its lower
side, notwithstanding, always became moist in
ON DEW, &c. 149
the course of the night ; and the same effect
was almost always observed, if the plate had
been placed horizontally in the air, a few inches
above the grass. While the undersides were
thus moist, the upper surfaces were very often
dry. If, however, the plate was elevated several
feet in the air, the condition of both sides was
always the same, whether this was dry or moist.
The remarks hitherto made, on the relation
of metals to dew, apply to the class generally;
but it is now to be mentioned, that they do not
all resist the formation of that fluid, with the
same force.
I saw, for example, platina one night dis-
tinctly dewed, while gold, silver, copper and
tin, though similarly situated, were entirely
dry ; and I have also several times seen these
four metals free from dew, while iron, steel,
zinc, and lead were covered with it.
I once supposed, in consequence of the dif-
ficulty with which metals are dewed, that they
might in all circumstances resist, in a greater
degree than other bodies, the condensation of
watery vapour upon their surface ; and I after-
wards found, that Le Roi* asserts this to be
the case. But having exposed at the same time,
to the steam of warm water, pieces of glass and
* Mem. de 1'Acad. Fran. 1751.
150 ESSAY
of metal, I did not see, that moisture formed
in the least more readily, upon the former than
upon the latter. I have since learned, that
Saussure* once entertained a similar suspicion,
which was also proved by an experiment to be
groundless.
All my experiments, hitherto spoken of, were
made in the country. But Le Roi having said,
that dew is never deposited by the air of cities,
I determined to ascertain, if his assertion was
just. With this view, I frequently exposed, at
night, 10 grains of wool upon a slight wooden
frame, placed in such a manner, between two
ridges of the top of my house, which is situated
in one of the most crowded districts of London,
as to be 3 feet distant from the nearest part of
the roof. The event was, that, upon clear and
calm nights, dew was always acquired by the
wool, though never in any considerable quan-
tity j probably, however, more from the wooden
frame being^ nearly surrounded by buildings,
much more elevated than itself, than from any
particular condition of the air in cities. The
formation of dew, in this situation, proceeded
much less regularly than in the country. For,
* Hygronometrie, page 829.
ON DEW, &c. 151
upon one evening, 10 grains of wool gained
in it 3 grains of moisture, in 1 hour and 18
minutes, though I scarcely ever knew a greater
quantity to be collected by a similar parcel of
wool, in the same place, during a whole night.
These experiments will no doubt seem to many
superfluous, since dew may be observed every
fine evening, upon grass in London. But as
dew upon grass is said by Le Roi to proceed
from the ground, and not from the atmosphere,
the argument derived from its appearance there,
in cities, against his assertion is thus eluded by
him.
The last subject, which I shall here touch
upon, is that of hoarfrost.
This substance has, I believe, from the time
of Aristotle*, been uniformly, and, according
to my observations, justly, considered as frozen
dew. I shall, therefore, frequently refer here-
after to the experiments of the late Mr. Patrick
Wilson of Glasgow respecting it, as if they had
been actually made upon that fluid. Indeed,
several of my experiments upon dew were only
imitations of some, which had been previously
made upon hoarfrost, by that ingenious and
most worthy man.
* Meteor. Lib. I. c, x.
152 ESSAY
SECTION II.
Of the Cold connected with the Formation of Dew*
DEW is often spoken of as being cold, by
popular writers. Thus Cicero and Virgil apply
to it the epithet of 'gelidus,' Milton that of
' chill/ and Collins that of « cold/ Of the same
import is a passage in Herodotus, in which it is
said, that, in Egypt, the crocodile passes a great
part of the day on dry land, but the whole of
the night in the Nile, this being warmer than
the atmosphere, and the dew. Among philoso-
phers, however, Mr. Wilson was the first, I be-
lieve, who ever suspected the existence of such
a conjunction.
In my experiments on the temperature of
bodies moistened with dew, small thermometers
were employed, (the largest being only 8 inches
long) having globular bulbs, which, in most of
them, were not more than from 2 to 2|- lines in
diameter. Their scales, which were marked in
the manner of Fahrenheit, were of ivory or
wood, and were furnished, almost all of them,
with hinges. They were always employed
naked, except I wished to know the effect of
covering them with any particular substance.
By means of these instruments I have very
ON DEW, &c. 153
many times, during serene and still nights, exa-
mined the temperature of dewed grass, and
have constantly observed it to be less than that
of the air, anywhere between 1 inch and 9 feet
above the ground, the latter being the greatest
height, at which I ever marked the heat of the
atmosphere, in these experiments. I generally,
however, compared the temperature of dewed
grass with that of the air 4 feet above the
ground ; and on nights, that were calm and
clear, very frequently found the grass, at the
ordinary place of my observations, 7, 8, or 9
degrees colder than the air at that height. Se-
veral times it was 10° and 11° colder than the
air, and once 12°. These differences are not
so great, as those related in Mr. Six's post-
humous work. But, in his experiments, the
temperature of grass was compared with that
of the air 7 feet above the ground, which, in
clear and calm nights, may be regarded as J a
degree warmer than the air at the height of 4
feet. Besides ; the most considerable differ-
ences, mentioned by Mr. Six, occurred in winter,
when he says a greater degree of cold is oc-
casioned by dew, than at any other time ;
whereas very few of my experiments, on the
temperature of grass, were instituted in that
season. In the last place ; my experiments
154 ESSAY
were almost always made on very short grass,
while Mr. Six's thermometers were laid upon
long grass bent, by strong pressure, towards
the earth ; in which state they marked a tem-
perature 1, 2, and 3 degrees lower, than that
shewn by similar thermometers placed upon
grass, less than an inch in height. Had it not
been for these circumstances, and the unfitness,
in various respects, besides the shortness of the
grass, for the production of a great cold, of the
common scene of my operations, I believe that,
in consequence of my thermometers being much
better adapted to mark a superficial, or transi-
tory cold, than those of Mr. Six, I should at
some time have seen a difference several degrees
greater, than the greatest ever seen by that
gentleman, which was one of 13j°. In con-
firmation of this opinion, I shall mention, that
having, during a short visit to a more distant
part of the country, exposed, in the evening, a
thermometer upon the surface of an open grass
field, I found it soon after, although the grass
was short, and the weather warm, 14° lower
than a similar thermometer, suspended in the
air, 4 feet above the grass. If to this quantity
be added ^ a degree, on account of the dif-
ference in elevation between our suspended
thermometers, the cold, connected with dew,
ON DEW, &c. 155
observed by me this night on grass, will exceed
the greatest ever observed by Mr. Six by 1
degree.
According to a few observations made by me,
the greater coldness of grass, than that of the
air, begins to appear, in clear and calm weather,
in places, sheltered in the afternoon from the
sun, but still open to a considerable portion of
the sky, soon after the heat of the atmosphere
has declined. A similar coldness continues
upon grass in still and serene mornings, for
some time after the rising of the sun, in places
shaded from its direct light, but otherwise open
to the sky. My experiments on this point have
also not been many, and none of them were
made in winter f which, I presume, are the rea-
sons, that I never observed a cold, from this
cause, later in the morning, than an hour after
sunrise. The surface of snow, however, was
once, in the depth of winter, observed by Mr.
Wilson of Glasgow to be considerably colder
than the air, till a little after midday*.
In cloudy nights, particularly if there was
wind, the grass was never much colder than
the air. On such nights, the temperatures of
both were sometimes the same ; at other times
that of the grass was the higher of the two,
* Paper in Phil. Trans. 1781.
156 ESSAY
even when the grass was wet from preceding
rain, and when, consequently, it must have
been, in some measure, cooled by evaporation.
On one such night, the grass was found to be
4° colder than the earth an inch beneath the
surface of the plat, which afforded a sufficient
reason for the grass itself being warmer than
the air. In windy weather, however, if the sky
was clear, some degree of cold, in addition to
that of the air, was always observed upon the
grass ; and in calm weather, very high clouds,
though sufficiently extensive and dense, to con-
ceal the sky completely, would yet frequently
allow of the grass being several degrees colder
than the air. I once observed, upon a night of
this kind, a difference of 5° between the tem-
peratures of those bodies.
If the night became cloudy, after having
been very clear, though there might be no
change with respect to calmness, a considerable
alteration in the temperature of the grass always
ensued ; and this sometimes very suddenly.
Upon one such night, the grass, after having
been 12° colder than the air, became only 2°
colder than it, the temperature of the air being
the same at both observations. On a second
night, grass became 9° warmer in the space of
an hour and a half. On a third night, in less
than 45 minutes, for the whole change occurred
ON DEW, &c. 157
while I was absent 45 minutes, the temperature
of the grass rose 15°, while that of the neigh-
bouring air increased 3j°. During a fourth
night, the temperature of the grass at half-past
9 o'clock was 32°. In 20 minutes afterwards,
it was found to be 39°, the sky having in the
mean time become cloudy. At the end of 20
minutes more, the sky being clear, the tempera-
ture of the grass was again 32°. These were
the most remarkable of my observations on this
subject ; but I may add to them, that I have
frequently seen, during nights that were gene-
rally clear, a thermometer lying on the grass-
plat rise several degrees, upon the zenith being
occupied only a few minutes by a cloud. On
the other hand, upon two nights I observed a
very great degree of cold to occur on the
ground, in addition to that of the atmosphere,
during short intervals of clearness of sky, be-
tween very cloudy states of it.
I did not speak in the preceding section of
another obscure state of the atmosphere, that
occasioned by fog, or mist, as the moisture de-
posited in it attaches to all bodies, indiscri-
minately ; on which account, I was unable to
determine, whether or not dew forms during its
continuance. But, with respect to the con-
nexion of this condition of the atmosphere with
cold, I have to remark, that I have several
158 ESSAY
times, on its appearance betwixt daybreak and
sunrise, found the difference between thermo-
meters on grass and in the air, which had been
considerable during the night, to diminish
greatly. I never, indeed, observed it to vanish,
but this I used to impute to the air being not
very much obscured. I have now, however,
reason to doubt the justness of this conclusion;
for on the evening of the 1st of January in the
present year, 1814, I found, during a dense fog,
while the weather was very calm, a thermo-
meter lying on grass, thickly covered with
hoarfrost, 9° lower than another suspended in
the air, 4 feet above the former. On the fol-
lowing evening, when the air was equally calm,
but the fog sufficiently attenuated to allow me
to see that the sky was almost entirely covered
with clouds, the difference between two ther-
mometers, similarly placed with the former,
was only 1°. On comparing the observations
of these two evenings, I conclude, that on the
first few or no clouds existed above the fog,
and consequently that fog, if there be no clouds
above it, may, in a very calm air, admit of the
appearance of a considerable degree of cold, at
night, upon the surface of the earth, in addition
to that of the atmosphere. Mr. Six, indeed,
says, while speaking of the cold connected with
dew, in his paper in the Philosophical Transac-
ON DEW, &c.
159
tions for 1788, " fogs did not, as far as I could
perceive, at all impede, but rather increase, the
refrigeration." But this was a mistake ; which
in all probability arose from his ascribing the
effect of a clear night to an ensuing foggy
morning, as he examined his thermometers only
in the daytime. He afterwards discovered his
error $ for, in his posthumous work, thick fogs
are ranked among the circumstances, which
always impede, and sometimes prevent alto-
gether, the appearance of a cold upon the sur-
face of the earth, greater than that of the atmo-
sphere. During a very dense fog, Mr. Wilson
found no difference, at night, between a ther*
mometer laid upon snow, and another suspended
in the air*.
When, during a clear and still night, different
thermometers were examined, at the same time,
which had been placed in different situations,
those which were situated, where most dew was
formed, were always found to be the lowest.
Thus, upon one such night, I found a thermo-
meter placed upon a little wool, lying upon the
middle of the upper side of the raised board, to
be 9° lower than another thermometer, in con-
tact with an equal quantity of wool, attached to
the middle of the underside of the board. On
* Edin. Phil. Trans. I. 170.
160 ESSAY
two other nights, the difference between two
thermometers in the same situations was 8°* I
found also, on two other serene and calm nights,
a spot of grass covered by the pasteboard roof,
and another spot surrounded by the earthen
cylinder, to be both 10° warmer than neigh-
bouring grass fully exposed to the sky. Think-
ing it possible, that the cylinder, which had
been exposed to the sun the preceding day,
might still possess some of the heat, which it
had then imbibed, I placed near to it, on an-
other night, a cylinder made of very thin paste-
board ; but this was equally efficacious with the
earthen one, in preventing cold from occurring
on grass. When the exposure was greater than
in the preceding examples, and more dew was
in consequence formed, the cold was also
greater, but still less than where the exposure
was complete. For instance, upon the night
during which 10 grains of wool, placed upon
the middle of the grass, which was sheltered
by the raised board, had gained 7 grains, and
the same quantity on grass fully exposed to the
sky had gained 10 grains, the difference be-
tween the temperatures of the two portions of
grass was only 2j°.
The same correspondence was observed, when
the differences in the quantity of dew did not
depend, as in the preceding instances, upon
ON DEW, &c. 161
any diversity of exposure to the sky. Thus, the
mercury in a thermometer placed upon wool,
lying on the raised board, was found to be at
the 44th degree, while that in another, pendent
in the air, at the same height from the ground,
and wrapped in wool, was at the 48th. Wool
also, on the raised board *, was commonly a little
colder than the same substance on grass, when
the night was very still ; and the leeward end
of that board was generally colder than the
windward extremity.
But, the most remarkable examples of this
kind were exhibited by the gravel walk, and the
bare garden mould. In still and serene nights,
the surfaces of these bodies were always warmer
than the neighbouring grass, and frequently
warmer than the air. On one night of this
description, I observed, 2|- hours after sunset,
the surface of the gravel walk to be 16J°, and
that of the garden mould to be 12j°, warmer
* The greater cold of the raised board, in my experiments,
most probably depended on the grass being very short; since
Mr. Wilson Found, that snow on the ground was colder than
the same body on a raised board. If 1 , 2, or 3 degrees were
added to the cold of the grass at my place of observation,
agreeably to the difference found by Mr. Six, between the
temperatures of long and short grass in dewy nights, the cold
on my raised board would, upon such nights, have been
always less than that of the grassplat.
M
162 ESSAY
than grass very near to them, and similarly ex-
posed to the heavens. As the night proceeded,
clouds formed and accumulated ; in consequence
of which the difference at sunrise, between the
temperatures of the grass and the gravel walk,
was only 6°, and between those of the grass and
the mould only 4°, the temperature of the grass
having in the mean time increased considerably,
while that of the other bodies had decreased a
little. At another time, shortly before sunrise,
a very clear morning having succeeded a cloudy
night, I found the gravel walk to be 10° and the
garden bed to be 9° warmer than neighbouring
grass, which was 8° colder than the air. Both
of these examples occurred in summer, and I
believe, that such considerable differences will
occur in that season only. It was on the first of
these two nights, that 10 grains of wool gained
only \ a grain of moisture on the mould, and
that the same quantity gained no weight on the
gravel walk. That the unfitness of the gravel
walk, however, to become cold, like its unfitness
to attract dew, arose from its situation, and not
from the nature of the substance of which it was
made, is proved by this circumstance, that river
^sand, placed on the raised board, was on 4 dif-
ferent nights, none of them highly favourable for
the production of cold, 7, 7, 8, and 8^ degrees
colder than the air at the same height.
ON DEW, &c. 163
It may be added here, that I have always
found, on dewy nights, the temperature of the
earth, \ an inch or an inch beneath its surface,
much warmer than the grass upon it. On five
such nights the differences were from 12 to 16
degrees. The earth, at the above-mentioned
depth, was also almost constantly warmer on
dewy nights than the air; sometimes it was con-
siderably so, for I once observed it to be 10°
warmer, at another time 9°, and at a third 7-J°.
An exception will no doubt occur, if very mild
weather should follow a lorig frost ; but of this
I have had no experience.
In the experiments upon my housetop in
London, I always found, during clear and calm
nights, wool lying on the wooden frame to be
colder than the air, at the same height; but the
difference was seldom more than 3°. On the
evening, however, during which dew formed
there more copiously than usual, the difference
was 5°. That the smallness of these differences
was not wholly occasioned by any thing special
in the air of cities was afterwards proved, by my
finding others much greater, in a garden nearly
in the middle of London, from which almost the
whole of the sky was visible.
Metals, likewise, furnish proofs of the con-
nexion of dew with a cold in the substance, on
M 2
ESSAY
\
which it forms, superior to that of the neigh-
bouring atmosphere. My observations, how-
ever, on the temperature of metals, when ex-
posed to the sky on dewy nights, were less
numerous, than those on several other subjects
treated in this Essay, by reason of the less fre-
quent opportunity I enjoyed of making them ;
and many of those, which I did make, were
afterwards found by me to have been impro-
perly conducted. I thought, for instance, for
some time, that the temperature of a metal, on
a dewy night, might easily be learned in the
way, in which I had been accustomed to ascer-
tain the temperature of dewed grass. But, ob-
serving dew one night on the glass tube of a
thermometer, which was lying on a metal placed
upon grass, while the metal itself was free from
moisture, I conceived it probable, that the cold
then indicated by the thermometer was not the
real temperature of the body, to which it was
applied. To determine the point, I placed on
the same metal a second thermometer, covered
with gilt paper, upon which this was found at
three observations to be 6J°, 7°, and 7° higher
than the other. In this experiment, the bulb
of the naked thermometer, from being very
small, did not project as far as the outer surface
of the scale, and, consequently, did not come
ON DEW, &c. 165
in contact with the metal. But even when the
ball of a thermometer was applied directly to a
metal, on a clear and calm night, a temperature
was marked by it, commonly 2 and 3, and some-
times more degrees less than that marked by
a similar , thermometer, inclosed in gilt paper,
and similarly placed. I found it likewise ne-
cessary, in this inquiry, to correct the tempera-
ture of the air, as given by a naked thermo-
meter. For, on still and serene nights, a ther-
mometer inclosed in a case of gilt or silvered
paper, and suspended in the air 4 feet above
the grassplat, was usually observed to be 1 J° or
2° higher than a bare thermometer, of the same
construction, suspended near to it. The dif-
ference of two such thermometers, thus placed,
was once observed by me to be 2|-°, and once
3^°. It may be thought, perhaps, that these
differences were caused by the metalled case
obstructing the transmission of the temperature
of the air to the inclosed instrument. But that
this was not the reason is shewn by my observ-
ing, that on cloudy nights there existed no dif-
ference between the two thermometers ; that,
even on clear nights, a thermometer contained
in a case of white paper, somewhat thicker than
the metalled, was always nearly of the same
temperature with a naked one which was sus-
pended close to it 5 and that, when a difference
166 ESSAY
did exist between the two latter, the thermo-
meter in the white paper case was commonly
lower than the other.
The estimation of the heat, both of air and
of metals, on a dewy night, is liable to errors
from other causes. As these, however, are
trifling, I shall not mention them, but proceed
to state the results of my observations, upon
the temperature of metals exposed to the sky at
night, though unable to vouch for their entire
accuracy.
Thin bright metallic plates, the least having
a surface of 25 square inches, and some of them
a surface of more that 100 such inches, were
several times observed, while lying on grass
which was attracting dew, to be 1 and 2, and
once 3, degrees warmer than the air 4 feet
above them. At other times, their temperature
was the same with that of the air. In both of
these cases their upper surfaces were always
free from dew. Metals thus situated were,
consequently, often much warmer than the
grass, which surrounded them. I made no ex-
periments on this point, during the nights, on
which occurred the greatest instances of cold
on grass, relatively to the temperature of the
air ; but I found, notwithstanding, during one
night, a metal on grass to be 10° warmer than
the exposed grass near to it. On two other
ON DEW, &c. 167
nights, the differences were 9° and 8°. The
superiority of the heat of metals on grass over
that of the air, when it did exist, was evidently
connected with the temperature of the grass,
which they covered, and this again with that of
the earth under the same portion of grass ; for
this portion was always a little warmer than the
metal, but not so warm as the earth.
On the other hand, metals, on which dew
was forming while they lay upon grass, were
always colder than the air. In like manner, if
one metal upon the grassplat were dewed, while
another similarly situated remained dry, the
former was always colder than the latter.
When a metal lying on the grassplat became
dewed, the grass under it was always colder
than that under another metal, which was un-
dewed.
A metal, while receiving dew, in consequence
of being elevated in the air, was always colder
than a similar metal, which remained undewed
on the grass.
The greatest instances of cold, observed by
me on metals, occurred at times, when other
bodies near to them had become considerably
colder than the atmosphere.
The cold, however, contracted by metals,
from exposure to the sky in a clear and still
night, was always less than that of other bodies
168 ESSAY
similarly situated, the greatest excess of cold
ever observed by me, in the larger metallic
plates, from this cause, over that of the air,
being not more than 3 or 4 degrees. If much
smaller pieces were placed upon grass, the re-
sult was different. For I have found a small
thermometer placed in this situation, while in-
closed in a sheath of gilt paper, to be only 3°
less cold than the surrounding grass, during a
night favourable to the production of cold on
the surface of the earth.
I collected only a few facts respecting the
comparative temperatures of different metals,
when they were exposed together to the sky,
on dewy nights ; but such as I did collect tend
to prove, that the most readily dewed metals
become colder than the air, sooner than those,
which receive dew with greater difficulty.
Many of the experiments, which have been
mentioned in this section, shew, that when bo-
dies, which had been equally exposed to the
night air, were examined at the same time,
those which were most dewed were also the
coldest. No such correspondence, however,
was found in the experiments of different
nights, or even of different parts of the same
night. Thus, during two nights, on which
ON DEW, &c.
grass was 12° and 14° colder than the air, there
was little dew; while on the night, which
afforded the most copious dew ever observed
by me, the cold possessed by the grass, beyond
that of the air, was for the most part only 3°
and 4° ; and I have always seen less dew about
sunset, than about sunrise, when the weather
has been calm and clear at both times, though
there is commonly, in this country at least, a
greater difference between the temperature of
grass and of air in the evening, than in the
morning. I had early observed, also, bodies
exposed to the sky, on a cloudy but calm night,
to be sometimes 2° or 3° colder than the air,
without having any appearance of dew; and
when two metals possessing different relations
to dew were exposed together, I have seen the
one, which was the fitter to attract that fluid,
colder than the other, though both were dry.
I shall conclude this part of my Essay, with
relating the results of some experiments, which
were made for the purpose of ascertaining the
tendencies of various bodies to become cold,
upon exposure to the sky at night. Unfor-
tunately, the weather was not always favourable
to my views ; but what occurred appears, to me,
notwithstanding, worthy of being related.
170 ESSAY
In the observations hitherto given by me on
the cold connected with dew, the temperature
of grass has been chiefly considered, partly be-
cause my first experiments had been made upon
it, and partly from a wish, which arose after-
wards, to compare my own experiments with
those of Mr. Six, which had been confined to
that substance. I found it, however, very unfit
to furnish the means of comparing the degrees
of cold produced at night on the surface of the
earth, at different times and places ; as its state
on different nights, on the same parts of the
plat I commonly made use of, and in different
parts of the plat on the same nights, was often
very unequal, in point of height, thickness and
fineness, all of which circumstances influenced
the degree of cold produced by it. I observed,
in consequence, a much greater uniformity in
the results of experiments made with various
other bodies, whose condition, when first ex-
posed to the air, was always the same. Of these,
the most productive of cold were the filamentous
and downy, as wool of moderate fineness, very
fine raw silk, very fine unspun cotton, fine flax,
and swandown, all of which were not only more
steadily cold, upon clear and calm nights, than
grass, but also gave rise to a greater degree of
cold, than was almost at any time observed upon
it, even in its best state. Among the bodies of
ON DEW, &c. 171
this class, wool produced the least cold, and I
formerly mentioned that it attracted less dew,
than silk, cotton, and flax. The last mentioned
substances, and swandown, were found equal,
or nearly so, in their tendency to become cold.
Swandown, however, exhibited the greatest cold
rather more frequently than any of the rest ; on
which account, and from its being more easily
managed, as it was used while adhering to the
skin of the bird, I at length scarcely ever em-
ployed any other body of the same class. On
the night, during which grass was observed to
be 14° colder than the air, swandown, lying
upon a neighbouring piece of grass, was still
one degree lower. This difference of 15°, be-
tween the temperature, at night, of a body on
the surface of the earth, and that of the air, a
few feet above the earth, is the greatest which
I have hitherto seen.
Fresh, unbroken straw, and shreds of white
paper, though not properly to be ranked among
filamentous substances, were also found to be a
little more productive of cold, than the wool
which I used.
The next class consisted of bodies in the state
of a powder, more or less fine. These were
clean river sand, glass, chalk, charcoal, lamp-
black, and a brown calx of iron. Chalk pro-
duced the least, and the three last substances,
172 ESSAY
the greatest cold. They were all, however, in-
ferior in this respect to bodies of the first class.
Solid bodies, having a surface exposed to the
sky, of at least 25 inches square, formed a third
class, on which such experiments were made.
The particular substances of this description,
subjected to trial, were glass, brick, cork, oak-
wood, and wax ; all of which were, likewise,
found inferior to the filamentous substances.
From these last experiments it follows, that
when a glass bulb of a thermometer is applied
at night to a body exposed to a clear sky, the
temperature exhibited by the instrument will
not be accurately that of the body in question,
except the disposition of the latter to become
cold, in such a situation, be the same as that of
glass. An example of this- fact has been given
in this Essay*.
My principal experiments, however, of this
kind were made with snow.
On the 25th of January 1813, the ground
being then covered with snow about an inch
deep, I went to my usual place of experiment
in the country ; but, during 8 hours that I at-
tended to my thermometers, the whole sky was
constantly overcast with clouds. The atmo-
sphere was, for the greater part of that time,
very still, and a thermometer on the snow was
* Page 164.
ON DEW, &c.
173
generally about £° lower, than another in the
air. That this difference was not owing to
evaporation was proved by the thermometer on
the snow always rising, from a half to a whole
degree, whenever the air was a little moved,
and falling the same quantity, as soon as a great
stillness again took place.
I had no opportunity of renewing my observa-
tions upon snow, before the beginning of the
present year, 1814. The state of my health
rendering it improper, that I should incur
much fatigue, or be long exposed to night air,
I restricted myself to the making a few experi-
ments, in the large garden in Lincoln's-Inn
Fields. I went thither, for the first time, on
the evening of the 4th of January, immediately
after a considerable snowfall had ceased, wish-
ing to begin my observations, before any cold
should arise on the snow's surface, from ex-
posure to the sky. This was desirable on an-
other account ; for Mr. Kirwan, in direct op-
position to indisputable facts, most clearly stated
by Mr. Wilson, had said, that the great cold,
observed by that gentleman on snow, was occa-
sioned by this substance having retained the
temperature of the high region, from which it
had fallen*. The result of my inquiry was,
that the surface of the snow, and the air 4 feet
* On Temperatures, p. 30.
174 ESSAY
above it, had precisely the same heat. The
depth of the snow was 4 inches.
My next experiment took place on the even-
ing of the 6th, the intervening day having been
snowy. The sky was clear, but the air had a
considerable motion. The heat of the atmo-
sphere, at the height of 4 feet, was at 9% h. 26°;
while that of the surface of the snow, and of
swandowri lying upon it, was 22°. The depth
of the snow was now about 5 inches.
On the 7th, a little after sunset, the heat of
the air in the garden was 23°, that of the surface
of snow 19°, but that of swaridown lying upon
the snow only 15°. There was then a gentle
breeze ; some parts of the sky were covered with
clouds, and the lower atmosphere was a little
obscure. While the exposed surface of the
snow was 19°> a part of its surface, which had
been covered, about 20 minutes, with a piece
of pasteboard, was 22°. Grass, at the bottom
of the snow, was 31°, and the earth an inch be-
neath the grass 32°.
After this, there was no fit time for observa-
tion until the 13th. The thermometers were
exposed at 8 h. on the evening of that day, the
sky being then without clouds ; but the stars
were not bright, and there was a perceptible
motion in the air. At 83- h. the temperature of
the air was 22^°, that of the surface of the snow
ON DEW, &c. 175
13°, and that of swandown, lying on the snow,
8°. At 9h. the air was 23J°, . snow 17°, and
swandown 15°. The sky being now, in great
measure, covered with high thin clouds, my
experiments ceased. At lO^h. the sky was
very bright, and the atmosphere very calm ; but
it was not then convenient to me to renew my
observations. Had I repeated them at that
time, I should probably have found a difference,
between the temperature of the swandown and
air, several degrees more considerable than
the one of 14^°, which had already occurred on
this evening, and consequently greater than the
greatest observed by Mr. Wilson, between the
temperatures of snow and of the atmosphere,
which was one of 16°.
The next favourable evening was that of the
21st. Much snow having in the meanwhile
fallen, its depth was now more than a foot.
The thermometers were observed 5 times be-
tween 4 h. 15m. and 4 h. 55 m. At 4 of those
times, the swandown was 13°, and at one of
them 13^°, colder than the air, the heat of
which at the 4 first observations was 26°, and
at the last 25^-°. The temperature of the sur-
face of the snow, during the whole period of
observation, was 17°, and consequently 4 times
it was 4°, and once 5°, less cold, than that of
the swandown. The atmosphere was altogether
176 ESSAY ON DEW, &c.
free from clouds, and nearly quite calm, but #
good deal hazy.
Before another proper evening arrived, my
health became so infirm, that I was obliged to
relinquish this pursuit. I conclude therefore
my account of it, with two remarks. 1. If Mr.
Wilson had been accustomed to examine the
temperature of swandown, or any similar sub-
stance, placed upon snow, he would, probably,
have observed a cold, on the surface of the
earth, exceeding that of the atmosphere by 20*
or more, on the night of his actually observing
an excess of 16°. 2. Since upon one evening,
when the atmosphere was neither very clear nor
very still, a difference of 14^° was found by me,
between the temperatures of air and of swan*
down, which is only \ a degree less than the
greatest difference I have ever observed, be-
tween the same substances on the stillest and
clearest nights in summer, a corroboration is
hence derived of a conclusion, made by Mr.
Six from his experiments, that the greatest dif-
ferences at night, in point of temperature, be-
tween bodies on the surface of the earth, and
the atmosphere near to it, are those which take
place in very cold weather.
PART II.
OF THE THEORY OF DEW.
DEW, according to Aristotle*, is a species of
rain, formed in the lower atmosphere, in conse-
quence of its moisture being condensed by the
cold of the night into minute drops. Opinions
of this kind, respecting the cause of dew, are
still entertained by many persons, among whom
is the very ingenious Mr. Leslie of Edinburgh f.
A fact, however, first taken notice of by Gers-
ten, who published his treatise on dew in 1733,
proves them to be erroneous ; for he found, that
bodies a little elevated in the air often become
moist with dew, while similar bodies, lying on
the ground, remain dry, though necessarily,
from their position, as liable to be wetted, by
whatever falls from the heavens, as the former.
Shortly after the appearance -of Gers ten's
treatise, Musschenbroek made the remark,
already mentioned in this Essay, that metals
will be free from dew, while other bodies attract
it copiously. This philosopher contented himself
* Meteor. Lib. 1. c. x. et De Mundo. c. iii.
f Relations of Heat and Moisture, p. 37, and 132.
N
178 ESSAY
with publishing his discovery, but his friend
Dufay concluded from it, that dew is an electric
phenomenon, since it leaves untouched the
bodies, which conduct electricity, while it ap-
pears upon those, which cannot transmit that
influence. If dew, however, were to form on
the latter only, its quantity would never be suf-
ficiently great, to admit its being distinctly seen -,
for the non-conductors, as soon as they became
in the least moist, would be changed into con-
ductors. Charcoal, too, it is now known, though
the best solid conductor of electricity after the
metals, attracts dew very powerfully ; and, in
the last place, contrary to the assertion of Du-
fay, dew frequently forms upon metals them-
selves.
Other authors have ascribed the production
of dew to electricity, for reasons different from
that of Dufay. But there are several considera-
tions, which seem to me to prove, that no such
opinion can be just. 1. When dew is produced
in a clear atmosphere, the portion of air, by
which it is deposited, must necessarily be un-
able, at that moment, to retain, in a state of pel-
lucid vapour, all the moisture, which it had
immediately before held in that form. But I
know of no experiment, which shows, that air,
by becoming positively electrical, which is said
to be its condition on the evenings, during which
ON DEW, &c. 179
dew is most abundant, is rendered less able, than
it had previously been, to contain watery vapour
in a state of transparency. 2. Bodies in similar
circumstances, as far as electricity is concerned,
acquire very different quantities of dew. Wool
placed on the raised board, for example, attracted
very much more dew, than wool attached to the
lower side of the same board, and even considera-
bly more than the same substance freely sus-
pended in the air, and entirely exposed to the
sky. 3. Dew forms in different parts of the
night, in quantities no way proportioned to the
degrees of electricity found in the atmosphere
at the same times. Thus, it is commonly more
copious in the morning than in the evening,
notwithstanding that the air is observed to be,
in the latter season, more highly electrical than
in the former. 4. I have several nights held
a glass bottle, upon which dew was forming,
close to the top of a Bennett's electrometer,
which had been previously kept in a dry place ;
but I never saw the slips of gold leaf to move
in consequence. It is very probable, however,
that more refined experiments will show, that
electrical appearances attend the production of
dew. These, perhaps, accompany every change
in the chemical form of bodies. But the facts,
which have been stated, seem sufficient to esta-
blish, that any such appearances, which may be
N 2
ISO ESSAY
hereafter remarked, during the formation of
dew, must be considered as effects, and not
as the cause, of the conversion of the watery
vapour of a clear atmosphere into a fluid.
A remaining argument applies equally to all
the theories, which have hitherto been made
public on the cause of dew. This is, that none
of them include the important fact, that its pro-
duction is attended with cold ; since no explana-
tion of a natural appearance can be well founded,
which has been built without the knowledge of
one of its principal circumstances. It may seem
strange to many, that neither Mr. Wilson, nor
Mr. Six, applied this fact to the improvement
of the theory of dew. But according to their
view of the subject, no such use could have been
made of it by them, as they held the formation
of that fluid to be the cause of the cold observed
with it. I had many years, as was formerly
mentioned, held the same opinion ; but I began
to see reason, not long after my regular course
of experiments commenced, to doubt its truth,
as I found that bodies would sometimes become
colder than the air, without being dewed ; and
that, when dew was formed, if different times
were compared, its quantity, and the degree
of cold which appeared with it, were very far
from being always in the same proportion to
each other. The frequent recurrence of such
ON DEW, &c. 181
observations at length converted the doubt of
the justness of my ancient opinion, into a con-
viction of its error, and at the same time occa-
sioned me to conclude, that dew is the produc-
tion of a preceding cold in the substances, upon
which it appears. Wishing, however, to obtain
proofs, more striking in degree, of the validity
of these inferences, than such as had been
afforded to me by casual observation, while
attending to other parts of my subject, I insti-
tuted the experiments which will be next re-
lated.
I had frequently remarked, early in the even-
ing, a considerable degree of cold on substances
exposed in calm weather to a clear sky, and I
had also sometimes seen, early in the evening,
the raised board altogether dry, while the grass
was much moistened. I therefore determined
to make the experiments in view on the raised
board, and to commence them as soon as the
sun should cease to shine upon it. The first
day I went to the country for this purpose, the
19th of August 1813, almost every circumstance
was favourable to its completion. There had
been no rain for three weeks; the wind was
northerly ; and the barometer was rising ; all
which indicated, that the atmosphere contained
little moisture. The air too was extremely still.
The only appearance in the least unfavourable
182 ESSAY
was, that the sky was not entirely free from
clouds; but these were few, of small extent,
thin, and high.
At 6h. 25m. immediately after the sun had
ceased to shine upon the spot, where my experi-
ments were to be carried on, though the time
of its setting was still 47 minutes distant, I
placed upon the raised board 1 0 grains of wool,
and a small bag, made of the skin of a swan's
breast with the down adhering, and stuffed with
wool, the whole weighing nearly 5 drachms.
On each of these substances the naked bulb of
a small and delicate thermometer was laid. A
similar thermometer, with its bulb also naked,
was suspended in the air, over the grassplat, at
the same height with the board. Two thermo-
meters were placed in other situations, as will
be seen in the annexed Table. After an ex-
posure of 20 minutes, the wool was 7° colder
than the air, but the swandown bag only 6°, no
doubt in consequence of its comparatively great
quantity of matter. Neither, however, had
gained the least weight, according to the scales
employed by me, which were sensibly moved
by the 16th of a grain. These observations
were repeated several times during the follow-
ing hour, as will be seen by the Table, at none
of which, except the last, was either the wool
or swandown found in the least heavier, than
ON DEW, &c.
183
when first placed on the board. At this last
observation, the wool, though 9i° colder than
the air, was still without any increase in weight ;
but the swandown, which was 1° colder than
the wool, had gained \ a grain. My experi-
ments now properly ceased ; but having suffered
the thermometers, which had been placed on the
wool and swandown, and in the air, to remain
in those situations, I examined them again at
8h. 45m., that is, 2h. £0m. after they had
been first exposed. The wool, which was still
9j° colder than the air, had gained somewhat
less than £ a grain ; and the swandown, which
was now llj° colder than the air, had gained
2 grains, including the J grain already men-
tioned. When these last observations were
made, the sky was entirely cloudless, and the
atmosphere very calm.
<
TABULAE VIEW OF OBSERVATIONS
on the Evening of dugust 19, 1813.
6h.45<n.
7h.
7h. 20m.
7b. 40m.
Bh. 46m,
Heat of air 4 feet above the grass
60f°
60$°
59°
58°
54*
• " •-• wool on the raised board
53$
54|
51*
48$
44*
swandowri on the same
54*
53
51
47|
49}
surface of the raised board
58
57
55f
— — grassplat*
53
51
4<$
49
42
* In these experiments, contrary to what usually happens,
the grass was almost constantly colder than the filamentous
substances, although they were placed upon the raised board
184 ESSAY
Similar experiments made at the same place,
on the evenings of the 25th of August and 17th
of September, in the same year, had results,
which were also similar but less in degree ; the
greatest difference between the temperature of
wool or swandown, while they were without any
increase of weight, and the temperature ^e
air, having been, on the first of those even .,s,
only 4°, and on the second only 5°. The rea-
sons were, in great measure, if not wholly, that
a considerable part of the sky was covered with
clouds, and that the air was commonly in that
state of motion, which is denominated a gentle
breeze.
On the evening of my first experiments, I had
omitted to measure the heat of the raised board,
before the thermometers were placed upon it.
This was attended to on the two latter evenings,
on the first of which its upper surface was found,
at the commencement of the experiments, 4°
warmer than the air ; on the second, both it and
the air were of the same temperature. Again ;
on the first of the latter evenings, 10 grains of
wool, to which 3 grains of water had been added,
having been laid on the raised board, near the
thermometers ; at the end of 45 minutes the
parcel was found to have lost 2^ grains of mois-
ture by evaporation, during the time, that dry
wool had become several degrees colder than
the air.
ON DEW, &c. 185
A fourth experiment of , this kind was made
by me on the 7th of January, 1814, in the
garden of Lincoln's-Inn-Fields, by placing 10
grains of wool on a sheet of pasteboard, which
lay upon the snow. At the end of 35 minutes
the wool was 5° colder than the air, without
possessing any additional weight.
Having thus shown the justness of my former
conclusion, that the cold, observed with dew,
is the previous occurrence, and, consequently,
that the formation of this fluid has precisely
the same immediate cause, as the presence of
moisture upon the outside of a glass or metallic
vessel, when a liquid considerably colder than
the air has been poured into it shortly before ;
I shall next apply this fact to the explanation
of several atmospherical appearances.
I. The variety in the quantities of dew, which
were found by me upon bodies of the same
kind, exposed to the air during the same time
of the night, but in different situations, is now
seen to have been occasioned by the diversity
of temperature, which existed among them.
II. Agreeably to the opinion of Mr. Wilson
and Mr. Six, the cold connected with dew
ought always to be proportional to the quan-
tity of that fluid ; but this is contradicted by
experience. On the other hand, if it be
granted, that dew is water precipitated from the
186 ESSAY
atmosphere, by the cold of the body on which it
appears, the same degree of cold, in the pre-
cipitating body, may be attended with much,
with little, or with no dew, according to the
existing state of the air in regard to moisture ;
all of which circumstances are found actually
to take place.
III. The formation of dew, indeed, not only
does not produce cold, but like every other pre-
cipitation of water from the atmosphere, pro-
duces heat. I infer this, partly because very
little dew appeared upon the two nights of the
greatest cold I have ever observed on the sur-
face of the earth, relatively to the temperature
of the air, both of them having occurred after a
long tract of dry weather ; and partly from the
most dewy night, which I have ever seen, having
been attended, during the greater part of it,
with no considerable degree of cold. On this
night, the difference between the temperatures
of grass and of air was at first 7^°, the dew
being then not very abundant. But, after the
dew had become very abundant, the difference
of those temperatures never exceeded 4°, and
was frequently only 3°.
With the view of obtaining, though indirectly,
some knowledge of the quantity of cold, which
had been prevented, by the formation of dew,
from appearing on the surface of the earth, in
ON DEW, &c. 187
the night just spoken of, I made the following
experiment. To 10 grains of wool having the
same form and extension, as the parcels em-
ployed for the collection of that fluid, were
added 21 grains of water, this being the quan-
tity of moisture, which had been attracted by
10 grains of wool, lying on the grassplat, in the
space of 8 hours on that night. The wet wool
having been then placed in a china saucer, laid
on a feather-bed in a room, the door and win-
dows of which were shut, its heat during the
following 8 hours was, at frequent examina-
tions, uniformly found to be about 4° less,
than that of a dry china saucer on the same
bed ; the temperature of the air in the room
not having altered more than \ a degree, in the
course of the experiment. At the end of the
8 hours, the wool still retained 2j grains of
moisture. If this quantity had also evaporated,
the cold uniformly produced during the 8 hours
would, in all probability, have been about 4j°.
From this experiment, therefore, I think it may
be inferred, that the mean quantity of cold,
which was prevented, by the formation of dew,
from appearing on the ground, during the night
which has been mentioned, was also aoout 4^°,
But, as the production of dew, during some
parts of the night, was at a greater rate, than
that of 211 grains for 8 hours, 1 or 2 degrees
188 ESSAY
may be added for those times, which will raise
the effect of the dew in diminishing the ap-
pearance of cold during them to about 6°, on
the supposition, which cannot be far from the
truth, that dew had been attracted as copiously
by the grass, as by wool which lay upon it.
The less difference commonly observed be-
tween the temperatures of grass and of air, in
the morning, than what occurs in the evening,
is likewise to be, in part, attributed to a greater
quantity of dew appearing in the former, than
in the latter season.
A more remarkable fact, deriving an explana-
tion from the same source, is the greater dif-
ference which takes place in very cold weather,
if it be calm and clear, between the tempera-
tures of the air and of bodies on the earth, at
night, than in equally clear and calm weather
in summer; since, in very cold weather, any
diminution of the temperature of a portion of
air, in contact with a cold body, will be at-
tended, in consequence of the well known re-
lations of the atmosphere to moisture, with a
much less formation of water, than an equal di-
minution would be in summer, supposing the
air, before it touches the cold body, to be at
both times equally near to its point of repletion
with moisture.
IV. In very calm nights, a portion of air,
ON DEW, &c.
which comes in contact with cold grass, will
not, when the surface is level, immediately quit
it, more especially, as this air has become spe-
cifically heavier than the higher, from a diminu-
tion of its heat, but will proceed horizontally,
and be applied successively to different parts of
the same surface. The air, therefore, which
makes this progress, must at length have no
moisture to be precipitated, unless the cold of
the 'grass which it touches should increase.
Hence in great measure is to be explained, why
on such nights, as have been just mentioned,
more dew was acquired by substances placed
on the raised board, than by others of the same
kind on the grass, though it began to form
much sooner in the latter than in the former
situation, those on the raised board having re-
ceived air, which had previously deposited less
of its moisture.
A reason is now also afforded, why a slight
agitation of the atmosphere, when very preg-
nant with moisture, should increase the quan-
tity of dew ; since fresh parcels' of air will
hence be more frequently brought into contact
with the cold surface of the earth, than if the
atmosphere were entirely calm.
V. Dew, in agreement with the immediate
cause which has been assigned by me for its
production, can never be formed, in temperate
190 ESSAY
climates, upon the naked parts of a living and
healthy human body, during the night ; since
their heat is never less in this season, in such
climates, than that of the atmosphere. I have,
in fact, never perceived dew on any naked part
of my own body at night, though my attention
was much occupied, for three years, with every
thing relative to this fluid, and though I had
been, during that period, much exposed to the
night air. On the other hand, in very* hot
countries, the uncovered parts of a human body
may sometimes, from being considerably colder
than the air, condense the watery vapour of
the atmosphere, and hence be covered with a
real dew, even in the day-time.
VI. Hygrometers formed of animal or vegeta-
ble substances, when exposed to a clear sky at
night, will become colder than the atmosphere ;
and hence, by attracting dew, or, according to
an observation of Saussure*, by merely cooling
the air contiguous to them, mark a degree of
moisture, beyond what the atmosphere actually
contains. This serves to explain an observation
made by Mr. De Lucf, that in serene and
calm weather, the humidity of the air, as de-
termined by an hygrometer, increases about,
* Hygronometrie, p. 25.
f Introduction a la Physique Terrestre, II, 491.
ON DEW, &c.
and after sunset, with a greater rapidity, than
can be attributed to a diminution of the general
heat of the atmosphere.
These examples are sufficient to show the
value of the fact, that bodies become colder than
the neighbouring air, before they are dewed,
in explaining many atmospherical appearances.
To this point, the investigation of the cause of
dew might have been carried at any time, since
the invention of thermometers ; but its com-
plete theory could not possibly, in my opinion,
have been attained, before the discoveries on
heat were made, which are contained in the
works of Mr. Leslie and Count Rumford.
The experience of most persons, respecting
the communication of heat among bodies in the
open air, is confined to what happens during
the day; at which time, those that are situated
near to one another are always found to possess
the same temperature, unless some very evident
reason for the contrary should exist. To
many, therefore, it may appear incredible, that
a perfectly dry body, placed in contact, on all
sides, with other bodies of the same tempera-
ture with itself, shall afterwards, without un-
dergoing any chemical change, become much
colder than they are, and shall remain so for
192
ESSAY
many hours \ yet these circumstances are found
to occur in substances attractive of dew, when
laid on the surface of the earth, in a still and
serene night, and are in perfect agreement
with the doctrine of heat, now universally ad-
mitted to be just.
To render this more easy of apprehension,
let a small body which radiates heat freely, and
possesses a temperature, in common with the
atmosphere, higher than 32°, be placed, while
the air is clear and still, on a slow conductor of
heat lying on the surface of a large open plain,
and let a firmament of ice be supposed to exist
at any height in the atmosphere ; the conse-
quence must be, that the small body will, from
its situation, quickly become colder than the
neighbouring air. For, while it radiates its
own heat upwards, it cannot receive a sufficient
quantity from the ice to compensate this loss ;
little also can be conveyed to it from the earth,
as a bad conductor is interposed between them^
and there is no solid, or fluid except the air, to
communicate it laterally either by radiation or
conduction. This small body, therefore, unless
it shall receive from the air, nearly as much
heat as it has emitted, which, considering the
little that can be communicated from one part
of the atmosphere to another, in its present
calm state, must be regarded as impossible, will
ON DEW, &c. 193
become colder than the air, and condense the
watery vapour of the contiguous parts "of it, if
they should contain a sufficient quantity to
admit of this effect. But events similar to these
occur, when dew appears in an open and level
grass field, during a still and serene night. The
upper parts of the grass radiate their heat into
regions of empty space, which consequently
send back no heat in return ; its lower parts,
from the smallness of their conducting power,
transmit little of the earth's heat to the upper
parts, which at the same time receiving only a
small quantity from the atmosphere, and none
from any other lateral body, must remain colder
than the air, and condense into dew its watery
vapour, if this be sufficiently abundant, in re-
spect to the decreased temperature of the grass*.
This subject may be further illustrated by a
reference to what happens in the experiment,
which has been used to prove the reflection of
cold.
In the simplest form of this experiment, a
* I have adopted in this explanation the hypothesis of Mr.
Prevost of Geneva, on the constant radiation of heat by bo-
dies in contact with the atmosphere, even at the time that
they are exposed to the influence of bodies warmer than them-
selves; as it appears to agree perfectly with all the pheno-
mena of the communication of heat, which do not depend
upon conduction. I shall hereafter make frequent use of this
hypothesis.
O «
194 ESSAY
small body, the bulb of a thermometer, possess-
ing the temperature of the atmosphere, is placed
before a larger cold body, rendered equal in
effect to one still larger, by means of a concave
metallic mirror. In this situation, the small
body radiates heat to the larger, without re-
ceiving an equivalent from it, and, in conse-
quence, becomes colder than the air through
which its heat is sent, notwithstanding that it
is continually gaining some heat, both from the
air which surrounds it, and from the walls and
contents of the apartment, in which the experi-
ment is made. Dew, therefore, would as readily
form upon the thermometer in this experiment,
as it would upon one suspended in the open air
at night, under a clear sky, provided that the
two instruments were equally colder than the
atmosphere, and that this was in both cases
equally near to being replete with moisture *.
* The invention of this experiment having been ascribed
a few years ago to Mr. Pictet of Geneva, various English
writers have shown, that it occurs in several much older
foreign authors. But I have not seen any mention made of
its having been also long since known in this country. That
it was so appears from the following extract of a letter,
written by Mr. Oldenburgh to Mr. Boyle in 1665. " I met
the other day in the Astrological Discourse of Sir Christopher
Heydon, with an experiment, which he affirms to have tried
himself, importing, that cold accompanies reflected light, by
employing burning spherical concaves, or parabolical sections,
ON DEW, &c. 195
Regarding now as established, that bodies
situated on or near to the surface of the earth
become, under certain circumstances, colder
than the neighbouring air, by radiating more
heat to the heavens, than they receive in every
way*, I shall in the first place offer a few re-
marks on the extent and use of this occurrence,
and shall afterwards apply the knowledge, of it
to the explanation of several more of the ap-
pearances described in the former part of this
Essay, and of some others, which have not
hitherto been mentioned by me.
Which, he saith, will as sensibly reflect the actual cold of snow
or ice, as they will the heat of the sun." Boyle's Works,
folio, vol. V. p. 345.
* Count Rumford offered the following conjecture, in a
paper printed in the Philosophical Transactions for 1804.
fe The excessive cold which is known to reign, in all seasons,
on the tops of very high mountains, and in the higher regions
of the atmosphere, and the frosts at night, which so fre-
quently take place on the surface of the plains below, in very
clear and still weather, in spring and autumn, seem to indi-
cate, that frigorific rays arrive continually at the surface of
the earth, from every part of the heavens." But he gave no
experiments to prove, that such a communication actually
exists between the heavens and the earth at night. Neither
does it appear from any of his writings which I have seen,
that he ever supposed, that the surface of the earth is more
cooled by these frigorific rays, than the air through which
they pass, or that some solid bodies are more cooled by them
than others.
O 2
196 ESSAY
Radiation of heat by the earth to the heavens
must exist at all times ; but, if the sun be at
some height above the horizon, the degree of
which is hitherto undetermined, and probably
varies according to season, and several other
circumstances, the heat emitted by it to the
earth will overbalance, even in places shaded
from its direct beams, that which the earth
radiates upwards. I suspended at midday, on
the 24th of July, 1813, in the open air over a
grassplat, while the sky was wholly covered
with very dense clouds, and the weather calm,
two delicate thermometers, one of which was
naked, but the other cased in gold paper. At
two observations, having an interval of 10 mi-
nutes between them, the thermometer in the
gilt case was 2° lower than that which was naked.
A white paper case was then drawn over the
gilt one, upon which, after 5 minutes, the
covered instrument was observed to be at the
same height with the naked. The outer white
case having, in the next place, been taken from
the covered thermometer, but that which was
gilt suffered to remain, the two instruments
were in a few minutes found again to differ 2°.
A thermometer on the grassplat was, during
these experiments, higher than the naked in-
strument in the air by 2°, and than that in the
gilt case by 4°. It is evident, therefore, that
ON DEW, &c.
heat radiated by the sun must, on this day,
have been transmitted in considerable quantity
through the thickest clouds ; since not only
was the earth's surface warmer than the air,
but a small body, covered with a substance not
readily admitting the entrance of radiant heat,
was colder than a similar body which was unco-
vered. In like manner, I observed at noon, on
the 2nd of January, 1814, during the prevalence
of a dense fog, a thermometer placed upon
swandown, which was lying upon grass thickly
incrusted with hoarfrost, to be 2° warmer than
the air, and 1° warmer than the grass*.
In a calm and serene night, however, when
consequently little impediment exists to the
escape, by radiation, of the earth's heat to the
heavens, and when no heat can be radiated by
the sun to the place of observation, an immense
degree of cold would occur on the ground, if
the following circumstances did not combine
to lessen it. 1 . The incapacity of all bodies to
prevent, entirely, the passing of heat, by con-
duction, from the earth to substances placed
upon them. Q. The heat radiated to these
* Another fact of the same kind, which occurred at the
same time, is that, although the temperature of the air was
3O°, the hoarfrost on trees rapidly decreased, the solid matter
of the trees intercepting radiant heat, which had penetrated
through 4he fog from the sun, and converting it into heat of
temperature.
198 ESSAY
substances by lateral objects. 3. The heat com-
municated to the same substances by the air.
4. The heat which is evolved, during the con-
densation of the watery vapour of the atmo-
sphere into dew.
The extent of the effect of all these checks
upon the production of cold, by the nightly
radiation of heat from bodies on the surface of
the earth, cannot, in the present state of our
knowledge, be properly estimated ; but facts
show that, notwithstanding their operation, the
cold originating in this source must be often
very considerable.
1. Mr. Wilson once observed a difference of
16°, from this cause, between the temperatures
of snow and of air. In taking the latter tem-
perature, however, he employed a naked ther-
mometer, on which account, in consequence of
what has already been mentioned by me, about
2° are to be added to the 16° noted by him, in
order to obtain the real difference between the
heat of the snow and the air at that time*.
2. If Mr. Wilson, as was formerly said, had
* As bright metals, when suspended in the air, and exposed
to a clear sky on a calm night, become colder than the sur-
rounding atmosphere, a thermometer covered with metalled
paper, and placed in the circumstances which have been just
mentioned, will mark a temperature less than that of the air
near to it. But, as the difference must be small, and as I
know of no way to estimate it accurately, I have hitherto
always neglected to consider it.
ON DEW, &c. 199
laid a thermometer on any downy substance in
contact with the snow, he would, in all proba-
bility, have found a cold indicated by it at least
20° greater than that of the air, as marked by a
naked instrument, and consequently at least 22°
greater than the real cold of the surrounding
atmosphere.
3. Mr. Wilson's place of observation was not
very favourable to the occurrence of a great
cold, from radiation of heat at night, it being
near to a large smoky city, in the immediate
vicinity also, as appears to me from what he
says of it, of one or more considerable buildings,
and in a climate abounding in moisture.
4. None of Mr. Wilson's experiments, in
which a very great degree of cold occurred,
were made within an hour or two after sunset,
during which time, according to my observa-
tion, the most considerable differences between
the temperatures of the air, and of bodies on
the surface of the earth, commonly happen.
If, then, such experiments should be made
in an atmosphere still colder than that, in which
Mr. Wilson made his, on a large plain remote
from any city, and free from objects of every
kind, that are elevated above the ground, and
in a country remarkable for the dry ness of its
air, all which circumstances may be found in
Russia during the winter j a difference of at
ESSAY
least 30° would probably appear, on some still
and serene night, between a small thermometer
placed with its bulb naked*, on the middle, or
leeward side of a stratum of a downy substance,
occupying a space upon a grass field, or bed of
snow, one or two square yards in extent, and a
similar thermometer inclosed in a case of gilt
paper, and suspended in the air a few feet above
the other. Two thermometers, thus placed,
would, I think, be sometimes found even in
this country to differ not much less than 30°.
I have myself never made any such experiments
with a downy substance, which had a surface of
more than a few square inches, or in a very cold
night, when the atmosphere was clear and calm,
and the scene of observation remote from large
masses of building.
But even a cold of 30° appears not to be the
greatest, that can be thought to occur, from the
radiation of heat to the heavens, at night, by
substances on the surface of the earth. For
experiments by Mr. Pictetf, Mr. Sixf, and I
may add by myself, establish that, in exception to
the common rule, the heat of the atmosphere in
clear and calm nights increases with the distance
* The effect would, perhaps, be a little increased, by cover-*
ing the bulb with a very thin layer of lamp-black,
f Essai sur le Feu, c. x.
J Phil. Trans. 1784, and
ON DEW, &c, 201
from the earth. Agreeably to Mr. Six's ex-
periments, the atmosphere at the height of 220
feet is often, upon such nights, 10° warmer than
what it is 7 feet above the ground. If, there-
fore, I am able to show, as I expect I shall be
in the course of a few pages, that the air at the
smaller height becomes colder than that of the
greater, from its vicinity to the surface of the
earth, previously rendered cold by radiating its
heat to the heavens, it will follow, that these
10° must be added to the quantity of cold
already mentioned ; and, consequently, that a
body on the ground may become, at night, at
least 40° colder than the air two or three hun-
dred feet above it, by the radiation of its heat
to a clear sky.
I shall add, with the greatest diffidence, a few
words upon a final cause of the radiation of heat
from the earth at night, and upon some of the
circumstances which modify its action, though
fully conscious of the danger of error, which is
always incurred in the attempt to appreciate
the works of our Creator.
The "heat which is radiated by the sun to the
earth, if suffered to accumulate, would quickly
destroy the present constitution of our globe *.
This evil is prevented by the radiation of heat
* Count Rumford says j ' ' May it not be by the action of
these [frigorific] rays, that our planet is cooled continually,
and enabled to preserve the same mean temperature for ages,
202 ESSAY
by the earth to the heavens, during the night,
when it receives from them little or no heat
in return. But, through the wise economy of
means, which is witnessed in all the operations
of Nature, the prevention of this evil is made
the source of great positive good. For the sur-
face of the earth, having thus become colder
than the neighbouring air, condenses a part of
the watery vapour of the atmosphere into dew,
the utility of which is too manifest to require
my speaking of it. I may remark, however,
that this fluid appears chiefly where it is most
wanted, on herbage, and low plants, avoiding,
in great measure, rocks, bare earth and con-
siderable masses of water*. Its production too,
notwithstanding the immense quantities of heat that are ge-
nerated at its surface, by the continual action of the solar
rays?" Phil. Trans. 1804, p. 181.
* I have no direct observations for the foundation of this
assertion concerning considerable masses of water. But, I
hold it, notwithstanding, to be just ; because, as soon as the
surface of the water is in the least cooled by radiation, the
particles composing it must fall downwards, from their in-
creased gravity, and be replaced by others that are warmer.
The whole mass, therefore, can never, in the course of a
single night, be sufficiently cooled to condense into dew any
great quantity of the watery vapour of the atmosphere. Be-
sides ; I have found, that even a small mass of water, as will
be more particularly mentioned in the last part of this Essay,
sometimes acquires no weight from the reception of dew, in
the space of a whole night favourable to the formation of
that fluid.
ON DEW, &c. 203
by another wise arrangement, tends to prevent
the injury, that might arise from its own cause ;
since the precipitation of water, upon the tender
parts of plants, must lessen the cold in them,
which occasions it. I shall observe in the last
place, that the appearance of dew is not confined
to any one part of the night, but occurs during
its whole course, from means the most simple
and efficacious. For after one part of the air
has deposited its moisture, on the colder surface
of the earth, it is removed, in consequence of
that agitation in the atmosphere which exists
during its stillest states, and gives place to an-
other having its quantity of water undiminished ;
and, again, as the night proceeds, a portion of
air, which had before deposited all the moisture,
which circumstances at that time permitted, is
rendered fit, by the general increase of the cold
of the atmosphere, to give out a fresh parcel,
when it comes anew into contact with the
ground.
I. The first fact, which I shall here attempt
to explain, is the prevention, either wholly or
in part, of cold, from radiation, in substances
on the ground, by the interposition of any solid
body between them and the sky. This evi-
dently appears to arise in the following manner.
The lower body radiates its heat upwards, as if
no other intervened between it and the sky j
204 ESSAY
but the loss, which it hence suffers, is more or
less compensated by what is radiated to it, from
the body above, the under-surface of which
possesses always the same, or very nearly the
same temperature as the air. In this way there-
fore, is to be accounted for the warmth of the
substances, which were sheltered from the sky
by the raised board, the pasteboard roof, and
the hollow cylinders of earth and pasteboard.
In these examples, the interposed substances
cannot be supposed to have remitted more heat
than they received. But in situations where
large masses of bare solid matter exist, which
are warmer than the atmosphere, from the heat
of the preceding day or other causes, a greater
heat will be received by the exposed body, than
what is radiated by itself. For example, it
seems certain to me, that the houses, surround-
ing LincolnVInn Fields, had an influence upon
my thermometers, during my experiments there
at night, beyond what arose from their merely
returning a quantity of heat, equivalent to that,
which they received from the surface of the
garden. It is not, however, absolutely requi-
site, that a body should be itself exposed to the
sky on a clear and calm night, in order to be-
come colder than the atmosphere ; exposure to
the influence of another body, so situated, is
sufficient for the production of a slight degree
ON DEW, &c.
of this effect. Thus, I have always found wool
attached to the underside of my raised board,
on such a night, to be a little colder than the
air ; and it has appeared to me a sufficient rea-
son for the fact, that the wool in this situation
was, in some degree, exposed to the influence
of grass, which had become considerably colder
than the atmosphere, by radiating its heat to
the sky.
II. No direct experiments can be made to
ascertain the manner, in which clouds prevent,
or occasion to be small, the appearance of a
cold at night, upon the surface of the earth,
greater than that of the atmosphere ; but it
may, I think, be firmly concluded, from what
has been said in the preceding article, that they
produce this effect, almost entirely, by radiating
heat to the earth, in return for that which they
intercept in its progress from the earth towards
the heavens. For although, upon the sky be-
coming suddenly cloudy during a calm night,
a naked thermometer, suspended in the air,
commonly rises 2 or 3 degrees, little of this rise
is to be attributed to the heat evolved by the
condensation of watery vapour in the atmo-
sphere, as was supposed by Mr. Wilson * ; since,
in consequence of the ceasing of that part of
the cold indicated by the thermometer, which
* Edin. Phil. Trans. I. 15;.
206
was owing to its own radiation to a clear sky,
the temperature of the atmosphere may seem
to increase 2°, or more, notwithstanding that it
has received no real addition. Besides; the
heat which is extricated by the condensation of
vapour, during the formation of a cloud, must
soon be dissipated ; whereas the effect of greatly
lessening, or preventing altogether, the appear-
ance of a superior cold on the earth to that
of the air, will be produced by a cloudy skyr
during the whole of a long night.
Dense clouds, near the earth, must possess
the same heat as the lower atmosphere, and will
therefore send to the earth, as much, or nearly
as much heat as they receive from it by radia-
tion. But similarly dense clouds, if very high,
though they equally intercept the communica-
tion of the earth with the sky, yet being, from
their elevated situation, colder than the earth,
will radiate to it less heat than they receive from
it, and may, consequently, admit of bodies on
its surface becoming several degrees colder than
the air. In the first part of this Essay, an ex-
ample was given of a body on the ground be-
coming at night 5° colder than the air, though
the whole sky was thickly covered with high
clouds*.
* Mr. Prevost of Geneva, in his work on Radiant Heat,
p. 382, has already in this way, conjecturally, accounted for
the effect of clouds, in diminishing, at night, the cold of the
ON DEW, &c. 207
Islands, and parts of continents close to the
sea, being, by their situation, subject to a cloudy
sky, will, from the smaller quantity of heat lost
by them through radiation to the heavens at
night, in addition to the reasons commonly
assigned, be less cold in winter, than countries
considerably distant from any ocean.
III. Fogs, like clouds, will arrest heat, which
is radiated upwards by the earth, and, if they
be very dense, and of considerable perpendi-
cular extent, may remit to it as much as they
receive. Accordingly, Mr. Wilson found no
atmosphere, and of the surface of the earth ; but he seems
not to have known, that their effect on the temperature of
the latter is much greater than that which they produce upon
the air. My explanation of this influence of clouds, on the
temperature of the surface of the earth, during the night, is
a direct consequence from the facts, which I had observed
respecting the prevention of cold on the ground from radia-
tion, by the interposition of solid bodies between it and the
heavens, and occurred to me in 1812. Mr. Prevost's work,
indeed, was published in IS 9; but I did not see it before
the summer of 1813 ; when it was lent to me by his relation
Dr. Marcet of London, who at the same time said, that he
believed there was no other copy of it in Great Britain, ex-
cept one, which had been sent by himself to Edinburgh.
Note to second edition.'] I did not know, until after the
first edition of this Essay was printed, that Mr. Prevost had
published his opinion on the effect of clouds in preventing
the occurrence of cold at night in the atmosphere, and upon
the surface of the earth, as early as 1792, in a work entitled
6 Recherches sur la Chaleur.'
208 ESSAY
difference at night, in very foggy weather, be-
tween the temperature of the surface of snow,
and that of the air. Several observations by
myself tend to confirm that of Mr. Wilson. An
instance, however, as was formerly said, oc-
curred to me of a difference at night of 9°
between the temperatures of grass crusted over
with hoarfrost, and of air, during a very dense
fog. A fact, remarked by Mr. Leslie, respect-
ing fogs, serves to explain this apparent ano-
maly. For it was found by that philosopher*,
from experiments made with his photometer,
that in mists and low fogs the diminution of the
sun's heat is small, when compared with what
occurs, when the sky is obscured by a dense
body of clouds ; and it will, I presume, be
readily granted, that the same state of the at-
mosphere, which allows the heat of the sun to
pass copiously, will also give a ready transit to
heat radiated by the earth. Now there are
several reasons for believing, that the fog, dur-
ing which grass was 9° colder than the air, did
not ascend far above the ground. 1. The baro-
meter had been falling for some days before,
and it is a matter of common observation, that
great fogs seldom occur, except it be high.
2. On the day preceding the observation, the
* On Heat and Moisture, p. 57.
ON DE\V, &c. 209
air, after having been extremely foggy for
nearly a week, had become clear enough to
allow the sun's being distinctly seen during the
whole of the afternoon, though there was still
a sufficient obscurity in the lowermost parts of
the atmosphere, to obstruct considerably the
view of objects on the ground and very near to
it. 3. On the day following the observation,
the fog was again much less ; on the next it
disappeared, and was succeeded by snow. It
is to be mentioned likewise, that on the even-
ing in question the state of the grass, which
was the subject of experiment, was unusually
favourable to the production of cold ; since,
contrary to general experience, it was as cold
as swandown. If, then, the latter substance,
from the much greater regularity of the ap-
pearances exhibited by it, be taken as the
standard, by which the occurrences of different
nights are to be compared together, it will
follow, that the fog of which I am speaking,
though it did not prevent, must have lessened,
the production of cold from radiation. For,
on the preceding evening, when there was little
fog, the atmosphere being equally still on both,
the difference between swandown and the air
was 12°; and on another, a fortnight after, the
difference at the same place of observation, be-
tween thermometers in the same situations, was
210 ESSAY
, the air being now free from fog. If the
atmosphere had been as still on this, as on the
former evenings, a greater difference would
doubtless have been seen. I conclude, there-
fore, that fogs do not in any instance furnish a
real exception to the general rule, that what-
ever exists in the atmosphere, capable of stop-
ping or impeding the passage of radiant heat,
will prevent or lessen the appearance at night
of a cold on the surface of the earth, greater
than that of the neighbouring air.
It follows also, from what has been said in
this article, that the water deposited upon the
earth, during a fog at night, may sometimes
be derived from two different sources, one of
which is a precipitation of moisture from a con-
siderable part of the atmosphere, in consequence
of its general cold \ the other, a real formation
of dew, from the condensation, by means of the
superficial cold of the ground, of the moisture
of that portion of the air, which comes in con-
tact with it. In such a state of things, all
bodies will become moist, but those especially,
which most readily attract dew in clear weather*.
I have had no opportunity, however, of trying
* The moisture observed at night by Musschenbroek in
Holland, and called by him dew, appears to me to have been
of this kind. See this Essay, p. 127.
ON DEW, &c.
211
this conclusion by the test of observation, since
it occurred to me.
IV. When bodies become cold from radia-
tion, the degree of effect observed must de-
pend, not only on their radiating power, but in
part also on the greater or less ease, with which
they can derive heat, by conduction, from
warmer substances in contact with them. Thus
grass, on a clear and still night, was constantly
colder, sometimes very much colder, than the
gravel walk, though a small quantity of sand,
placed upon grass, was always nearly as cold as
this substance. In this case, the difference in
temperature, between the gravel walk and sand,
evidently depended on the different quantities
of heat, which they received from the parts
beneath. A like reason is to be given for dew
appearing in greater quantity on shavings of
wood, than on the same substance in a more
dense and compact form ; and for filamentous
and downy substances becoming colder than all
others, even than lampblack, which is placed
by Mr., Leslie, at the head of the best solid
radiators of heat. For the lampblack exposed
by me, being about 2 lines in depth, possessed,
in consequence, a fund of internal heat, which
would more readily pass to its cold surface,
than the heat of the lower parts of the downy
substances would to their upper surface.
212" ESSAY
This subject is illustrated by the following
experiment. On a dewy evening, I depressed
into soft garden mould a drinking glass, having
a thick flat bottom, until its brim was upon a
level with the surrounding earth, and at the
same time placed a similar vessel, with its
cavity also towards the sky, on the surface of
the mould. In the morning, the inside of the
depressed glass was entirely dry, while that
of the other was dewed. I then applied the
bulb of a small thermometer to the inside of
the bottom of each vessel, on which I found the
heat of that part of the depressed one to be 56°,
but of the same part of that which stood on
the mould only 49 J°. At this time the tem-
perature of the air was 53°. The cause, there-
fore, was evident, both of the wetness of the
first vessel, arid of the dryness of the second.
From this source also is to be derived the
reason, why the prominent parts of various
bodies were observed by Mr. Wilson to be
crusted with hoarfrost, while their more re-
tired and massy parts were free from it *.
V. Bodies, exposed in a clear night to the
sky, must radiate as much heat to it during the
prevalence of wind, as they would do if the air
were altogether still. But in the former case,
* Paper in Phil. Trans. 178O.
ON DEW, &c. 213
little or no cold will be observed upon them
above that of the atmosphere, as the frequent
application of warm air must quickly return a
heat equal, or nearly so, to that which they had
lost by radiation. A slight agitation of the air
is sufficient to produce some effect of this
kind ; though, as has already been said, such
an agitation, when the air is very pregnant with
moisture, will render greater the quantity of
dew, one requisite for a considerable produc-
tion of this fluid being more increased by it,
than another is diminished.
VI. A small body, as a thermometer, sus-
pended in the air, will even in the calmest night
exhibit but little cold from radiation, since it is
continually exposed to the application of fresh
parcels of warmer air, both from the progres-
sive motion of this fluid, and from the down-
ward motion produced in it by the superior
gravity of such portions, as have been cooled
by contact with the suspended body. On. the
other hand, a thermometer upon a board, raised
above the earth and possessing a surface of
several square yards, will have its cold from
radiation much less diminished than the former,
as it is exposed to no loss from a downward
motion of the air, and as the air, which ap-
proaches it horizontally, must, almost always,
have had its temperature previously lowered,
214 ESSAY
by passing over another part of the board. The
reason then of the lee side of the raised board
being often colder than the windward is ob-
vious.
VII. There is a remark by Theophrastus *,
which has been confirmed by other writers, that
the hurtful effects of cold occur chiefly in hollow
places. If this be restricted to what happens
on serene and calm nights, and it does not, I
believe, hold true in any other circumstances,
two reasons from different sources are to be
assigned for it. The first is, that the air being
stiller in such a situation, than in any other, the
cold, from radiation, in the bodies which it
contains, will be less diminished by renewed
applications of warmer air ; the second, that
from the longer continuance of the same air in
contact with the ground, in depressed places
than in others, less dew will be deposited, and
therefore less heat extricated during its forma-
tion. It will be seen in the last part of this
Essay, that, in the East Indies-, depressions in
the earth are artificially made, for the purpose
of increasing the cold, which appears in serene
nights. On this subject, however, it is to be
observed, that if the depressed or hollow places
be deep, in proportion to their horizontal ex-
tent, a contrary effect must follow ; as a case
* Lib. v. c. xvi.
ON DEW, &c. 215
•will occur more or less similar to that which
existed in some experiments formerly related
by me, in which a small portion of grass was
surrounded by a hollow cylinder.
VIII. An observation closely connected with
the preceding, namely that, in clear and still
nights, frosts are less severe upon hills, than in
neighbouring plains*, has excited more atten-
tion, chiefly from its contradicting what is com^
monly regarded an established fact, that the
cold of the atmosphere always increases with
the distance from the earth. This inferior cold
of hills is evidently a circumstance of the same
kind, with that ascertained 'by Mr. Pictet and
Mr. Six, respecting the increasing warmth, in
clear and calm nights at all seasons of the year,
of the different strata of the atmosphere, in pro-
portion as these are more elevated above the
earth. As the greater cold of the lower air is
the less complicated fact, I shall attempt to ex-
plain it in the first place. Mr. Pictet, indeed,
furnishes an explanation himself, by ascribing it
to the evaporation of moisture from the ground.
But to show that this is not just, it need only
be mentioned, that the appearance never occurs
in any considerable degree, except upon such
* Theophrastus also remarks, that it freezes less on hills
than on plains, but without mentioning, that this happens
only on calm and serene nights. Lib. v. c< xx.
216 ESSAY
nights as are attended with some dew, and that
its great degrees are commonly attended with a
copious formation of that fluid ; since it cannot
be thought, that the same stratum of air will
deposit moisture on the ground, from an in-
sufficiency of heat, at the very time it is re-
ceiving moisture from the ground, in the state
of pellucid vapour, as this presupposes, that it
is not yet replete with water.
Our atmosphere has been very generally re-
garded, as incapable of being heated directly
by the rays of the sun, principally because these
give no heat to any particular portion of it, in
which they are brought to a focus. I do not
know, whether this experiment was ever made
with all the accuracy of which it is susceptible ;
but, granting that it has been thus made, my
opinion is, notwithstanding, that no reliance
can be placed in it. For as air, if heated at all
by concentrated sunbeams, must be heated by
them in a very slight degree, during the time
that their focus may be looked upon as sta-
tionary, otherwise the present question would
not have arisen, it is necessary for conducting
the experiment properly, that, during the whole
of it, the same individual small portion of air
shall constantly receive that focus ; but this,
for various manifest reasons, cannot possibly
happen. Viewing, therefore, the argument
ON t)EW, &c. 217
founded upon this experiment as without force,
I shall now offer several considerations, which
seem to prove, that air is actually heated by the
sunbeams, which enter it.
1 . Air both reflects and refracts light, and all
other bodies, as far as I know, acquire heat,
while they act thus on the light of the sun.
2. Air suffocates or absorbs the sun's light,
which it cannot be supposed to do, without in-
creasing in temperature.
3. If air, considered as an uniform fluid,
were even incapable of gaming heat directly
from the sun's rays, heat would be communi-
cated by them to it, through the intervention
of the innumerable particles of solid matter,
which the trivial experiment of receiving a sun-
beam into a darkened room shows to be present
in the atmosphere. Should it be said, that this
appearance may occur only in the neighbour-
hood of the earth, from the accidental admix-
ture of solid matter raised from its surface by
winds, or in any. other way, the answer is, that,
as my inquiry is concerning the existence of a
certain condition of the atmosphere, it matters
not how this originates. Nothing more can be
demanded, than that it should always be found,
which I believe to be the case ; since, if I can
trust my memory with respect to what took
place many years ago, I should say, that such
218 ESSAY
particles are to be seen, by means of the sun's
light, in the air over the middle of the Atlantic
ocean. These particles then must receive heat
from the sunbeams, which impinge upon them,
and this they will communicate to the con-
tiguous pellucid air.
4. Unless it be admitted, that the atmosphere
is capable of intercepting part of the heat,
which is radiated into it by the sun, and of
converting this into heat of temperature, I deem
it impossible to find a sufficient reason, for the
great warmth which exists, after a long calm,
in air incumbent upon the Atlantic and Pacific
oceans, at the distance of a thousand miles or
more from any considerable body of land. It
cannot be derived from the neighbouring water,
since this is colder than the lower atmosphere ;
and no one will suppose it to be the same heat,
which the air had acquired from the last con-
tinent it had passed over, many days before.
But, if even this were supposed, another difc
ficulty would remain to be removed, which is,
that, during the whole of the calm, the air is
cooled every night, and again becomes warm
in the day *.
* One reason is hence apparent for the great coldness of
the high regions of the atmosphere ; since the air in them
must be less fit, than that of the lower strata, to arrest heat
which is radiated into it.
ON DEW, &c. 219
Should what has been said be thought suf-
ficient to establish, that the air arrests part of
the sun's heat, which is radiated into it bound
up with light, two consequences must also be
allowed. The first is, that air will exert a
greater power of the same kind upon heat ra-
diated into it without light, since the sun's heat
passes instantaneously through many bodies,
which refuse a similar way to heat radiated by
terrestrial substances ; the other, that air must
be as capable of becoming cold by radiating its
own heat*, as of becoming warm from heat
radiated into it, as these two properties are uni-
formly observed to exist together, and to be
proportional to each other. The truth of the
latter conclusion may also be inferred from this
fact, that in still and calm weather the heat of
the air, a few feet above the earth, will some-
times decrease, even in this country, 18 or 20
degrees between sunset and sunrise, though no
change of wind has in the meantime occurred >
for the inconsiderable conducting power, which
air is now known to possess, will permit only
a small part of this diminution to arise from
heat passing, by means of that power, from the
* Mr. Prevost says : " On peut supposer queles molecules
«3e 1'air rayonnent." Du Calorique Rayonnant, p. 24.
220 ESSAY
atmosphere to the colder earth. Mr. Leslie *, in-
deed, ascribes this effect to the descent of cold
air from the higher regions of the atmosphere ;
but if this were just, a less cold ought to be
found, on a clear and still night, in the lower
than in the higher strata, which is contrary to
the uniform results of numerous experiments
by Mr. Pictet and Mr. Six. Winds too, which
produce such a mixture, always lessen the noc-
turnal decrease of temperature in the lowermost
part of the atmosphere.
Having thus shown, that air is capable, both
of absorbing heat, which is radiated into it, and
of radiating heat, which had before formed a
part of its temperature, I proceed to apply the
knowledge of these facts, to the explanation of
the phenomenon observed by Mr. Pictet and
Mr. Six.
This phenomenon occurs on those nights
only, which permit bodies, on the surface of
the earth, to become cold by radiating their
heat to the heavens. On other nights, when
.bodies, thus situated, were not colder than the
air, I have observed the atmosphere, within the
limits of 9 feet from the ground, the boundary
of my own experiments, to decrease a little in
* On Heat and Moisture, p. 11, and 132.
ON DEW, &c. 221
temperature, as the distance from the earth
increased. Mr. Six likewise found, that, on
cloudy nights, the air was sometimes colder
220 feet above the ground, than at the distance
of 9 feet from it. When, therefore, the earth
has become colder, from radiation, than the
neighbouring air, in consequence of the latter
having, by reason of its small radiating power,
emitted a less proportion of its heat to the
heavens, the warmer air must radiate a part of
its heat to the earth, without receiving a full
compensation, and will therefore become colder,
than it otherwise would have been. In propor-
tion too as the air is nearer to the earth, must
the cold of the former from this cause be the
greater. My own conception of this matter is
facilitated*, by contemplating the occurrence
of an opposite effect, when the earth is warmer
than the air. Let it be supposed then, that
while the earth, in this state, radiates upwards
a quantity of heat, a foot in depth of the in-
cumbent air is capable of stopping a 1000th of
what it hence receives, and of converting it
into heat of temperature. The consequence
must be that the next foot, from receiving only
999 parts of what had been emitted by the
earth, will not be so much heated as the first
* The same facility is afforded by considering cold as a
body.
222 ESSAY
foot, though it should absorb the same pro-
. portional quantity of what enters it. In this
way, every successive foot will acquire a less
quantity of heat than the preceding, and a
state of the atmosphere be produced, like to
that which is actually observed in a calm and
sunny day. In the day, however, the pheno-
mena, from the heating of air by rays from the
earth, are somewhat confused by the warmed
portions rising upwards, and mixing with what
is colder ; whereas, at night, the air, which has
been cooled by radiating heat to the earth, is
rendered, by an increase of gravity, the more
fit to retain its low position. I have here, for
the sake of simplifying the argument, taken no
notice of the cooling of any considerable mass
of the air, in consequence of the actual contact
of its lowermost stratum with the earth, or by
the conduction of the temperature of one por-
tion of it to another. But, in a calm state of
the atmosphere, these effects must be incon-
siderable, though it appears to me impossible,
in the present state of our knowledge, to de-
termine them with any precision.
According to the view, which has been given
by me of this subject, the heat of the air, in a
clear and calm night, ought to increase, within
the limits of the phenomenon, in some de-
creasing geometrical ratio, as the atmosphere
ON DEW, &c. 223
ascends ; and this conclusion is so far con-
firmed, by the observations of Mr. Pictet and
Mr. Six taken together, that the increase of
temperature is found to be greater in a given
space very near to the earth, than in an equal
space more remote from it.
To return to the immediate object of this
article, the fact is certain, whatever may be
thought of my explanation of it, that, in every
clear and still night, the air near to the earth is
colder than that which is more distant from it,
to the height at least of 220 feet, this being the
greatest to which Mr. Six's experiments relate.
If then a hill be supposed to rise from a plain,
to the height of 220 feet, having upon its
summit a small flat surface covered with grass ;
and if the atmosphere, during a calm and serene
night, be admitted to be 10° warmer there, than
it is near the surface of the low ground, which
is a less difference, according to the observa-
tions of Mr. Six, than what sometimes occurs
in such circumstances, it is manifest, that,
should both the grass upon the hill, and that
upon the plain, acquire a cold of 10° by radia-
tion, the former will, notwithstanding, be 10°
warmer than the latter.
But the equality here supposed to be in the
cold acquired by grass, in two such situations,
224 ESSAY
can seldom exist. For, according to an ob-
servation made by Aristotle*, and since fre-
quently repeated, the air of high places is much
more agitated, than that upon low ground.
The frequent renewal, therefore, from this
cause, of the air in contact with the grass on
the hill, will prevent it from ever becoming
much colder than the general mass of the atmo-
sphere, at the same height. Consequently, any
diminution in this way of the 10° of cold, for-
merly supposed to occur there from radiation,
must be added to the difference of temperature
in the grass in the two situations.
What has hitherto been said refers only to
the occurrences on the very summit of the hill.
With respect to its sides, these can be only a
little colder than the atmosphere upon a level
with them, even in its calmest state. For, in
fche first place, they do not enjoy the full aspect
of the sky; and, in the second, the air, whicli
is cooled by contact with them, will, from its
increased gravity, slide down their declivity,
and thus make room for the application of new
and warm parcels to the same surface. The
motion too, thus excited in the air, near to the
sides of the hill, must occasion a motion in that
* Meteor, lib. 1. c. x.
ON DEW, &c. 225
Upon the summit, which may, in some measure,
account for the last-mentioned observation of
Aristotle, as far as relates to what happens in a
clear night.
The height of the hill, in this example, has
been supposed to be small, to make it accord
with that of the stations, whose temperatures
were compared by Mr. Six with the heat of the
air near the ground. But observations of the
same kind will apply to hills of much greater
elevation. For granting, first, that the air at
the height of 220 feet is never more than 10°
colder, than that near to the earth, which is not
probable, and is indeed contradicted by some
of Mr. Six's observations ; and again, that the
increase of the air's heat, in a calm and serene
night, ceases precisely at the greatest height,
to which Mr. Six carried his observations, which
is also improbable ; still a reduction, to the ex-
tent of 10°, in the temperature of the air near
to the earth, will render the cold of this low
portion of the atmosphere greater than that of
any other portion, which is not more than 2500
or 3000 feet above the former, if the estimate
be just, which makes a declension in the heat
of the atmosphere of 1° for every 250, or 300
feet of its height, when no counteracting cause
exists.
The remarks, however, which have been
Q
226 ESSAY
offered on the greater warmth of hills at night,
in a certain state of weather, are strictly ap-
plicable to those only, which are insulated, and
of inconsiderable lateral extent ; and it is upon
such chiefly, if not solely, that this phenomenon
has been observed. The superiority of the cold
of a low plain, from radiation, over that of a
wide expanse of hilly ground, will, for obvious
reasons, be less; and no superiority of this kind
will probably exist in the former situation, when
the high ground is not only extensive, but flat
on the top, forming what is called a table-land ;
unless indeed, which seems to be actually the
case, the air of such an elevated country should
be commonly more agitated, than that of lower
places equally level.
An explanation may be now easily given of
an observation by Mr. Jefferson of Virginia *,
which, however, had also been made by Ari-
stotle t, and Plutarch J, that dew is much less
copious on hills, than it is upon plains. For
allowing, at first, the surface of the ground to
be in both situations equally colder than the air
which is near to it ; still, as the production of
dew must be in proportion to the whole de-
pression of the temperature of the air which
* Notes on Virginia, p. 132. f Meteor. Lib. 1 . c. x.
J De Primo Frigido.
ON DEW, &c. 227
furnishes it, below what its heat had been in
the preceding day, and as one part of this de-
pression, the general cooling of the atmosphere,
is much more considerable on the plain than on
the hill, moisture must necessarily be deposited
more copiously in the former than in the latter
place. If the greater agitation of the atmo-
sphere, and the less quantity of moisture, during
clear weather, in its higher region than in the
lower, be added, it may readily be inferred,
that dew shall sometimes be altogether wanting
on a hill, though abundant on a plain at its
foot, agreeably to what has been actually ob-
served by Mr. Jefferson.
IX. The leaves of trees often remain dry
throughout the night, while those of grass are
covered with dew. As this is a similar fact to
the smallness of dew on hills, I shall in account-
ing for it do little more, than enumerate the
circumstances on which it depends.
1. The atmosphere is several degrees warmer
near the upper parts of trees on dewy nights,
than close to the ground. 2. The air in the
higher situation is more agitated, than that in
the lower. 3. The air at a little distance from
the ground, from being nearer to one of its
sources of moisture, will on a calm evening
contain more of it, than that which surrounds
the leaves of elevated trees. 4. Only the leaves
228 ESSAY
of the very tops of trees are fully exposed to the
sky. 5. The declension of the leaves fron* an
horizontal position will occasion the air, which
has been cooled by them, to slide quickly away,
and be succeeded by warmer parcels. 6. The
length of the branches of the trees, the tender-
ness of their twigs, and the pliancy of the foot-
stalks of their leaves, will cause in the leaves an
almost perpetual motion, even in states of air
that may be denominated calm. I have hence
frequently heard, during the stillness of night,
a rustling noise in the trees, which formed one
of the boundaries of the ordinary place of my
observations, while the air below seemed with-
out motion.
Nearly in the same manner is to be ex-
plained, why shrubs and bushes also receive
dew more readily than lofty trees.
X. Bright metals, exposed to a clear sky in a
calm night, will be less dewed on their upper
surface than other solid bodies ; since of all
bodies they will, in such a situation, lose the
smallest quantity of heat by radiation to the
heavens, at the same time that they are capable
of receiving, by conduction, at least as much
heat as any others from the atmosphere, and
more than any others from the warmer solid
substances, which they happen to touch.
If the exposed pieces of metal be not very
ON DEW, &e. 229
small, another reason will contribute somewhat
to their being later and less dewed than other
solid substances. For, in consequence of their
great conducting power, dew cannot form upon
them, unless their whole mass be sufficiently
cold to condense the watery vapour of the
atmosphere ; while the same fluid will appear
on a bad conductor of heat, though the parts a
very little beneath the surface are wanner than
the air *.
From the same ready passage of heat from
one part of a metal to another, a metallic plate
suspended, horizontally, in the air several feet
above the ground, will be found dewed on its
lower side, if the upper has become so ; while
the lower surface of other bodies, more attrac-
tive of dew, but worse conductors of heat, are
without dew in a similar situation.
A metal placed at night in the air, near to
the ground, is, for the most part, sufficiently
cold to condense, on its underside, the vapour
which arises from the warmer earth ; though
* I hence think it probable, that dew will sometimes form
on the bulb of a thermometer, before the mercury in it is
cooled below the temperature of the air. It seems certain
to me, also, that dew may appear upon substances, which,
from the thinness of the layer of matter their cold is con-
fined to, will produce little or no sensible effect upon a ther-
mometer that is applied to them.
230 ESSAY
its upper surface may be dry, from possessing
the same, or almost the same temperature, as
the atmosphere near to it.
As the temperature of metals is never much
below that of the neighbouring air, a slight
diminution of their cold from radiation will
often occasion them to evaporate the dew,
which they had previously acquired, though
other substances, which had been more cooled
by radiation, are still attracting dew. For a
like reason, a metal, which has been purposely
wetted, will often become dry at night, while
other substances are becoming moist.
A substance highly attractive of dew, such
as wool, if laid upon a metal, will derive heat
from it, and will therefore acquire less dew,
than an equal portion of the same substance
laid upon grass.
A large metallic plate will be less readily
dewed while lying on grass, than if it were
placed in the air, though only a few inches
above the grass ; because, in the former situa-
tion, it receives freely, by means of its great con-
ducting power, heat from the earth ; whereas,
when placed in the air, it powerfully resists by
another property, possessed in a great degree
by bright metals, the entrance of heat radiated
towards it by the grass beneath. Besides ; the
grass under the metal possesses now less heat,
ON DEW, &c. 231
tlian when this substance was in contact with
it, partly from having a small oblique aspect of
the sky, and partly from receiving air, which
has been cooled by passing over other grass
fully exposed to the heavens.
When a piece of metal, having closely applied
to its under surface a substance of some thick-
ness, which attracts dew powerfully, and, there-
fore, imbibes readily heat that is radiated to it,
is exposed to the sky at night, the heat supplied
by the attached substance, both from its own
original store, and from what it has acquired
through the radiation of the ground to it during
the exposure, will enable this piece to resist
longer, than a bare piece, the formation of dew,
or even than another piece, which has only a thin
coat of matter considerably attractive of dew
attached to its underside. The experiment
with the wooden cross, covered with gilt paper,
affords an example of the latter fact.
A very small metallic plate, suspended in the
air, is less readily dewed than a large one,
similarly situated, as it receives, in proportion
to its size, more heat from the atmosphere.
On the other hand, a very small plate laid
upon grass, rendered cold by radiation, will be
sooner dewed than a larger one in the same
situation, from presenting a greater propor-
tional circumference to the surrounding grass,
and therefore losing more quickly its heat by
232 ESSAY
conduction. It will be also sooner dewed than
another very small plate suspended in the air;
since the latter, like other small bodies similarly
placed, must be continually acquiring more heat
than the former, in the manner described above
in this Essay *.
A piece of metal, applied to different por-
tions of cold grass in succession, will sooner be-
come cold itself, than another piece, which is
suffered to remain constantly upon one portion
of the same grass, and will in consequence be
sooner dewed.
If the bare side of a piece of metalled paper
be exposed to a clear and calm sky at night, it
will become cold, by radiation, and receive, by
conduction, the heat of the inferior metallic
surface ; whence, if this surface be afterwards
made the upper one, it will sooner acquire dew
than a similar metallic surface, which has been
exposed to the sky during the whole of the ex-
periment.
When a metal covers, in part only, the upper
surface of a piece of glass, the uncovered por-
tion of the glass quickly becomes cold by radia-
tion, on exposure to $ serene sky in a still night,
and then, by deriving to itself a part of the heat
of the metal, occasions this body to be more
readily dewed, than if the whole of the exposed
* Page 213.
ON DEW, &c. 233
surface had been metallic. In this experiment,
the outer edge of the metallic surface, from
being nearest to the colder glass, will be the
first and the most dewed, while the parts of the
uncovered glass, which are contiguous to the
warmer metal, will be the last and the least
dewed, of their respective substances.
A piece of glass, covered on one side with a
metal, being placed on grass, with this side
down, its upper surface attracts dew as readily
as if no metal were attached to it ; since the
metal, in this situation, has no power to lessen
the radiation of heat from the upper surface of
the glass. I conclude, however, from general
principles, for I have not made the trial, that if
the same piece of glass, having its metallic side
still undermost, were raised in the air a little
above the grass, it would be more readily dewed
on its upper surface, than if it had been without
a metallic coating on the lower, as this coating
must resist the introduction of heat radiated by
the warmer grass, and thus preserve nearly un-
diminished the cold acquired, from radiation
of heat to the sky, by the bare upper surface.
The preceding remarks apply to the whole
class of metals ; but the discoveries of Mr.
Leslie, respecting the difference in the capa-
cities of these bodies to radiate heat, furnish
an explanation of a diversity among themselves,
in regard to attraction for dew, which was
234 ESSAY
noted in the foregoing part of this Essay. Gold,
silver, copper and tin, are there said to resist
the formation of dew more strongly, than other
substances of the same class ; but these metals,
according to Mr. Leslie, radiate heat the most
sparingly. -On the other hand, lead, iron and
steel, which, according to the same author,
radiate heat more copiously than the former
metals, were found by me to acquire dew more
readily. I do not know, if the radiating power
of platina has been ascertained by direct ex-
periments ; but, as its conducting power is
small, its radiation must be great, since these
qualities exist always in opposite degrees in the
same substance ; and I have accordingly ob-
served it to be dewed, while the four first-men-
tioned metals were dry. I am ignorant both of
the radiating and the conducting power of zinc,
as determined by ordinary experiments ; but I
infer, from its being more easily dewed than
gold 'or silver, that it radiates heat more co-
piously than they do ; unless indeed, the pieces
which I used, from having had their surfaces
roughened by friction with sand, which was
employed to brighten them, had acquired a
radiating power, greater than that possessed by
polished pieces, agreeably to the results of some
of Mr. Leslie's experiments *.
* I once intended to subjoin here an explanation of some
very curious observations by Mr, Benedict Prevost on
ON DEW, &c. '235
XI. Thinking it probable, that black bodies
might radiate more heat to the sky, at night,
than white, I placed upon grass, on five different
evenings, equal parcels of black and white wool.
On four of the succeeding mornings, the black
wool was found to have acquired a little more
dew than the white ; whence I inferred that it
had, in consequence of its colour, radiated a
little more heat. But I afterwards remarked,
that the white wool was somewhat coarser
than the black ; which circumstance alone was
which were published, first in the 44th volume of the French
Annals of Chemistry, and afterwards by Mr. Peter Prevost
of Geneva, in his Essay on Radiant Heat ; but fearing to be
very tedious, I have since given up the design. I will say,
however, that, if to what is now generally known on the
different modes, in which heat is communicated from one
body to another, be added the two following circumstances 5
that substances become colder, by radiation, than the air,
before they attract dew ; and that bright metals, when ex-
posed to a clear sky at night, become colder than the air
much less readily than other bodies j the whole of the ap-
pearances observed by Mr. Prevost may be easily accounted
for.
Note to second edition."] I found, shortly after the publica-
tion of the former edition of this Essay, that the learned Dr.
Young had, several years before, in his great work on Na-
tural Philosophy, employed the principle of the radiation of
heat to account for several of the facts observed by Mr. B.
Prevost. On the subject of Dr. Young's explanation, I have
spoken somewhat fully in the 28th number of Dr. Thomson's
Annals of Philosophy.
236 ESSAY
sufficient to occasion a difference in their quan-
tities of moisture. Another night, I laid on
the raised board a piece of pasteboard covered
with white paper, and close to this a second
piece similar to the former in every respect,
except that it was covered with paper black-
ened with ink. At daylight, I saw hoarfrost
upon both pieces ; but the black seemed to have
a greater quantity than the white. A doubt,
however, afterwards arose upon the accuracy
of this experiment likewise ; for, as the light
was faint, when I viewed the two surfaces, the
quantity of hoarfrost, though equal on both,
might have appeared greater on the black, than
on the white, from the contrast of its colour
with that of the former surface. But trials of
this kind, as Mr. Leslie* has observed, never
afford firm conclusions ; since a black body
must always differ from a white in one or more
chemical properties, and this difference may of
itself be competent to produce a diversity in
their powers to radiate heat.
With the view to render the subject less com-
plicated, I have hitherto treated of dew, as if it
* On Heat, p. 95.
ON DEW, &c. 237
were altogether derived from watery vapour pre-
viously diffused through the atmosphere ; this
appearing to me to be by far its most considera-
ble source, and none of my conclusions of any
importance being liable to be affected, even by
the establishment of a contrary opinion. Other
writers, however, have regarded dew as being
entirely the product of vapour emitted, during
the night, by the earth and plants upon it.
According to this theory, dew is said to rise.
The first trace, which I have found of the
opinion, that dew rises from the earth at night,
occurs in the History of the Academy of
Sciences for 1687. It is mentioned there briefly
and obscurely, and was, probably, shortly for-
gotten ; for Gersten, who advanced it anew in
1733, held himself to be its author. Muss-
chenbroek and Dufay embraced it immediately
after Gersten ; but the former soon admitted,
that dew sometimes falls. As far as I have
learned, no writer upon dew has since ascribed
its total production to vapour, emitted by the
earth at night, except Mr. Webster of New
England*. But this opinion is frequently ad-
vanced in conversation by persons, not much
accustomed to philosophical pursuits, chiefly, I
think, because it contradicts a popular belief.
* Mem. of American Acad. vol. III.
238 ESSAY
The only argument used by the French aca-
demicians, in support of their opinion, is, if I
understand it rightly, that as much dew is ob-
served under an inverted glass-bell, as in any
other situation. But admitting, for a moment,
this to be true, they would not thus prove,
that the ground is the only source of that
fluid.
Gersten was led to think, that dew rises from
the earth, by often finding grass, and low shrubs,
moistened with it, while trees were dry. Re-
specting this fact, I shall add nothing to what
I have lately said upon it. But his chief argu-
ment is derived from another fact related in the
first part of this Essay, which is, that a plate of
metal, laid upon bare earth on a dewy night,
will remain dry on its upper surface, while it
becomes moist on the lower. This also is easily
explicable by what has already been mentioned
by me. For the lower side of the metal, in con-
sequence of the upper being in contact with the
air and being exposed to a clear sky, is colder
than the earth a little below the surface, and
therefore condenses the vapour, which strikes
against its bottom ; while the upper side, from
being frequently warmer, and never more than
a little colder than the air, is for the most part
unable to condense the watery vapour of the
ON DEW, &c. 239
atmosphere*. Gersten, moreover, describes
several appearances himself, which refute his
opinion. He mentions, for example, that the
higher parts of shrubs are more dewed than the
lower j that metallic plates, placed horizontally
in the air, are as much dewed on their superior,
as on their inferior surfaces ; and that convex
and cylindrical bodies, suspended in the air, the
latter having a position parallel to the horizon,
are dewed only on their upper parts.
The principal reason given by Dufay for the
rising of dew is, that it appears more early on
bodies near to the earth, than on those which
are at a, greater height. But this fact readily
admits of an explanation on other grounds, that
have already been mentioned. 1. The lower
air, on a clear and calm evening, is colder than
the upper, and will, therefore, be sooner in a
condition to deposit a part of its moisture.
2. It is less liable to agitation than the upper.
3. It contains more moisture than the upper,
from receiving the last which has risen from
the earth, in addition to what it had previously
* I have, in like manner, observed, on a cloudy night, a
piece of glass, laid over an earthen pan containing water and
placed upon the ground, to be wet on its lower side, while
the upper was dry j the glass being, in this situation, suf-
ficiently cold to condense the vapour of water heated by the
earth, but not enough so to condense the watery vapour of
the atmosphere,
240 ESSAY
possessed, in common with other parts of the
atmosphere. Dufay attempted to strengthen
his argument, by exposing, on three dewy
nights, similar substances at different heights
from the ground, expecting that the lower
would always acquire more moisture than the
upper ; but, upon all the nights, some one of
the lower substances acquired less moisture,
than some one of the higher.
Mr. Webster has advanced no new fact in
favour of the opinion, of which I am speaking.
Enough having been said to prove, that dew
is not entirely the product of vapour rising from
the earth at night, I shall next show, that it
often occurs, when this cause can have little or
no operation.
1. It appears from Hasselquist and Bruce,
that in Egypt, shortly before the rising of the
Nile, and consequently when the ground there
is in its driest state, dew becomes exceedingly
plentiful, though little or none had formed be-
fore, while the earth was somewhat less dry.
The cause evidently is, as was formerly men-
tioned, the moist air brought from the Me-
diterranean by the north wind, which then
prevails.
52. Mr. Webster, speaking of hoarfrost, which
he properly regards as frozen dew, candidly
says, though it overthrows his opinion : " This
ON DEW, &c. 241
frost appears, when the surface of the earth is
sealed with frost, and of course the vapour of
which it is formed, cannot at the time, perspire
from the earth."
3. I have myself, at all seasons of the year,
frequently observed wool, upon the middle of
the raised board, and therefore out of the way
of vapour rising from the ground, to acquire
more dew, than wool laid upon the grassplat.
4. The bodies, that condense the rising va-
pour, must necessarily be colder than it ; but,
as they are likewise, according to the opinion
under view, of the same temperature with the
air surrounding them, this also should condense
the rising vapour. Dew, therefore, should never
appear in any considerable quantity, without
being accompanied with fog or mist. Now I
can assert after much attention to this point,
that the formation of the most abundant dew
is consistent with a pellucid state of the atmo-
sphere. Hasselquist makes a similar observa-
tion, with regard to Egypt j where, during the
season remarkable for the most profuse dews,
" the nights," he says, " are as resplendent with
stars, in the midst of summer, as the lightest
and clearest winter nights in the north."
But, although these facts prove, that copious
dews may occur with little or no contribution
by vapour immediately rising from the earth, it
242 ESSAY
must yet be admitted, that some of the moisture,
which forms during clear and still weather, on
bodies situated upon or near its surface, is in
most cases to be attributed to this source ;
since, in my experiments, substances on the
raised board became much later moist than
others on the ground, though equally cold with
them. The quantity from this cause, however,
can never be great. For in the first place, until
the air be cooled by the substances attractive
of dew, with which it comes in contact, below
its point of repletion with moisture, it will be
always in a condition to take up that which has
been deposited upon grass, or other low bodies,
by warm vapour emitted by the earth ; just as
the moisture formed upon a mirror by our breath
is, in temperate weather, almost immediately
carried away by the surrounding air. Accord-
ingly ; I have sometimes, in serene and still
weather, observed dew to appear sparingly
upon grass in the shade, several hours before
sunset, and to continue in nearly the same
quantity till about sunset, when it would in-
crease considerably, at the time that the same
fluid began to show itself on the raised board.
In the second place; though bodies situated
on the ground, after they have been made suf-
ficiently cold, by radiation, to condense the
vapour of the atmosphere, will be able to retain
ON DEW, &c. 243
the moisture, which they acquire by condensing
the vapour of the earth ; yet, before this hap-
pens, the rising vapour must have been greatly
diminished, by the surface of the ground having
become much colder. These considerations,
added to the fact, that substances on the raised
board attracted rather more dew, throughout
the night, than similar substances lying on the
grass, warrant me to conclude, that on nights,
favourable to the production of dew, only a very
small part of what occurs is owing to vapour
rising from the earth ; though I am acquainted
with no means of determining the proportion
of this part to the whole. On the other hand,
however, in a cloudy night, all the dew that
appears upon grass may sometimes be attributed
to a condensation of the earth's vapour ; since
I have several times, in such nights, remarked
the raised board to be dry, while the grass was
moist. These nights were calm, and evapora-
tion from the grass consequently not copious.
When evaporation on cloudy nights was assisted
by wind, dew has never, as was mentioned in
the first Part of this Essay, been any where
observed by me*.
* The interval between the first appearance of dew in the
afternoon on grass, in shaded places, and sunset, was formerly
said by me., on the authority, however, of only a few ob-
servations, to be considerably greater, than that between
244 ESSAY
Agreeably to another opinion, the dew found
upon growing vegetables is the condensed va-
pour of the very plants, on which it appears.
But this also seems to me erroneous for several
reasons. 1. Dew forms as copiously upon dead
as upon living vegetable substances, 2. The
transpired humour of plants will be carried
away by the air which passes over them, when
they are not sufficiently cold to condense the
watery vapour contained in it ; unless, which is
almost never the case if mist does not already
exist, the general mass of the atmosphere be
incapable of receiving moisture in a pellucid
form. Accordingly, on cloudy nights, when
the air, consequently, can never be cooled more
than a little below the point of repletion with
sunrise, and the ceasing of the formation of dew upon grass
in the morning. These observations were made on spots
exposed during the greater part of the day to the sun. In
such places, the heat acquired, from the sun, by the upper-
most layer of earth, will be longer retained, than that ac-
quired by the grass, which will, therefore, be sufficiently
cool, soon after the heat of the day has declined, to condense
a part of the vapour then copiously rising from the earth;
whereas in the morning, both less vapour will rise, the sur-
face of the earth having now lost a great part of its heat, and
a less proportion of that which does rise will be condensed
by the grass, as the temperature of this body now more
nearly approaches that of the ground, from first receiving
the heat of the sun reflected from the atmosphere and other
substances.
ON DEW, &c. 245
moisture, by bodies in contact with it, dew is
never observed upon any plants, that are ele-
vated a few feet above the ground. 3. If a
plant has become, by radiating its heat to the
heavens, so cold, as to be enabled to bring the
air in contact with it below the point of reple-
tion with moisture, that which forms upon it,
from its own transpiration, will not then, in-
deed, evaporate. But other moisture will, at
the same time, be communicated to it by the
atmosphere ; and when the difference in the
copiousness of these two sources is considered,
it may, I think, be safely concluded, that almost
the whole of the dew, which will afterwards
form on the plant, must be derived from the
air ; more especially when the coldness of a
clear night, and the general inactivity of plants
in the absence of light, both lessening their
transpiration, are taken into account.
An experiment, however, has been appealed
to in proof, that the dew of plants actually does
originate from fluid transpired by them ; that
namely, in which a plant, shut up in an air-tight
case, becomes covered with moisture. But this
experiment, if attentively examined, will be
found to have little weight. First ; the in-
closed plant, being exempt from the cold, which
its own radiation would have produced in its na-
tural situation, on a dewy night, will transpire
246 ESSAY ON DEW, &c.
a greater quantity of fluid, than a similar plant
exposed at the same time to the open air.
Again ; the small quantity of air, contained in
the case, must soon be replete with moisture,
after which, the whole of what is further
emitted by the plant will necessarily assume
the form of a fluid, whatever may be the con-
dition of the external atmosphere ; whereas,
during even the clearest night, only a part of
the smaller quantity of moisture, emitted by
the exposed plant, will be condensed on its
surface. In the last place ; notwithstanding
the circumstances, which favour the appearance
of moisture upon inclosed plants from their own
transpiration, still the quantity observed on
them is said to be, for I have made no experi-
ment myself respecting this matter, much less
considerable, than what is seen upon plants of
the same kind, exposed to the air for the same
time, during a calm and serene night.
PART III.
OF SEVERAL APPEARANCES CON-
NECTED WITH DEW.
1HERE are various occurrences in nature,
which seem to me strictly allied to dew, though
their relation to it be not always at first sight
perceivable. The statement and explanation
of several of these will form the concluding part
of the present Essay.
I. I observed one morning, in winter, that
the insides of the panes of glass in the windows
of my bedchamber were all of them moist, but
that those, which had been covered by an inside
shutter, during the night, were much more so,
than others which had been uncovered. Sup-
posing, that this diversity of appearance de-
pended upon a difference of temperature, I ap-
plied the naked bulbs of two delicate thermo-
meters to a covered and uncovered pane 5 on
which I found, that the former was 3° colder
than the latter. The air of the chamber, though
no fire was kept in it, was at this time 11 5°
248 ESSAY
warmer than that without. Similar experiments
were made on many other mornings, the results
of which were ; that, when the warmth of the
internal air exceeded that of the external, from
8° to 1 8°, the temperature of the covered panes
would be from 1° to 5° less than that of the
uncovered ; that the covered were sometimes
dewed, while the uncovered were dry ; that at
other times both were free from moisture ; that
the outsides of the covered and uncovered panes
had similar differences with respect to heat,
though not so great as those of the inner sur-
faces ; and that no variation in the quantity of
these differences was occasioned by the wea-
ther's being cloudy or fair, provided the heat
of the internal air exceeded that of the ex-
ternal equally in both of those states of the
atmosphere.
The remote reason of these differences did
not immediately present itself. I soon, how-
ever, saw, that the closed shutter shielded the
glass, which it covered, from the heat, that was
radiated to the windows by the walls and furni-
ture of the room, and thus kept it nearer to the
temperature of the external air, than those parts
could be, which, from being uncovered, received
the heat emitted to them by the bodies just
mentioned.
In making these experiments, I seldom
ON DEW, &c. 249
observed the inside of any pane to be more than
a little damped, though it might be from 8° to
12° colder than the general mass of the air in
the room ; while, in the open air, I had often
found a great dew to form on substances, only
3° or ,4° colder than the atmosphere. This at
first surprised me ; but the cause now seems
plain. The air of the chamber had once been
a portion of the external atmosphere, and had
afterwards been heated, when it could receive
little accession to its original moisture. It con-
sequently required being cooled considerably,
before it was even brought back to its former
nearness to repletion with water ; whereas the
whole external air is commonly, at night, nearly
replete with moisture, and therefore readily
precipitates dew, on bodies only a little colder
than itself.
When the air of a room is warmer than the
external atmosphere, the effect of an outside
shutter, on the temperature of the glass of the
window, will be directly opposite to what has
been just stated; since it must prevent the
radiation, into the atmosphere, of the heat of
the chamber transmitted through the glass.
II. Count Rumford* appears to have rightly
conjectured, that the inhabitants of certain hot
* Phil. Trans. 1804. p. 182.
250 ESSAY
countries, who sleep at nights on the tops of
their houses, are cooled, during this exposure,
by the radiation of their heat to the sky ; or,
according to his manner of expression, by re.
ceiving frigorific rays from the heavens. An-
other fact of this kind seems to be the greater
chill, which we often experience upon passing,
at night, from the cover of a house into the
open air, than might have been expected from
the cold of the external atmosphere. The
cause, indeed, is said to be the quickness of
transition from one situation to another. But,
if this were the whole reason, an equal chill
would be felt in the day, when the difference,
in point of heat, between the internal and ex-
ternal air, was the same as at night, which is
not the case. Besides ; if I can trust my own
observation, the feeling of cold from this cause
is more remarkable in a clear than in a cloudy
night, and in the country, than in towns. The
following appears to be the manner, in which
these things are chiefly to be explained.
During the day, our bodies while in the open
air, although not immediately exposed to the
sun's rays, are yet constantly deriving heat from
them, by means of the reflection of the atmo-
sphere. This heat, though it produces little
change on the temperature of the air which
it traverses, affords us some compensation for
ON DEW, &c. 251
what we radiate to the heavens. At night also,
if the sky be overcast, some compensation will
be made to us, both in towns and in the
country, though in a less degree than during
the day, as the clouds will remit towards the
earth no inconsiderable quantity of heat. But
on a clear night, in an open part of the country,
nothing almost can be returned to us from
above, in place of the heat which we radiate
upwards. In towns, however, some compensa-
tion will be afforded, even on the clearest
nights, for the heat which we lose in the open
air, by that which .is radiated to us by the sur-
rounding buildings.
To our loss of heat by radiation, at times
that we derive little compensation from the ra-
diation of other bodies, is probably to be at-
tributed a great part of the hurtful effects of
the night air. Descartes * says that these are
not owing to dew, as was the common opinion
of his cotemporaries, but to the descent of cer-
tain noxious vapours, which having been ex-
haled from the earth during the heat of the
day, are afterwards condensed by the cold of a
serene night. The effects in question certainly
cannot be occasioned by dew, since that fluid
* Meteorolog. c. vi.
252 ESSAY
does not form upon a healthy human body, in
temperate climates ; but they may, notwith-
standing, arise from the same cause, that pro-
duces dew on those substances, which do not,
like the human body, possess the power of ge-
nerating heat, for the supply of what they lose
by radiation or any other means.
III. I had often, in the pride of half know-
ledge, smiled at the means frequently employed
by gardeners, to protect tender plants from
cold, as it appeared to me impossible, that a
thin mat, or any such flimsy substance, could
prevent them from attaining the temperature
of the atmosphere, by which alone I thought
them liable to be injured. But, when I had
learned, that bodies on the surface of the
earth become, during a still and serene night,
colder than the atmosphere, by radiating their
heat to the heavens, I perceived immediately a
just reason for the practice, which I had before
deemed useless. Being desirous, however, of
acquiring some precise information on this sub-
ject, I fixed, perpendicularly, in the earth of a
grassplat, 4 small sticks, and over their upper
extremities, which were 6 inches above the
grass, and formed the corners of a square, the
sides of which were 2 feet long, drew tightly
a very thin cambric handkerchief. In this
ON DEW, &c. 253
disposition of things, therefore, nothing existed
to prevent the free passage of air from the ex-
posed grass, to that which was sheltered, except
the 4 small sticks, and there was no substance
to radiate heat downwards to the latter grass,
except the cambric handkerchief. The tem-
perature of the grass, which was thus shielded
from the sky, was upon many nights afterwards
examined by me, and was always found higher
than that of neighbouring grass which was un-
covered, if this was colder than the air. When
the difference in temperature, between the air
several feet above ihe ground and the un-
sheltered grass, did not exceed 5°, the sheltered
grass was about as warm as the air. If that
difference, however, exceeded 5°, the air was
found to be somewhat warmer than the shel-
tered grass. Thus, upon one night, when fully
exposed grass was 11° colder than the air, the
latter was 3° warmer than the sheltered grass j
and the same difference existed on another
night, when the air was 14° warmer than the
exposed grass. One reason for this difference,
no doubt, was that the air, which passed from
the exposed grass, by which it had been very
much cooled, to that under the handkerchief,
had deprived the latter of part of its heatj an-
other, that the handkerchief, from being made
colder than the atmosphere by the radiation of
254 ESSAY
its upper surface to the heavens, would remit
somewhat less heat to the grass beneath, than
what it received from that substance. But still,
as the sheltered grass, notwithstanding these
drawbacks, was upon one night, as may be col-
lected from the preceding relation, 8°, and upon
another 1 1°, warmer than grass fully exposed to
the sky, a sufficient reason was now obtained
for the utility of a very slight shelter to plants,
in averting or lessening injury from cold, on a
still and serene night.
In the next place ; in order to learn whether
any difference would arise from placing the
sheltering substance at a much greater distance
from the ground, I had 4 slender posts driven
perpendicularly into the soil of a grass field,
and had them so disposed in other respects,
that their upper ends were 6 feet above the
surface, and formed the angular points of a
square having sides 8 feet in length. Lastly ;
over the tops of the posts was thrown an old
ship flag of a very loose texture. Concerning
the experiments made by means of this arrange-
ment of things, I shall only say, that they led
to the conclusion, as far as the events of dif-
ferent nights could rightly be compared, that
the higher shelter had the same efficacy with
the lower, in preventing the occurrence of a
cold upon the ground, in a clear night, greater
ON DEW, &c. 255
than that of the atmosphere, provided the
oblique aspect of the sky was equally excluded
from the spots on which my thermometers were
laid.
On the other hand ; a difference in tem-
perature, of some magnitude, was always ob-
served on still and serene nights, between bodies
sheltered from the sky by substances touching
them, and similar bodies, which were sheltered
by a substance a little above them. I found,
for example, upon one night, that the warmth
of grass, sheltered by a cambric handkerchief
raised a few inches in the air, was 5° greater,
than that of a neighbouring piece of grass,
which was sheltered by a similar handkerchief
actually in contact with it. On another night,
the difference between the temperatures of two
portions of grass, shielded in the same manner,
as the two above-mentioned, from the influence
of the sky, was 4°. Possibly, experience has
long ago taught gardeners the superior advan-
tage of defending tender vegetables, from the
cold of clear and calm nights, by means of sub-
stances not directly touching them ; though I
do not recollect ever having seen any con-
trivance for keeping mats, or such like bodies,
at a distance from the plants, which they were
meant to protect.
256 ESSAY
Walls, I believe, as far as warmth is con-
cerned, are regarded as useful, during a cold
night, to the plants which touch them, or are
near to them, only in two ways ; first, by the
mechanical shelter which they afford against
cold winds, and secondly, by giving out the
heat which they had acquired during the day.
It appearing to me, however, that, on clear and
calm nights, those on which plants frequently
receive much injury from cold, walls must be
beneficial in a third way, namely, by prevent-
ing, in part, the loss of heat, which they would
sustain from radiation, if they were fully ex-
posed to the sky, the following experiment was
made for the purpose of determining the just-
ness of this opinion.
A cambric handkerchief having been placed,
by means of two upright sticks, perpendicularly
to a grassplat, and at right angles to the course
of the air, a thermometer was laid upon the
grass close to the lower edge of the handker-
chief, on its windward side. The thermometer
thus situated was several nights compared with
another lying on the same grassplat, but on a
part of it fully exposed to the sky. On two of
these nights, the air being clear and calm, the
grass close to the handkerchief was found to be
4° warmer, than the fully exposed grass. On a
ON DEW, &c. 257
third, the difference was 6°. An analogous
fact is mentioned by Gersten, who says, that an
horizontal surface is more abundantly dewed,
than one which is perpendicular to the ground.
IV. The covering of snow, which countries
in high latitudes enjoy during the winter, has
been very commonly thought to be beneficial to
vegetable substances on the surface of the earth,
as far as their temperature is concerned, solely
by protecting them from the cold of the atmo-
sphere. But were this supposition just, the
advantage of the covering would be greatly cir-
cumscribed ; since the upper parts of trees and
of tall shrubs are still exposed to the influence
of the air. Another reason, however, is fur-
nished for its usefulness, by what has been said
in this Essay ; which is, that it prevents the oc-
currence of the cold, which bodies on the earth
acquire, in addition to that of the atmosphere,
by the radiation of their heat to the heavens
during still and clear nights. The cause, in-
deed, of this additional cold, does not constantly
operate ; but its presence, during only a few
hours, might effectually destroy plants, which
now pass unhurt through the winter. Again ;
as things are, while low vegetable productions
are prevented, by their covering of snow, from
becoming colder than the atmosphere in con-
sequence of their own radiation, the parts of
258 ESSAY
trees and tall shrubs, which rise above the snow,
are little affected by cold from this cause. For
their outermost twigs, now that they are de-
stitute of leaves, are much smaller than the
thermometers suspended by me in the air, which
in this situation very seldom became more than
Q° colder than the atmosphere. The larger
branches too, which, if fully exposed to the sky,
would become colder than the extreme parts,
are, in a great degree, sheltered by them ; and,
in the last place, the trunks are sheltered both
by the smaller and the larger parts, not to men-
tion that the trunks must derive heat, by con-
duction through the roots, from the earth kept
warm by the snow *.
In a similar way is partly to be explained the
manner, in which a layer of earth or straw pre-
serves vegetable matters in our own fields, from
the injurious effects of cold in winter.
V. The bare mention of the subject of this
article will be apt to excite ridicule, it being an
attempt to show, in what way the exposure of
animal substances to the moon?s light promotes
their putrefaction. I have no certain knowledge,
* It may be remarked here, however, that a thick covering
of snow, while it renders the surface of the earth warmer
that it would otherwise be, must occasion the lower atmo-
sphere to be colder, by preventing the passage of the heat of
the ground to the air, either by radiation or conduction.
ON DEW, &c. 259
that such an opinion prevails any where, at pre-
sent, except in the^West Indies; but I con-
clude, from various circumstances, that it exists
-also in Africa, and that it was carried thence
by negro slaves to America. It was entertained,
however, by persons of considerable rank and
intelligence among the ancients ; for Pliny *
affirms it to be true, and Plutarch, after making
it a subject of discussion in one of his Sym-
posia f, admits it to be well founded.
As moonbeams communicate no sensible heat
to the bodies, on which they fall, it seems im-
possible, that they can directly promote putre-
faction. But still a reason, for ascribing such
a power to them, may be derived from their
being received by animal substances, at the
very time that a real, but generally unnoticed,
cause of putrefaction, in warm climates, (and
it is in these alone the opinion I am treating of
has ever prevailed) is taking place, which ceases
to act, as soon as the moon's light is excluded.
The nights, on which a steady moonshine
occurs, must necessarily be clear ; and nights,
which are clear, are almost always calmt. A
* Lib. ii. §. civ. f Lib. iii. Prob. x.
I Mr. De Luc has remarked, that clouds frequently dis-
appear soon after sunset. Idees sur la Meteorologie, II. £8.
I have often observed this myself, and at the same time
another fact of which he takes no notice ; namely, that the
s 2
260 ESSAY
moonshiny night, therefore, is one, on which
dew forms plentifully; hence the expressions
' roscida* and * rorifera luna' employed by Virgil
and Statius ; and hence also an opinion, held,
as appears from Plutarch, even by philosophers
among the ancients, that the moon communi-
cates moisture to the bodies, which are exposed
to its light *.
Animal substances are among those, which
acquire dew in the greatest quantity. To do
this, indeed, they must previously become colder
than the atmosphere ; but, having acquired the
moisture of dew, in addition to their own, they
will, on the following day, be in that condition,
which is known, by experience, to favour putre-
faction most powerfully in hot climates*
The immediate cause assigned here, for the
quick putrefaction of animal substances, which
have been exposed to the moon's rays in a hot
country, is the same as that given by Pliny and
Plutarch ; but they attributed the origin of this
atmosphere is then calmer than it had been before sunset.
This calmness of the air very commonly, if not always, pre-
cedes the dissipation of th6 clouds.
* Akin to this opinion of the ancients respecting the
humefying quality of the moon, is one, which has been held,
by modern writers as well as ancient, upon that planet's
being a cause of cold to the bodies, which receive its rays .
though I know of no author who has taken notice of this
affinity.
ON DEW, &c. 261
immediate cause, the additional moisture, to
the peculiar humefying quality, which they
supposed that luminary to possess. This false
theory has, probably, contributed to discredit,
with the moderns, the circumstance which it
was employed to explain.
VI. The last fact, of which I shall treat in
this Essay, is the formation of ice, during the
night in Bengal, while the temperature of the
air is above 32°.
I have seen only two original descriptions
of this process, both of which are contained in
the Philosophical Transactions ; the first, by Sir
Robert Barker, in the 65th volume ; the other
in the 83rd, by Mr. Williams.
According to the method followed by Sir R.
Barker's ice-maker, square excavations, 2 feet
deep, and 30 wide, having been formed in a
large open plain, their bottoms are covered with
sugar-cane, or stems of Indian corn, dried, to
the thickness of 8 inches or 1 foot. On this
layer, are afterwards placed, in rows, near to
each other, small, unglazed earthen pans, i ,of
an inch thick, and 1 inch and -^ deep, filled
with boiled soft water. The pans are sufficiently
porous to allow their outer surface to appear
moist, after water has been poured into them.
Sir R. Barker adds ; that the nights, the most
favourable for the production of ice, are those,
262 ESSAY
which are the calmest and most serene, and on
which very little dew appears after midnight ;
that clouds and frequent changes of wind, are
certain preventives of its formation; and that,
although ice is thus very readily procured by
art in Bengal, during the winter, it scarcely
ever occurs there naturally.
The process described by Mr. Williams must,
from its extent, 300 persons being employed in
it, have been carried on for profit, and would,
consequently, be conducted in the most eco-
nomical manner. A piece of ground, nearly
level, containing about 4 acres, was divided
into square plats, from 4 to 5 feet wide, which
were surrounded by little mounds of earth, 4
inches high. In these inclosures, previously
filled with dry straw, or sugar-cane haum, were
placed as many broad, shallow, unglazed earthen
pans, containing unboiled pump water, as they
could hold. The air was generally very still,
when much ice was formed ; wind prevented
its formation altogether. In the morning, be-
tween 5 and 6 h., at which time alone, Mr. Wil-
liams made his observations, a thermometer,
with its bulb naked, placed on the straw, amidst
the freezing vessels, was never found by him
lower than 35° ; and he has observed ice, when
a thermometer so placed was 42°. Another ther^
mometer, suspended 5j feet above the ground,
ON DEW, &c. 263
was commonly 4° higher than that among the
pans. It is possible, therefore, that Mr. Wil-
liams may have seen ice, a little before 'sunrise,
when the temperature of the air was 46°. But
granting this were the fact, it would not hence
follow, that the ice was formed, while the air
possessed that heat. For, although the air is
generally held to be in all countries colder
about sunrise, than at any other time, I know
from my own observations, that this is not
always the case in England ; and similar ex-
ceptions may occur in Bengal. Sir H. Davy
has said, in his Elements of Chemistry, that ice
will form in Bengal, when the temperature of
the air is not below 50° ; but he has given no
authority for this assertion.
The formation of ice, in the circumstances
which have been just mentioned, was attributed
by Sir R. Barker altogether, and by Mr. Wil-
liams in great measure, to cold produced by
evaporation. Sir R. Barker's opinion has since
been adopted by some of our most distinguished
writers on Natural Philosophy, as Watson,
Thompson, Young, Davy and Leslie, apparently,
however, without their having fully considered
it, as I shall now attempt to show.
1. It is necessary for the complete success of
the process, that the air should be very still ;
wind, which so greatly promotes evaporation,
264 ESSAY
prevents the freezing altogether. Sir R. Barker
admits, that the excavations in the earth are
made to increase the stillness of the air in con-
tact with the water in the pans ; but, with the
view to explain the utility of this stillness he
supposes, in opposition to all experience, that
water kept very quiet freezes more readily,
when other circumstances are the same, than if
it were a little agitated.
2. No proof is given, that evaporation from
the pans actually does occur, at the times which
are the most favourable for the appearance of
ice. At any rate it cannot be considerable ;
since, agreeably to what is mentioned by Sir R.
Barker, dew forms in a greater or less degree
during the whole of the nights, the most pro-
ductive of ice ; and it is not to be thought, as
was said upon a former occasion, that one por-
tion of air will be depositing moisture, from
possessing a superabundance of it, while another
in the immediate vicinity is receiving moisture
in great quantity, in the state of pellucid vapour;
as the latter fact can exist only when the air
is far removed from a state of repletion with
water.
3. If evaporation produced the cold under
consideration, the wetting of the straw or other
matter, upon which the pans are placed, would
tend to increase it; and, accordingly, Sir H.
ON DEW, &c. 265
Davy affirms this to be the case. But Mr. Wil-
liams, who must here be regarded as the better
authority, says, that it is necessary to the suc-
cess of the process that the straw be dry ; in
proof of which he mentions, that when the
straw becomes wet, by accident, it is replaced ;
and that when he purposely wetted it in some
of the inclosures, the formation of ice there
was always prevented. The reasons are clear.
The water, by softening the straw, renders it
easily compressible by the weight of the pans,
and at the same time fills up what would other-
wise be vacant spaces among its parts. The
straw, therefore, in this condensed state, must
afford a ready passage to heat from the earth to
the pans, the hindrance of which is allowed by
every person to be the use of it, in this process,
when dry. Again ; the moisture, which passes
through the straw to the earth it covers, will
rise afterwards in the form of vapour, having
the same temperature with the warm ground,
and will communicate heat to the pans. In the
last place ; a part of this vapour will be con-
densed into water by the pans, in consequence
of which heat must be extricated.
4. It is mentioned both by Sir R. Barker and
Mr. Williams, in support of their opinions, that
the pans, when new, are so porous, that they
readily permit water to transude them 5 and
266 ESSAY
that old pans, which permit this in a less de-
gree, are less fit for the making of ice. But the
argument, which is hence derived by them, is
completely refuted by a fact related by Mr.
Williams himself; for he says, that the pans are
greased before they are used, to prevent the
adhesion of the ice to their sides ; since, if this
purpose be answered, the water can never be in
contact with the pans, and therefore can never
pass through them.
The real reason of the less fitness of old pans
for the making of ice is perhaps the following.
The production of the cold, which occurs in
this process, must take place in the water;
since neither the straw upon which the pans
are placed, nor the air above them, was ever
found by Mr. Williams of so low a temperature
as 32°. l' Whatever, therefore, obstructs the
passage of heat from the straw to the water,
must favour the freezing of the latter. But
this will be less effectually done by an old than
by a new pan, as the density of the former is
greater, from the grease forced into it by rub-
bing, and from the slime and sand that will
enter with the water into its pores, when these
are not entirely closed by the grease ; which
must often happen, as the smearing is performed
only once in three or four days. The difference,
however, in effect betwixt old and new pans
ON DEW, &c. 267
must be very small ; as it does not appear that
the old are ever laid aside on account of their
unfitness.
In a like way may be explained, without the
aid of cold produced by the evaporation of
moisture from the outsides of the pans, another
fact mentioned by Mr. Williams, that ice was
often found by him in those vessels, while water
contained in a china plate, surrounded by them,
had none ; since the thin and dense substance
of the plate must have transmitted more readily,
than the thick and rare substance of the pans,
the heat of the straw to the water.
5. In accounting for the making of ice in
Bengal, it is requisite to show, not only how the
first film is produced, but also, in what way the
thickness of this film is afterwards increased.
If evaporation be the cause of this increase, it
follows, that a plate of ice in the night-time,
and in the stillest air, both unfavourable to that
process, must yet emit as much moisture, as is
necessary for the production of a cold, accord-
ing to Mr. Williams, of at least 14°, and accord-
ing to Sir H. Davy of at least 18°; a conclu-
sion, as it appears to me, of itself sufficient to
destroy the credit of the theory, from which it
is drawn.
While attending to this subject, I became
desirous of acquiring some knowledge of the
268 ESSAY
degree of cold, which might be produced by
evaporation from water contained in a shallow
vessel. With this view, I placed on a feather-
bed, situated between the door and window of
a room in my house in London, two china plates,
into one of which as much water was poured,
as covered its bottom to the depth of ^ of an
inch. The other plate was kept dry. The
bulb of a small thermometer being then applied
to the inside of the bottom of each plate, I ob-
served upon many days, in various seasons of
the year, the difference between these instru-
ments while the door and window were open.
I found, in consequence, that when the tem-
perature of the air in the room was 75°, the
highest at which any experiment was made, the
thermometer in the plate, containing water, was
between 6 and 7 degrees lower than the one in
the dry plate ; that the difference between these
thermometers diminished gradually as the air
became colder ; and that when the temperature
of the air was 40°, the lowest for which I have
any observation, the difference was only lj°.
At 32°, therefore, it would have been very
small, and at a few degrees below 32 it would
probably have vanished. This supposition agrees
with an observation made by Mr. Wilson of
Glasgow, who found, that no cold was pro-
duced by evaporation from snow possessing a
ON DEW, &c. 269
temperature of 27°, though the air in the im-
mediate neighbourhood was purposely much
agitated by him.
The conclusions here given by me, respecting
the cold produced by the evaporation of water,
were drawn from experiments made in the day,
while the sky was clear, the air very calm, and
the temperature of the atmosphere stationary.
At night, and during a cloudy day, the differ-
ences were less. On the other hand, if there
was any perceptible motion in the air, they
were greater. They were also greater if the
heat of the atmosphere was increasing; but
less, if this was decreasing.
Having thus, I think, placed beyond doubt,
that the formation of ice in Bengal is not occa-
sioned by evaporation, I shall now state several
reasons, which have induced me to believe, that
it depends upon the radiation of heat to the
heavens.
1. This cause not only exists, but exists in a
degree, sufficient for the production of the effect,
which I attribute to it. For Mr. Wilson found
the surface of snow, during a clear and calm
night, to be 16° colder than air £ feet above it,
the temperature of the latter being taken by a
naked thermometer ; whereas the greatest heat
of the atmosphere ever observed by Mr, Wil-
liams, at the distance of 5j feet from the ground,
270 ESSAY
during the time that he supposed ice to be form-
ing, was only 14° higher than the freezing point
of water. I need say nothing of the difference
of 18° related by Sir H. Davy, as he does not
speak from his own observation, and as he gives
no authority for what he advances ; though even
this difference is considerably less, than what I
have attempted to show must sometimes occur,
from the radiation of heat at night, between
the temperature of air, a few feet above the
earth, and that of bodies placed on its surface.
It is to be mentioned here also, that, accord-
ing to Mr. Leslie*, the power of water to ra-
diate heat, exceeds, perhaps, that of all other
substances.
2. Ice is chiefly formed in Bengal during the
clearest and calmest nights ; and it is on such
nights that the greatest cold, from radiation, is
observed on the surface of the earth. In Sir R.
Barker's more refined mode of conducting the
process, an unusual stillness of the air, in con-
tact with the water to be frozen, is procured,
by placing the pans containing it a little below
the level of the ground ; in which situation, it
was formerly shown, bodies must grow colder
from radiation to the heavens at night, than in
any other.
* On Heat, p. 80.
ON DEW, &c. 271
3. The cold, by means of which ice is pro-
duced in Bengal, appears, as I think may be
inferred from what is said by Sir R. Barker, in
its greatest degree, like cold from radiation
in other substances, on those still and serene
nights, during which little dew is deposited by
the atmosphere.
4. Clouds and wind prevent the formation of
ice in Bengal ; and the same states of the atmo-
sphere either prevent, or considerably diminish,
the occurrence of cold from the radiation of
heat at night by bodies on the ground.
I shall close this subject, by giving some ac-
count of a few attempts to procure the freezing
of water at night, in this country, by exposing
it to air of a temperature, higher than that of
32°. These were made by me in 1812, at my
usual place of experiment, which was formerly
stated to be not well adapted for the appearance
of a great cold from radiation, and on nights
not among the most favourable to such an un-
dertaking, even of those which occur in this
country. It is proper also to mention, that I
was then less able to conduct such experiments,
and to make use of them, than I afterwards be-
came, from a longer attention to similar objects.
'
272 ESSAY
EXPERIMENT 1st.
With a view to imitate the method of making
ice described by Sir R. Barker, I had a pit dug,
on the evening of the 3rd of May, in the middle
of the garden so often spoken of, 4^ feet long,
3 wide and 2 deep. It consequently had the
same depth as the excavations mentioned by
that gentleman, but was considerably less in its
other dimensions. Clean dry straw was then
strewed, to the height of a foot, over the bottom
of the pit. On the straw were next laid a num-
ber of small shallow earthen pans, a part of
which were glazed, and a part unglazed. In
the last place ; all the pans were filled with soft
water, which had been boiled on the same even-
ing. Contrary to my expectation, the unglazed
pans remained as dry on the outside, after water
had been poured into them, as those which were
glazed. I conclude, therefore, that the former
were more dense in their substance, than the
unglazed pans used in India; and that their
density was probably the reason, why ice did
not afterwards form in them, sooner than in
the glazed pans, which were employed by me.
Two pans, containing boiled water, were set
upon the grassplat, at a little distance from the
pit. A watch-glass filled with boiled water was
ON DEW, &c. 273
also placed upon the grassplat, and another was
laid upon the raised board, which had been
thinly covered with sand. All these arrange-
ments were not completed before lOh. at night.
At 1 h. in the morning, ice appeared in the
watch-glasses on the grassplat and raised board ;
the heat of the air, as measured by a naked
thermometer, being then, at 4 feet above the
ground, 39^°, and at 7 feet, 40 J°. At 2h. ice
was observed in the pans in the pit, while a
thermometer in the air, 2j feet above the
ground, was 36J0. Shortly afterwards, ice
began also to form in the pans upon the grass-
plat. The temperature of grass, fully exposed
to the sky, was at the same time 30°, while that
of the earth an inch below the bottom of the
grass was 45°. During the time of these ob-
servations dew formed copiously.
EXPERIMENT
My next attempt was in the manner men-
tioned by Mr. Williams.
On the evening of the 22nd of May, I en-
compassed a square piece of level ground, the
sides of which were 3 feet long, with a border
of earth 4 inches high, and filled the area with
dry straw. On this were placed several of the
earthen pans, which had been formerly used,
T
274 ESSAY
and a few smaller vessels, all containing unboiled
water. After an exposure of little more than
an hour, water in a watch-glass upon the straw
was found frozen, the temperature of the air,
£ feet above the straw, being then 37°. In
half an hour more, ice began to appear in the
earthen pans, while a thermometer 5^ feet
above them, this being the height at which Mr.
Williams used to suspend his instrument, was
36°. The air soon after became colder; but
its temperature was never less than 33°, though
taken by a naked thermometer, which, as was
before said, upon a clear and calm night,
occasions the air to seem about 2° colder than
it really is.
It might be inferred, from what is mentioned
by Mr. Williams, that the temperature of the
straw beds, on which the ice-pans were set at
Benares, was always found by him above the
freezing point, for this reason, that the straw,
from containing no moisture, could not, like
the water, grow cold by evaporation. I had,
therefore, been surprised, during the first ex-
periment, for I had then but little acquaintance
with the phenomena of cold observed with dew,
that a thermometer, laid upon an exposed part
of the straw, was always below the freezing
point, after ice had b§gun to form in the pans.
On reading, however, his account of the process
ON DEW, &c. 275
a second time, with increased attention, my
wonder ceased. For, as the pans he speaks of
were large, and touched one another, and as all
the pans employed in India, for the making of
ice, widen as they rise from the bottom, like
our milk-pans, the thermometer, placed by him
on the straw, must have been secluded from all
view of the sky, and would therefore mark a
temperature much higher, than if it had been
laid, as in my experiment, upon straw fully
exposed to the heavens. On this, the second
night, therefore, I placed a thermometer under
the edge of one of the pans lying on the straw
bed, and found it some time afterwards 6° higher,
than a similar instrument upon a part of the
straw bed which was uncovered. Generally,
however, the difference was not so great. If
my pans had been large, like those of Mr. Wil-
liams, I should, no doubt, have observed more
considerable differences ; for, in consequence
of their smallness, I could not lay a thermo-
meter on the straw bed, so as to be fully screened
from the sky by the edge of any of them, with-
out its being almost in contact with the vessel,
every part of which was always colder than the
sheltered straw.
Much dew formed in the course of this night.
The greatest difference remarked by me, during
it, between the temperatures of grass and of air,
T 2
276 ESSAY
was 6°, and between those of air and a fully
exposed part of the straw bed 9°.
EXPERIMENT 3rd.
This was begun on the evening of the 16th
of October, and was likewise made agreeably
to the method related by Mr. Williams,
Ice appeared in the pans, when the tempera-
ture of the air, at the height of 5J feet, was,
according to a naked thermometer, 37°.
On this night, I placed upon the straw bed a
dry earthen pan, among those which contained
water, and found the inside of its bottom to be
as much colder than the air, as the water was
in the other pans, before ice appeared in them.
After the water had begun to freeze, no proper
comparison could be made between its tem-
perature and that of the empty pan. This pan,
in the course of the night, attracted moisture,
which was afterwards converted into a film of
ice.
But the chief fact established by the present
experiment was, that water may freeze at night,
in air of a temperature higher than 32°, not
only without any loss of weight from evapora-
tion, but with a gain of weight from an opposite
process.
I had observed that water, exposed early in
ON DEW, &c. 277
the evening in the open air to the sky, lost a
little weight, in the course of a ..clear night.
This I imputed to evaporation taking place,
before the water had been cooled enough to
condense the vapour of the atmosphere, and to
the weight gained afterwards being insufficient
to compensate the previous loss. I exposed,
therefore, on this night, water to the influence
of the sky, until it was cooled to 34°. Of this
I put 2 ounces into each of two china saucers?
which had also been exposed to the air, and
then placed the saucers upon the straw bed.
In the morning, a thin cake of ice was found in
both saucers, one of which had gained 2^, and
the other 3 grains, in weight. Dew was also
copious on this night. At one time, grass was
9j°, and the exposed part of the straw bed 12°,
colder than the air*.
It must be evident to every person, that the
formation of ice, in the three preceding experi-
ments, was the effect of a natural operation,
similar to that by which the same substance is
produced in Bengal. These two facts must,
therefore, have a common cause, and this has
* The greater cold, observed in this and the preceding
experiment, upon straw than upon grass, is to be referred
to the shortness of the latter, by reason of which heat was
readily communicated to its upper parts by the earth.
278 ESSAY
been shown, by the last experiment, independ-
ently of what; was said before in this Essay, not
to be evaporation. It is also clear, that the
cold, induced on the water in those experi-
ments, had a common cause with that observed,
at the same time, upon the grass and the straw ;
which latter cold must, in consequence of proofs
formerly given, be admitted to have arisen from
the radiation of the heat of those substances to
the heavens. A necessary inference, therefore,
appears to be, that the formation of ice in Ben-
gal, in the circumstances described by Sir R.
Barker and Mr. Williams, must likewise be
attributed, in by far the greater measure, if
not altogether, to a loss of heat, which the
water suffers by its own radiation, while situated
in such a manner, that it can receive little heat
from other bodies, either by radiation or con-
duction*.
* On the evenings preceding the nights, during which ice
is produced in Bengal, the temperature of the water exposed
in the pans is, probably, often 60° or more. But water of
the heat of 60°, if exposed in a shallow earthen vessel to air
of the same temperature, during the day, while the weather
is calm and clear, will lose about 3° of heat by evaporation.
A cold from this cause may, therefore, concur with that from
radiation, and, consequently, may, in Bengal, accelerate
somewhat the formation of ice. The influence, however, of
evaporation there, in this respect, should the state of the air
. ON DEW, &c. 279
with regard to moisture still permit it, which must often not
be the case while dew is forming, will, as the night proceeds,
gradually diminish, and at length almost disappear, before
the freezing of the water commences; since I have lately
shown, that evaporation from water of 32° produces very
little cold, even in the day-time. Indeed, it seems to me
much more probable, that on a clear and calm night, though
in a dry winter of Bengal, water at the temperature of 3$t°
will acquire warmth from the formation of dew upon it, than
that it will become cold from evaporation.
280 ESSAY
CONCLUSION.
i
The experiments which were made by me
on dew, and other subjects treated of in the
preceding Essay, were unavoidably attended
with many inconveniences, which were the
more felt, as my health had long been feeble,
and as my professional duties obliged me often
to return to London in the morning, without
having previously taken rest, after the whole'
of a night had been spent in attending to the
objects of my pursuit. The inconveniences
here alluded to were, indeed, so great, that I
was twice or thrice obliged to intermit my
labours for several months together, and at
length found it necessary to cease from them
entirely, before I had nearly completed the
plan, which I had formed. I take the liberty
of mentioning these things, to excuse, in part,
the imperfections, which will be observed in
what I have written, as some of them would,
no doubt, have been removed by a further in-
terrogation of Nature *.
London, September 25, 1815.
* Of the experiments related in the beginning of the second
Part of this Essay, with the view of proving, that the forma-
tion of dew is an effect of previous cold in the substances on
ON DEW, &c. 281
•which it appears, those of only one evening were remarkable
for the greatness of their results, the weather upon the other
evenings not having favoured much my purpose. I took
advantage, therefore, of being in the country, at the distance
of a few miles from London, on the 2 1st of the present
month, the last day but one of an unusually long tract of
dry weather, to expose to the sky, 28 minutes before sunset,
weighed parcels of wool and swandown, upon a smooth,
unpainted, and perfectly dry fir table, 5 feet long, 3 broad,
and nearly 3 in height, which had been placed an hour
before, in the sunshine, in a large level grass-field. At this
time, and throughout my experiments, the air was very still,
and the sky very serene. The atmosphere, too, in all pro-
bability, contained but little moisture, in consequence of the
long absence of rain ; and the surface of the ground appa-
rently contained none. The wool, 12 minutes after sunset,
was found to be 14° colder than the air, the temperature of
the latter being measured by a naked thermometer suspended
4 feet above the ground, and to have acquired no weight.
The swandown, the quantity of which was much greater
than that of the wool, was at the same time 13° colder than
the air, and was also without any additional weight. In 2O
minutes more, the swandown was 144-° colder than the
neighbouring air, and was still without any increase of its
weight. My experiments now ceased from a failure of
daylight.
In my former experiments of this kind, the greatest cold
observed by me from radiation, without the appearance of
dew, was only 9^°.
While making the experiments on wool and swandown,
I attended frequently to the temperature of the grass, and
found it at one time 15° colder than that of the air 4 feet
above the ground. This difference is 1° greater, than any
I had ever before seen between the temperatures of the same
282 ESSAY ON DEW, &c.
substances, and is equal to the greatest which I had ever
known to occur, between those of the atmosphere and of
swandown lying upon grass. I had this evening placed no
swandown upon grass.
These experiments were not made till nearly the whole of
the present edition of my Essay was printed, and could not,
therefore, be mentioned in their proper place.
A
LETTER
TO
THE RIGHT HONOURABLE
LLOYD, LORD KENYON,
RELATIVE TO SOME CONDUCT OF THE
COLLEGE OF PHYSICIANS OF LONDON,
POSTERIOR TO THE
DECISION OF THE COURT OF KING'S BENCH
IN THE
CASE OF DR. ST ANGER;
AND CONTAINING
OBSERVATIONS ON A PRINCIPAL GROUND OF THAT
DECISION.
TO
DAVID HUME, ESQ.
ADVOCATE, PROFESSOR OF SCOTCH LAW IN THE
UNIVERSITY OF EDINBURGH, &C.
MY DEAR FRIEND,
When you requested some months ago,
that I would proceed no farther in the letter, which I
had informed you I was writing to the Chief Justice of
the Court of King's Bench, it appeared to me that you
had, in a great measure, mistaken its object. I therefore
considered myself entitled to continue my undertaking,
and have accordingly now brought it to a conclusion. I
readily admit, that, in one point of view, I may have been
imprudent ;
fe <rr£§sra.if nfi$ ?' aur^a-iv dtyeoc,
But I trust that, as far as the more important parts of
moral character are concerned, you will find nothing in
what I have done unworthy of the friendship between
us, which, from its commencement, now nearly thirty
years ago, when our boyish fancies gilded every prospect
before us, has been ever my pride, and often, in the storms
of life, the chief anchor of my hope.
As a piece of composition, my letter will no doubt be
deemed faulty by you in many respects. You will per-
ceive, for instance, a considerable want of unity in the
execution, should indeed the great rules of criticism be
286
thought applicable to such a trifle as the present, from
the introduction of circumstances, which must seem both
trifling and irrelevant, if the rank and character of the
person to whom they are communicated be considered.
My excuse for part of them is, that, it being one of my
intentions to give information to some of my own profes-
sion, I conceived it allowable to mention various things
for this purpose alone.
I shall anticipate only one other of your observations
regarding my letter, and this refers to the lateness of its
appearance. I confess myself much ashamed, that almost
a twelvemonth has passed away since the occurrence of
the event, which especially gave rise to it. But accidents,
which would appear ridiculous in narration, whatever
their eifects may have been, often interrupted my labour,
and indolence often pleaded, for a time, irresistibly against
the performance of an ungrateful task, which duty had
imposed. The delay, however, has necessarily tended to
diminish the probability of there being many considerable
errors in what I have advanced.
Forgive me for employing this mode of conveying my
sentiments to you, and accept my warmest wishes for
your welfare.
I remain,
Your most affectionate friend,
WILLIAM CHARLES WELLS.
London,
September 1, 1799.
LETTER
TO
THE RIGHT HONOURABLE
LLOYD, LORD KENYON, &c.
La fede unqua non deve esser corrotta,
O data a un solo, o data insieme a mille ;
E cosi in una selva, in una grotta,
Lontan dalle cittadi, et dalle ville;
Come dinanzi a tribunal!, in frotta
Di testiraon, di scritti, e di postille ;
Senza giurare, o segno altro piu espresso,
Basti una volta, che s'abbia proraesso.
MY LORD,
IF confidence can be placed in the accuracy
of the well-known writer of short-hand, Mr.
Gurney, the decision of your Lordship, and the
other Judges of the Court of King's Bench, in
the case of Dr. Stanger, rested principally upon
this ground — that he might readily obtain by a
direct application to the College of Physicians,
what he then prayed the court to enjoin that
body to grant. — -Every person, your Lordship
said, has already a right io address himself to
the honourable feelings pf those breasts, to
288 LETTER TO
whicli Dr. Stanger must at last address him-
self, if the mandamus were issued. The same
sentiment was immediately after expressed by
you a second time ; " if any one proposes him" — •
I venture to repeat your Lordship's words —
" the question is submitted to a majority. It
goes then to that tribunal, which, 1 hope and
believe, is the sanctuary of honour and good
faith, and he may as well address himself to
them now, as if this mandamus went." I am,
my Lord, one of those persons, whom you thus
declared to have a right to address themselves
to the honourable feelings of the College of
Physicians of London. I have exercised that
right. — I have applied to the sanctuary of honour
and good faith, for a completion of those as-
surances, which your Liordship regarded so
deeply imbedded in truth, that you erected
upon them a decision, wbich was to affect the
reputation and fortunes of many of your fellow-
subjects, of no mean rank in society, and from
which there could be no appeal. Of the suc-
cess of this application f now think it my duty
to inform you, as it originated in your counsel.
The counsel was given in open court ; the nar-
ration of its consequences ought, therefore, in
my opinion, to be made with equal notoriety,
if my feebleness would permit ; and this con-
sideration will, I hope, induce your Lordship
LORD KENYON. 289
to pardon the unusual liberty which I take in
addressing you thus publicly.
But it seems to me proper, before entering
upon this narration, that I should speak at
greater length of the case of Dr. Stanger.
Your Lordship's attention must have been so
much occupied by the many important affairs,
in which you have been engaged since its oc-
currence, that the traces left in your memory
by some of its circumstances, the knowledge of
which is necessary to the right understanding
of what I have to say respecting myself, are
now perhaps nearly, if not altogether effaced.
On the 26th of January, 1796, Dr. Christopher
Stanger, a physician of eminence in this metro-
polis, made oath in the Court of King's Bench,
that he had shortly before applied to the College
of Physicians, to be examined for admission
into their order of candidates; and that this
examination had been refused to him in con-
sequence of a by-law, which he conceived con-
trary to the intention of the charter and acts
of Parliament, by which their corporation had
been established. The next day, a rule was
granted by the court for the college to show
cause, why a mandamus should not issue to
compel them to examine that gentleman. In
the beginning of the following April, Sir George
Baker, president of the college, and Mr. Roberts,
290 LETTER TO
their attorney, made each of them an affidavit,
to justify the refusal to admit Dr. Stanger to
the examination he required. In these affidavits
it was stated, that the college, in pursuance of
a power granted by their charter, had from
time to time prescribed certain qualifications
and conditions, as requisite for the admission
of persons into the commonalty or fellowship,
and into the order of candidates ; that, by one
of their statutes then in force, no person could
be admitted into that order, unless he were a
doctor in medicine of Oxford or Cambridge j
that Dr. Stanger was not a graduate of either
of those universities ; and that there were two
by-laws of the college, by which licentiates of
certain descriptions might be received into the
fellowship, without their previously entering
into the order of candidates. Such were the
general grounds on which the refusal of the
college to examine Dr. Stanger was to be de-
fended. It was, however, clearly seen from Sir
George Baker's affidavit, that if the reasons
hitherto alleged should be found insufficient,
an attempt would be made to show, that the
applicant was unworthy of reception into any
society, from having violated the faith which,
it was said, he had solemnly pledged to the col-
lege, upon being admitted a licentiate.
The by-law, which restricted admission into
LORD KENYON. 291
the order of candidates to the graduates of Ox-
ford and Cambridge, had been decided by Lord
Mansfield to be bad ; and according to the con-
fession of the counsef of the college, the two
by-laws, which allowed licentiates to enter the
fellowship, had been framed in consequence of
the censure passed by that judge upon the
former system of admission, and of his recom-
mendation that a more liberal one should be
adopted. Their real defence, therefore, as
having regard to the possible applications of
persons in whom they could not pretend to find
the smallest appearance of blame, rested en-
tirely upon the two last-mentioned by-laws.
These by-laws were recited at length in the
affidavit of Mr. Roberts. By one of them, the
president was allowed once in two years, but
not oftener, to propose a licentiate often years
standing, to be admitted into the college without
examination of his fitness. If he chose, how-
ever, to omit the exercise of this privilege*, as
the present president has repeatedly done, it was
not to devolve upon any other person. But
when Lord Mansfield condemned the whole of
the former system of admission, there existed a
much more liberal statute for the reception of
licentiates, through favour ; for according to it
every licentiate of three years standing, who
had taken the degree of doctor in medicine,
292 LETTER TO
after studying four years in any university,
might in this way be admitted a member of the
college : one at least, therefore, of the new
by-laws, certainly afforded no corrective to the
evil, of which that great man complained.
The remaining by-law was consequently the
only source, from which such a corrective could
be expected. It declared, that licentiates of
seven years standing, and who had completed
the thirty-sixth year of their age, might be ad-
mitted into the fellowship of the college, should
they be found fit upon examination. I shall not
trouble your Lordship, at this time, with any
observations upon the numerous fetters, by
which the action of the pretended principle of
this by-law was impeded. I have at present
nothing in view but to show, that this was the
only measure of any importance the college had
adopted for the purpose of removing the re-
proach, which had been thrown upon them by
Lo*d Mansfield ; and that it therefore afforded
the only good ground for their resisting the
issue of the mandamus which Dr. Stanger so-
licited.
Accordingly, when the question of the man-
damus came to be argued before the Court of
King's Bench, on the 23d of April, 1796, Mr.
Erskine, the leading counsel of the college, was
found to derive from this by-law his chief
LORD KENYON. 293
reasons against the proceeding of that writ. It
can scarcely be thought, that so ingenious and
eloquent an advocate would confine the defence
of his clients to any single point. It was not
surprising, therefore, that he should make a
show of resistance at various parts. But still
it was evident, that this by-law was regarded
by him as his only secure position. How could
he indeed act otherwise, consistently with the
deference which was due to the opinion of
Lord Mansfield? Dr. Stanger had applied for
a mandamus to the Court of King's Bench, be-
cause the College of Physicians refused to ex-
amine him for admission into their order of
candidates. But the by-law, in which they
grounded this refusal, had been decided to be
bad by the late chief judge of that court. No
other defence then could well be offered there
for such conduct, than that, in compliance with
the advice with which his censure was accom-
panied, a new by-law for the admission of
members had been framed, which so qualified
the former, as to take away from it all appear-
ance of illiberality ; and that if Dr. Stanger
chose to apply under the new statute, he would
readily be received into the college.
This appears to me a just summary of the
chief, if not the only argument, of Mr. Erskine
upon that occasion. But to avoid all suspicion
294 LETTER TO
of error, I shall now take the liberty of showing
in what manner Mr. Erskine represented hi^
own argument, and what assistance he expected
to gain from it, in his attempt to prevent the
issue of the mandamus. I shall at least prove
by this procedure, that I can have no intention
to deceive.
EXTRACTS* from Mr. ErskmJs Speech in the Court
of Kings Bench, April 23, 1796, ii\ the Case of Dr.
Stanger against the College of Physicians.
\ " Subsequently to the time when Dr. Fother-
gill's case was before the court, there was a re-
vision of the statutes of this learned body, who
took the very best and the most eminent advice
which this kingdom could furnish them."
" They made two by-laws — in which there
is a power given for any fellow at the ordinary
comitia majora, after Michaelmas, to propose a
licentiate of seven years standing, who is thirty-
six years of age, for examination, who, if ap-
proved of by the majority of the fellows then
present, is to be examined at the three next
comitias, and then, if approved, to be admitted
* From Mr. Gurney's Report, taken in short-hand.
LORD KENYON. 295
a candidate, though he has not studied at either
of the English universities.
" Your Lordship will observe, that Dr. Stanger
could not have this mandamus under this by-
law* and therefore I admit I must support the
by-law Sir George Baker sets forth in his affi-
davit, because no person, except he be of one
university or the other, can possibly be ex-
amined, but upon the proposition of one of the
fellows that he should be examined ; and upon
the proposition of one of the fellows, if he be a
licentiate of seven years standing, and thirty-
six years of age, though he has not that qualifica-
tion which is required in the by-law set forth in
Sir George Baker's affidavit, yet this door is open
to him. And can it be supposed, or will any
gentleman stand up and say it is consistent
with probability, that a man of eminent learn-
ing and high qualifications, who, notwithstand-
ing he has not had that species of education,
which I will show from the time of the charter
to this day has been constantly adopted; yet, if
he be a person who has undoubtedly qualifica-
tions for it, is it to be supposed that there is not
one fellow of the whole college who would pro-
pose such a person ?"
" In the by-law which I have just stated, any
one fellow may propose the examination of an
296 LETTER TO
individual, though such individual could not,
according to the ordinary by-laws of the col-
lege, be admitted to examination. But to leave
the door open, and to prevent the observations
that were made in the case of Dr. Fothergill,
and under the auspices of the most learned
men of the profession, this by-law was made ;
therefore, I think, I have gone the length of
showing, that the present by-laws can be at-
tended with no possible inconvenience."
" I will only ask my friends, by-and-by, to
explain to your Lordship, how it is consistent
with reason or common sense, to say, that the
public can suffer, or this learned profession be
affected in its dignity or advantages, if no per-
son should have an opportunity to force him-
self into their college, unless he comes within
the scope of their by-laws, sanctioned from all
antiquity, and comes within the sense of these
by-laws; although no door is shut against them
at all, but any one fellow of the college may,
notwithstanding that statute, propose them for
examination, &c. It is not easy to conceive,
that a man can be entitled to so much favour,
because of his eminent qualifications, as that
he can supersede all the rules and provisions
of the country, and yet shall not be able to
find one person within the walls of a college,
LORD KENYON. 297
consisting of near a hundred members, to pro-
pose him, although such a man would add dig-
nity and lustre to the college."
" Will any man say that these things are at-
tended with any inconvenience to the public ?
They are not at all ; for, in the first place, if
the gentleman who proposes himself to ex-
amination has studied at either of the English
universities, then this does not apply ; if he has
not studied at either of the universities, and
can find one fellow in the college who knows
any thing of him, and thinks him a fit person to
be proposed, then this by-law does not stand
in his way."
Such, my Lord, was the use which Mr.
Erskine made of this by-law, in resisting the
issuing of the mandamus. The pleadings ceased
almost immediately after he had finished his
speech, and the rule was discharged, in con-
sequence of an error which was discovered in
the mode of Dr. Stanger's application to the
college. While it was in doubt, however, whe-
ther this error was of sufficient importance to
put a stop to the proceedings, some conversa-
tion took place between the judges and Dr.
298 LETTER TO
Stanger's counsel, an exact relation of a part of
which will demonstrate more strongly than I
can possibly do, that the court uniformly re-
garded the conditions, which were required by
the by-law for admitting licentiates into the
college, merely as cautionary measures against
the entrance of improper persons into their
body ; and constantly supposed, that if any
licentiate of good character, and possessing the
qualifications marked by the statute, could pre-
vail upon a fellow to propose him, no obstacle
would afterwards exist to his admission. How
far these opinions were well founded, will here*
after appear to your 'Lordship.
Extract from Mr. Gurnets Report.
Mr. Justice Lawrence. " Where is the diffi-
culty of a gentleman's getting some one fellow
of the college to propose him ?"
Mr. Law. "There has been no person ad-,
mitted — there have been many trials, but no-
body has ever got through that wicket, nor
ever will.*'
Mr. Justice Lawrejice. " Do you imagine, if
they think Dr. Stanger, or any other physician,
is a fit person, that they will not propose him?"
Lord Kenyan. " There is a wicket of that
LORD KENYON. 299
kind put in our own profession — for, as I un-
derstand, all the four inns of court have for
some time insisted, that one of their body shall
propose a gentleman to be called to -the bar,
and that precaution has been attended with ex-
tremely good consequences. I am sorry, from
what one hears, that it has not been quite a
sufficient guard now and then, through a little
inadvertence or misinformation ; but certainly
it is attended with good consequences."
Mr. Law. " That is a delegation of the
power of inquiry to one whose special business
it is, and I believe every gentleman upon whom
that delegation falls, discharges his duty pro-
perly, and makes that inquiry,"
Mr. Justice Grose. " But why should not
this supposed duty be as honourably and as
well executed by them, as in our profession*?"
* Students of law, as the author is informed, are admitted
to the bar by the benchers of the inns of courts, who, for the
following reasons, may be supposed to execute their trust
with fairness and impartiality. 1. As they are few in number,
each of them must feel himself responsible for the acts of the
whole. 2. They are either of advanced age, and little con-
nected with the practice of their profession, or of considerable
rank in it. None of them, therefore, can well be jealous of
any person who may apply for admission. 3. The applicants
for admission are, for the most part, very young men, who
for this reason cannot have exhibited such talents as are
300 LETTER TO
The mode of Dr. Stanger's first application
to the college having been determined to be
likely to excite jealousy in persons much their seniors, were
these even liable to be affected with that passion. 4. The
profession of law includes so many individuals, that the ac-
cession of one more to it can scarcely excite fear in any former
member, that his profits may hence be diminished. 5. Since
none are allowed to practise as advocates before admission at
an inn of court, an applicant cannot, in the previous exercise
of his profession, have given umbrage to any of those who
are to decide upon his fitness. 6. So many gentlemen of
great figure and independent fortune embrace the profession
of law, either with the view of preparing themselves for the
discharge of various duties incident to their rank in society,
or in the expectation of obtaining some high office in the
state, that it is natural to infer that great liberality exists in
its government. Stronger reasons may no doubt be given
by persons better acquainted with the subject, but these seem
to the author sufficient to explain the fact, that every person,
who possesses the prescribed qualifications, is morally certain,
upon application to any of the inns of court, of being admitted
to the bar.
Similar reasons cannot be given, why fairness and impar-
tiality should be found in the decisions of the College of
Physicians, upon the applications of licentiates for admission
into their body. For, 1 . Not a few of the fellows, but all of
them indiscriminately, determine the fate of every such ap-
plication. 2. The greater part of the voters are consequently
not of such a rank in their profession as to be above the
reach of jealousy. The proportion of such persons at the
meetings of the college is further increased by their having
little to do elsewhere. 3. As the seven years of the applicant's
licentiateship will, in all probability, have been spent in the
metropolis, it is surely not unlikely, that some of the voters
LORD KENYON. 301
wrong, in the June following he presented him-
self a second time to them, requesting permis-
sion to undergo any examination which might
ascertain his fitness to be & fellow of their body.
An examination was again refused. Dr. Stanger
having made oath of this, a new rule was ob-
tained on the 26th of November, from the Court
of King's Bench, for the college to show cause
why a mandamus should not issue against them.
may have become jealous of his talents or success. 4. The
members of the college are very few in number when com-
pared with the barristers belonging to all the different inns
of court. In the list for 1798, there are only twenty-seven
fellows who exercise their profession in London, and some of
these are very aged, and take little concern in practice. Any
advantage, therefore, to be derived by a licentiate from being
admitted into the college, will probably be regarded by some
of the former members as tending to diminish, or prevent
the increase of their own emoluments. 5. In the course of
seven years passed in the exercise of a profession, in which,
above all others, misunderstandings are apt to arise among
its different members, it is almost impossible that a licentiate
should not have given umbrage to some of those who are to
decide upon this application. 6. Physicians in this country
are almost universally taken from the middle ranks of men.
They cannot therefore be expected to conduct themselves, as
a body, in the same liberal manner as the members of a pro-
fession, which contains a considerable number of persons of
high birth and large hereditary fortunes. Other circum-
stances, tending in like manner to produce unfair and partial
decisions in the College of Physicians, when licentiates apply
to them to be examined, will be mentioned hereafter.
302 LETTER TO
On the 23d of January, 1797, Dr. Gisboriie*
then president of the college, made an affidavit
in answer to Dr. Stanger's, the purport of
which was the same as that of the affidavits of
Sir George Baker and Mr. Roberts, in the
former cause. In the new trial which followed
on the llth of May, 1797, the leading counsel
of the college, as in the preceding one, was
Mr. Erskine, who quickly abandoning all weak
points, again fixed upon the by-law for the
admission of licentiates, after an examination
of their fitness, as the only ground which was
fit to bear his works of defence. To prove
that I am here also justifiable in attributing
such conduct to him, I proceed to insert several
passages from his speech upon this second oc-
casion.
EXTRACTS from Mr. ErsJcine's Speech in the Court
of King's Bencft, May 11, 1797, in the Case of Dr.
Stanger against the College of Physicians.
" Your Lordship will take it that this last
statute which I have read, [that restricting
admission into the order of candidates to gra<
duates of Oxford and Cambridge] and which
still is in existence, and which is qualified by
others I am about to state, was the last in exist-
ence at the time when the cases of the King v.
LORD KENYON. 303
Dr. Askew and Dr. Fothergill, and those other
cases came before the Court of King's Bench,
as reported in Sir James Burrow. Since that
time your Lordship will find that other by-laws
have been introduced, greatly qualifying those
previous by-laws, and as I have been given to
understand, framed under the advice, and with
the assistance of some of the most eminent and
learned persons in this kingdom, in the profes-
sion of the law." [Mr. Erskine then recited
the by-law, by which licentiates might enter
the college upon being examined in regard to
their fitness, and afterwards proceeded thus :]
" Your Lordship observes then, that by the last
by-law which I have just now stated, though
a man had never seen either of the universities,
yet if he can find out of the whole college of
physicians, any one person who is a fellow of
the college, to usher him in for an examination,
he is, notwithstanding the other statute, of
which this statute, your Lordship observes, is
a great qualification, entitled to undergo the
ceremonies which the college has thought fit
to prescribe ; and which I will show your Lord-
ship, by and by, it has, and it cannot be denied
that it has, a right to prescribe for its own go-
vernment ; he may be admitted."
304 LETTER TO
" Then what are we assembled here upon ?
Why upon this grave and notable question —
whether the by-laws which I have read to your
Lordship, taken altogether as one body ; those
that are subsequent qualifying, restraining, and
modifying those that are antecedent ; whether all
these taken together constitute a reaso able body
of' by-laws^ within the meaning of the charter^
granted by the king, and confirmed by an act
of the legislature. Orl, whether these by-laws
shut out any persons who had a right by some
privilege inherent in them as British subjects,
under this charter, and this act of parliament,
from becoming members of this grave and
learned body.*'
" Would a mandamus go to the Bishop of
London if he refused ordination to a person
who set forth his learning, but had not been at
either of the universities, and was therefore re-
jected ? But have the college of physicians done
that ? No ; they have done no more than this^ — •
if you have been at the university, and have
acquired a degree and testimonials, without
dispensation, we presume that you are learned
from the place from whence you came, and the
'discipline you have been engaged in, and we
examine you at once j but if you have not, do
LORD KEXYtfN. 305
rce reject you? No\ but we require that you
should be introduced for examination by some one
of the fellows of the. college, and then we will exa-
mine you. Is it consistent with common sense
to say, that there is any thing unreasonable in
that?"
" Your Lordships have the same authority,
assembled in your judicial capacity, as judges
over our voluntary societies, as you have over
a college by mandamus. I apprehend, if a
person were to apply to your Lordships, and
say, I have been rejected at Lincoln* s-Inn ;
why ? because I could find nobody who would
give in my name to the benchers to be called
to the bar ; you would reject such petition with
indignation. You would say, that those learned
bodies, who have a jurisdiction exactly similar,
only that it is directed and referred to a different
profession, in the regulation, and in the learn-
ing and integrity of the members of which, the
public have a similar interest, inasmuch as they
exercise a profession very important in every
view of it ; your Lordships would say, that he
ought not to be admitted, who could not find
one person to propose him as fit to be examined ;
(and that is all that we are contending for] be-
cause if a man can find any one fellow of the
x
306 LETTER TO
college to propose him, he may be admitted,
provided they think him fit."
" Now I will consent to the learned Serjeant
making this rule absolute, if he can prove that
this by-law is unreasonable ; for we are here
upon the reasonableness of the by-law. I read
that part of the charter which gives them au-
thority to make by-laws, and I defy the wit or
imagination of man to put another question
upon the court here, than — Whether this class
of by-laws, taken altogether, be unreasonable.
I consent to the rule being made absolute, if
any one of my friends, or all of them together,
can, in their imagination ; I do not appeal to
any experience they can bring ; but if they can
in their imaginations, however fertile they may
be, figure to themselves an inconvenience that
may arise from them. They may say, Oh, there
may be a conspiracy which may exclude a vir-
tuous and enlightened man ! Setting aside the
main improbability, that members of a learned
body could league themselves in a conspiracy so
base and so scandalous, as to refuse to examine
a man proposed to them by one of their own
order, under their own laws, from a professional
jealousy, lest they should be eclipsed by that
person, &c."
LORD KENYON. 307
" And yet what is the argument, that, when
bowing to the great learning and ability of Lord
Mansfield upon that occasion, when the college
having no other end and object in the world ;
and what other end and object can they have,
than the regulation of a profession, which I will
say — and let Dr. Stanger take part of the honour
if he pleases — is a profession which not only
preserves the health of our relations and friends,
and gives greater security to human life, but
which, I say, also gives us a class of men who
are an ornament to society and to this country,
with a knowledge of the languages and the
various branches of philosophy, which gives
that insight into nature and its works which are
acquired in the learned institutions, which now
are to be broken down, and all sort of persons
are to be suffered to do — What ? Not to practise
physic, for they practise it already, but they
are to be let in for the purpose of governing
one of the wisest and the most learned bodies ;
of governing men who, one and all of them,
almost, are deeply skilled in every thing that
learning and science have brought forth in any
age ; and yet, forsooth, it is to be considered,
as if the charter and acts of parliament were
likely to suffer, because a man has kept his learn-
ing so much to himself, that nobody could everjind
it out) so as to be able to think it was wise or decent
308 LETTER TO
to propose him\ or else, that he is such a pheno-
menon in human shape, that there must be a
conspiracy among them to keep him out, lest
he should eclipse them all. I am sure Dr.
Stanger does not wish to represent himself as
such a person ; but I am certain that if Dr.
Stanger would have applied to the college, as men
of the first learning in every age have applied
to it, he would have been admitted.39
These were the arguments, which the by-
law for admitting licentiates to examination
furnished to Mr. Erskine, against the issuing
of the mandamus, upon Dr. Stanger's second
application. That they were the only argu-
ments, upon which he placed the least depend-
ance in preventing that writ from going forth,
is demonstrated by the concluding sentence of
his speech, in which he collects to a single point
the scattered tendencies of all that he had
before advanced. " My proposition is," said
Mr. Erskine, " that it is reasonable the college
should say ; if you are of the universities we
will examine you at once ; if not of the universi-
ties^ we do not refuse to examine you, but we
consider it reasonable to point out the mode
in which that examination should go forward ;
otherwise we must examine all the world : and
LORD KENYON. 309
we conceive that the regulation which we have
imposed, in order to prevent frivolous examina-
tions, is not inconsistent with the reasonable
exercise of discretion ; and which, therefore, is
warranted by the charter, which entitles us to
make these statutes."
Having already occupied so much of your
Lordship's time in citing passages from Mr.
Erskine's speeches in the case of Dr. Stanger,
I feel averse to give extracts from those of the
remaining counsel of the college upon the same
occasion. I shall, therefore, only recal to your
Lordship's tecollection, that two of them, Mr.
Gibbs and Mr. Dampier, made use of the same
by-law to convince the court, that the manda-
mus ought not to proceed.
I know not, my Lord, exactly in what light
the declarations of counsel in a court of law are
to be regarded, or how far they may be thought
binding upon the persons in whose behalf they
are made ; but if they are ever held to impose
an obligation upon a client, and without admit-
ting that they do, it is difficult to conceive how
the business of a court of judicature can be
carried on, those in Dr. Stanger's case ought
to have produced this effect. The principal
advocate, of a body termed by your Lordship,
the sanctuary of honour and good faith, declares
310 LETTER TO
to the Court of King's Bench, that his clients
are willing to examine Dr. Stanger, or any other
person of the same description, should applica-
tion be made to them under a particular by-
law which he recites. This pledge, for so I
must call Mr. Erskine's declaration, was given
in April, 1796. The same cause was tried a
second time in May, 1797. If, therefore, Mr.
Erskine had gone beyond his instructions in
giving that pledge, sufficient time had surely
intervened, to have allowed the college to make
the discovery, and to warn him against commit-
ting the same error a second time. Did this
happen ? His subsequent conduct proves that
it did not ; for in his second speech he repeats
the pledge, in language still stronger than that
which was formerly employed by him. But
it may be said, that inveterate obstinacy, or
unconquerable ignorance on the part of Mr.
Erskine, might occasion the repetition. Some
of a committee of the college appointed to
conduct the law-suit hear Mr. Erskine's second
speech. Do they then caution the remaining
counsel to avoid the rock, which had twice en-
dangered the safety of the vessel committed to
his care ? We can here also only judge frpm the
event. The two who speak next, vouch, like
Mr. Erskine, for the willingness of the college
LORD KEN YON. 311
to examine Dr. Stanger, or any other person of
similar qualifications, for admission into their
body.
But it seems superfluous to offer proof, that
the college were bound by .the repeated and
unchecked declarations of their counsel, to a
prompt and honourable execution of the statute
for the admission of licentiates to examination,
when it is considered in what light that statute
was regarded by the court. For in delivering
your opinion on Dr. Stanger's case, your Lord-
ship, after speaking of the by-law which had
formerly restricted admission into the college
to the graduates of Oxford and Cambridge, was
pleased to express yourself in the following
manner :
" If it, [the restrictive statute] had been a
sine qua non, if it had controlled the parties
who are to form their judgment, and taken
from them all power of decision upon candi-
dates, it would have had that seed of death in it,
which Lord Mansfield found in that by-law* which
he decided to be bad. But this is not so ; here
* The only difference between the present restrictive sta-
tute, and that which was in existence in the time of Lord
Mansfield, is, that foreigners, who have taken degrees at
Oxford or Cambridge, are not now prevented from entering
the college : but it is evident that tl^is relaxation can affect
very few persons, perhaps not more than one in a century.
312 LETTER TO
every person has a right to address himself to the
honourable feelings of those breasts, to which Dr.
Stanger must at last have addressed himself, if
this mandamus went. If they find him to be,
(as I am inclined to believe he is from what I
hear of him) possessed of all the requisites of
medical learning and moral character, he will
address as powerful arguments to those gentle-
men, every individual of whom is called upon to
exercise his opinion upon the subject. He is not
to wait to be seconded; the by-law does not
require that ; if any one proposes him the ques-
tion is submitted to a majority. It goes then
to that tribunal, which, I hope and believe, is
the sanctuary of honour and good faith, and he
may as 'well address himself to them now as if this
mandamus went ; they are not bound to admit,
all they are bound to do is to examine."
One of your brethren on the bench, Mr.
Justice Grose, refused the mandamus on the
same ground as your Lordship. Another, Mr.
Justice Lawrence, had several times, in the
course of the two trials, declared his confidence
in the readiness of the college to admit any
licentiate, in the situation of Dr. Stanger, to
an examination, and for this reason probably
thought it unnecessary to repeat the same opi-
nion, when he gave his reasons for refusing the
mandamus. The remaining judge, Sir William
LORD KENYON. 313
Ashurst, was the only one who did not, at some
period or other of Dr. Stanger's applications to
the court, approve of the by-law for the admis-
sion of licentiates into the college upon exa-
mination, and express his belief that it would
be carried into execution, whenever an applica-
tion should be made in consequence of it. What
he said, however, at the close of the trial,
afforded no reason to suppose, that he enter-
tained sentiments on these subjects, different
from those of his brethren.
I have now, my Lord, finished the relation of
those parts of Dr. Stanger's case, which seem to
me to form a proper introduction to what I shall
say concerning myself. I may have been tire-
some by minuteness of detail ; but if I have
been at the same time accurate, as I believe I
have, I trust that I / shall readily receive your
Lordship's forgiveness ; more especially .when
it is considered with what view that statement
has been given. It is to point out, in a manner
not to be questioned, what conduct the college
were bound to pursue upon the application of
a licentiate for examination, before 1 describe
the conduct which they actually did pursue,
when such an application was made. It is to
exhibit a picture from the masterly hand of
314 LETTER TO
your Lordship, guided rather by the suggestions
of a warm and virtuous fancy, than by an ac-
curate knowledge of the object to be repre-
sented, before I produce another picture of the
same object copied from nature, by an artist,
rude indeed and unskilful, but whose diligence
and fidelity may have compensated his want of
genius and taste.
Before the decision of the Court of King's
Bench, in the case of Dr. Stanger, I had with
many others believed, that the fellows of ih&
college never meant to admit any licentiate to
an examination. But when I had become ac-
quainted with the declarations of their counsel,
respecting the by-law for examining licentiates,
and with your Lordship's opinion, that it fur-
nished a remedy for the evil in the former system
of admission, equal to that which even a man-
damus could afford, I concluded with some
firmness, that although my belief had been
originally well founded, still they would scarcely
be hardy enough to refuse to examine a licen-
tiate, while the circumstances of Dr. Stanger's
cause were recent in the memory of every one.
Not having been in court myself during the
trial of that cause, my first knowledge of the
proceedings in it was derived from verbal re-
ports. Fearing, however, that these might be
incorrect, I thought it prudent not to form any
LORD KENYON. 315
plan in consequence of what had passed there,
before I should see an account of the proceed-
ings, which Mr. Gurney was then preparing
from his notes in short-hand. When I had
perused that account which from various cir-
cumstances I did not receive until several
months after the trial, I hastened to inquire,
whether any licentiate, wrho came within the
conditions of the by-law, meant to avail himself
of it : but, finding that there was none, I deter-
mined to apply f >r an examination of my own
fitness to be a fellow of the college, whatever
reason I might have for being fearful of its
issue, rather than allow the grounds of the de-
cision to run any hazard of being forgotten,
from want of an early appeal to them. I men-
tioned this intention to two of my friends among
the fellows, Dr. David Pitcairn, and Dr. Mat-
thew Baillie, who, by immediately offering to
propose me, removed the first, and in the opi-
nion of the judges of the Court of King's
Bench, the only difficulty in the way of obtain-
ing my object. On the 29th of September,
1797, a motion was accordingly made at the
college by Dr. Pitcairn, and seconded by Dr.
Baillie, not that I should be admitted a fellow,
but merely that I should be examined concern-
ing my fitness to become one hereafter. If
your Lordship's surprise would have been
316 LETTER TO
excited, as surely it must, by any opposition
whatever to this proposal, to what height will
it be carried, when you learn the ground of that
which was actually made ? Could your Lordship
have even imagined, that a by-law of the college
of Physicians, which, by the declaration of their
counsel in the Court of King's Bench, had been
framed in 1778, with the best legal advice this
country could afford, for the express purpose of
removing the blame which had been thrown
upon them by Lord Mansfield ; that a by-law,
which, if before forgotten, had been recalled to
their recollection in 1789, by an application
under it from Dr. Sims ; that a by-law, to whose
existence they had twice sworn before your
Lordship, once in April 1796, and again in
January 1797; that a by-law, upon which they
had rested their chief defence in a recent trial
before the Court of King's Bench ; that a by-
law, to the beneficial operation of which Dr.
Stanger had, in the course of that trial, been
advised by one of the judges upon the Bench
to trust implicitly, instead of applying to the
court for a mandamus ; and, lastly, that a by-
law, which your Lordship had expressly said
bound them to examine every person who ap-
plied under it, should in September 1797, be
declared a dormant by-law, the propriety of
who'se revival formed a question of very great
LORD KENYON. • 317
concern, and was consequently not to be de-
cided upon before it had undergone much se-
rious consideration ? In the midst of your in-
dignation against such conduct in a body of
men, formerly styled by your Lordship, the
sanctuary of honour and good faith, it must yet
afford you some consolation to know, that many
of the members were free from its guilt ; and
that when a motion was made to get rid of Dr,
Pitcairn's proposal, by what is termed the pre-
vious question, out of twenty-three, the whole
number at the meeting, ten voted against it.
Few men are so lost to shame, as not to de-
sire that their most iniquitous acts should wear
an appearance of justice. It is not, therefore,
wonderful, that the college of Physicians should
have attempted to palliate the conduct which
has been mentioned. With this view they
maintained, that proper notice had not been
given of the intended proposal by Dr. Pitcairn.
But no such notice was either required by the
by-law which authorised that proposal, or had
been established by custom. What end indeed
would the giving of notice in the case before
them have served? Not surely to afford time
for their considering, whether they were to re-
ceive what they were bound to receive. If it
were to have relation to the character of the
person to be proposed, all that could be well
318 * LETTER TO
known of it was already in their possession ^
since, during the nine years of his being a licen-
tiate, he had never absented himself from Lon-
don an entire day, and had in the same time
associated more with fellows of the college than
with licentiates. Besides, the mere admission
to an examination did not prevent the free ex-
ercise of their judgment at any one of the four
other ballots which were to take place, before
he could be received into their body ; and as
the last of those ballots was not to be held till
twelve months after the admission to be ex-
amined, those of the tenderest consciences were
allowed sufficient time for the most scrupulous
inquiries respecting him.
But not to dwell longer upon this mode of
answer to their pretext, I proceed ito assert,
that notice was given to the college of Dr. Pit-
cairn's intended proposal. If the proper person
for receiving it was absent from his duty, the
fault lay with him. Among the many illiberal
circumstances of the by-law for admitting licen-
tiates to an examination, is this ; that no person
can be proposed under it, except upon one day
in the year ; namely, at the general meeting of
the college, immediately after Michaelmas. I
had not been able before the 20th of September,
to ascertain whether or not I could be proposed
in 1797. Two days after this, and seven days
LORD KENYON. . 319
before the meeting of the college, I went to the
president's house in London, to inform him of
what was intended, being desirous that, although
such a notification was not required, it should
not afterwards be said, that an attempt had
been made to surprise the judgment of the col-
lege. I was told there that he was in the
country, at a considerable distance from Lon-
don, but that he was expected to return in a
day or two. Upon this, I wrote a letter at his
house, which I left there, to acquaint him with
the object of my visit. Three days after, how-
ever, I learned that he was still out of town,
and probably would not come to it till the day
preceding the meeting of the college. In con-
sequence of this information, I immediately sent
a letter to him in the country, to make known
what was meant to be done. On the same day
I called upon the officer of the college, whose
business it is to summon the fellows to their
meetings, and authorised him to acquaint those
whom he should see, that I was to be proposed
for examination. I gave the same information
myself to one fellow, my colleague, Dr. George
Fordyce. If I did not give it to more, this was
from fear, lest the doing so might be regarded
as an indirect solicitation of votes. Yet, not-
withstanding all this supererogatory care to ap-
prize the president and fellows of the college
320 LETTER TO
of what was intended by Dr. Pitcairn, they
were bold enough to refuse even to allow his
proposal to proceed to a ballot, on this pre-
tence, among others, that it had not been pro-
perly notified to them.
Amongst the voters against a ballot on the
proposal of Dr. Pitcairn, was Dr. John Burges*,
whose conduct in this matter seems worthy of
particular notice, as he had himself only a few
years before made a similar motion regarding
another licentiate. I dispute not here the claim,
which that gentleman makes to ancient faith,
and purity of manners, and most exemplary
zeal for the honour of the college : but as an
humble inquirer into the principles of human
* I here, and perhaps shall elsewhere, venture to say, upon
which side of a question a particular member of the college
has voted, though it be the custom of tjiat body to collect
suffrages by ballot. But ballots are so little adapted to the
freedom of Englishmen, that they are seldom in this country
attended with the concealment, which is probably derived
from them among the crafty and dissembling Italians. When
votes are collected here, in this way, many of those who give
them openly mention the side which they support; others,
though they do not make a direct confession, yet by the
tenour of their conversation, leave little doubt upon the
same point ; and by these means, as tittle doubt at length
remains in regard to the few, \vho have endeavoured to in-
volve their conduct in mystery. I shall be very ready, how-
ever, to correct any mistake which I may fall into upon this
subject.
LORD KENYON.
nature, I think myself entitled to say a few
words upon his acting so differently at different
times, in circumstances apparently the same.
Travellers inform us, that many of the in-
habitants of the isthmus, which connects the
two great continents of America, labour under
a most strange depravation of sight. When
the sun has arisen above the horizon, and has
enabled other men by its light to pursue their
ordinary occupations, these people become blind,
and retire into caverns and dark woods, there to
pass the day in quiet and repose. But, as soon
as night has descended upon the earth, and the
face of nature is to other eyes covered with
darkness, their sight is restored, and they then
come forth from their hiding-places, to exercise
the labours, and enjoy the pleasures of life.
I know not whether a similar infirmity has hi-
therto been observed in the mental perceptions
of man ; if it has not, I announce the existence
of an undescribed disease, and produce the con*
duct of Dr. Burges in proof of my discovery.
This gentleman, some years ago, saw so clearly
the propriety of carrying into execution the
by-law of the college, for admitting licentiates
to examination, that he proposed Dr. James
Sims, as a candidate under it. To the other
fellows, however, the propriety of the measure
was then involved in so great darkness, that no
Y
322 LETTER TO
one of them could be induced to second his
motion. In 1797, the arguments of the ad-
vocates of the college, and the speech of your
lordship, diffused such light over this subject,
that when another licentiate is proposed for ex-
amination, ten fellows, without the smallest so-
licitation from any person, and in direct oppo-
sition to the suggestions of ancient prejudice,
declare their opinion, that it ought to be granted.
But the light which now enables men of healthy
minds to discern merit in a measure, in which
they formerly could see none, overpowers by its
excess the infirm perception of Dr. Burges. In
this distress, groping in darkness, he begs for
time to consider, whether the by-law for the
examination of licentiates, ought even in any
instance to be carried into effect.
At the time that I was made acquainted with
the fate of Dr. Pitcairn's proposal, I was also
told, that since it had not been put to a vote, it
could not be said to have been rejected, but was
rather to be supposed still lying on the table
of the college. Hence I concluded, that if a
charge were now brought against them of dis-
regard to the decision of your Lordship, they
might attempt to evade it by maintaining, that
the consideration of Dr. Pitcairn's motion had
LORD KENYON. 323
been only suspended. I resolved, therefore, to1
bring their sincerity to trial afresh, by having
myself proposed a second time for examination.
Upon mentioning this determination to Dr. Pit-
cairn, he offered his aid in accomplishing it, by
repeating his former motion in September 1798,
before which, in consequence of what has al-
ready been observed, it could not be received.
During that interval, the college proceeded
to impose a new restriction upon the admission
of licentiates into their body, as if to demon-
strate the truth of the allegation against them,
which had been so scornfully repelled by the
judges of the Court of King's Bench, that their
by-law for the examination of persons of that
class was altogether illusory, and had been
framed with the intention, that no one should
ever be received by it. — -The new restriction
was, that whoever meant to propose a licentiate
for examination, should give notice of this at
a preceding quarterly meeting of the college.
Its professed object was to allowr time to the
fellows for inquiry into the character of the
person to be proposed. The pledge to be given
by a fellow upon proposing a licentiate, the
candidate's residence for at least seven years in
the midst of them, and the interval of a twelve-
month between the first and last ballots upon
his fitness, were consequently declared to be
S24 LETTER TOT
insufficient barriers against the entrance of un-
worthy persons into the corporation: But it
seems to me more difficult to admit that this
was in truth their opinion, than to believe, that
the real object of the new regulation was very
remote from the one exhibited; and what I
shall immediately say, will probably incline
your Lordship to form a similar conclusion.
In the end of June 1798, Dr. Pitcairn, though
much debilitated by a dangerous illness, under
which he had lately laboured, attended at the
college to give notice, that he should in the
following September again propose me for ex-
amination. To this notice he premised, that he
conceived it to be unnecessary, since the merits
of his first proposal had not yet been considered.
But unfortunately for mankind and himself, he
was shortly after again taken ill, and was in
consequence obliged to leave London for the re-
covery of his health, a few days before the time
arrived for making his motion. Previously to
his departure, however, he wrote a letter to
Dr. Baillie, in which, after stating his own in-
ability to propose me, he delegated that office
to him. Accordingly, Dr. Baillie produced this
letter at the meeting of the college in Sep-
tember, and then proceeded to execute his
trust. This was resisted by the same men, who
had opposed the former motion for my being
LORD KENYON. 325
^examined. It was urged -by them, that die new
by-law required the proposal to be made 'by the
vary person who had given notice of it. To
this it was answered, that as the avowed object
of the notice was to allow time for inquiry into
the character of the person to be proposed, the
spirit of the by-law prescribing it had, in the
present ^case, been completely satisfied. And
it was asked, whether a delegation had never
formerly been received, when he who had de-
clared his intention of bringing forward any
measure was prevented 'by illness, or the un-
avoidable duties of his profession, from attend-
ing .at the college to propose it. No reply was
made ; *but a question was immediately put,
whether the present delegation should be ad-
mitted. A ballot being taken, twelve votes
were found against the delegation, and nine in
favour of it.
An attempt was then made to bring in a dif-
ferent way before the college the original ques-
tion of examination. It was maintained, that
the first proposal by Dr. Pitcairn was still upon
their table, as it had never been decided upon,
and that it ought now to receive their de-
termination. The minutes of the meeting in
September 1797, were in consequence called
for and read, upon which it was declared, that
Dr. Pitcairn's proposal had -then been finally
326 LETTER TO
disposed of and rejected. No cloud, therefore,
now hangs over the conduct of the college ;
nothing now intervenes to alter its natural co-
lours, or to distort the light by which it is seen,
Your Lordship, perhaps, notwithstanding the
facts which have been described in the fore-'
going narrative, will scarcely think it possible,
that the college of Physicians should have in-
tentionally violated their engagement, or have
advisedly acted in contempt of the grounds of
a decision in the Court of King's Bench ; and
hence you may imagine, that they were in^
fluenced by some well-founded objection to the
person proposed for examination, though from
ignorance or inadvertence, they might have
given to their conduct the appearance of a de-
sertion of principles, which they were bound to
maintain. Lest, therefore, you should be in^
duced by your ancient respect for the college,
to form an opinion so unfavourable to my cause,
I will now attempt to prove, that no well-
founded objection did exist to my being ex-
amined by them ; I mean, no objection, which
any of those who resisted the proposal for an
examination, would venture publicly to avow.
In such an undertaking I must necessarily
speak much of myself ; but for this I hope I
LORD KENYON. 327
shall readily be pardoned, since I stand now
before your Lordship in the situation of one
accused, and hence acquire a right of pro-
ducing whatever testimony I can collect in my
favour.
By the charter of the college, the qualifica-
tions required for its members, are learning and
good character. In addition to these, a by-law
demands from licentiates that they be of seven
years standing, and thirty-six years of age, be-
fore they can be proposed for admission by
means of an examination. Since, therefore, I
possessed, without dispute, the latter requisites,
all the avowable objections, which, in ordinary
times, could possibly be brought against me by
the college, are reducible to two kinds ; one
containing those which have any relation to my
learning; the other, such as are connected with
my moral reputation.
When I was proposed at the college by Dr.
Pitcairn, all that was asked was, that they would
examine whether I possessed the proper degree
of knowledge for a fellow. Nothing more,
therefore, on this head, could reasonably be re-
quired by them, before the trial, than a strong
presumption of my being able to undergo it.
Now this presumption was manifested to them
in various ways. Their own advocates had as-
serted, in the case of Dr. Stanger, that the
328 LETTER TO
charter of incorporation, though it divided the
physicians of London into two classes, members
of the corporation and licentiates, demanded
however the same learning from both ; and that
the college would act contrary to their duty, if
they gave equal liberty to practise medicine to
descriptions of men possessing unequal degrees
of ability*. But, nine years previously to my
being proposed by Dr. Pitcairn, I had under-
gone the trials of fitness, to which licentiates
are subjected before admission to practise, and
if I may venture to credit what was said by Sir
* This is a dictate of common sense ; but though found
by the counsel of the college, in the charter which was
granted to them nearly three hundred years ago, its justness
was not acknowledged when the late Dr. Fothergill became
a licentiate ; for he was permitted to exercise his profession in
London, under a by-law which declared, that one reason for
constituting a class of licentiates was, that many persons who
were fit to practise medicine, had not, however, sufficient
learning to be fellows. But there is reason to believe, that
the late admission, on the part of the college, of equality in
point of learning between the fellows and licentiates, was
merely to serve a particular purpose during the trial of Dr.
Stanger's cause. For in the testimonials of fitness to prac-
tise, which they give to licentiates, they still refuse to style
them doctors of physic, though they constantly bestow that
title on fellows j and it was, I suppose, in consequence of
this distinction, that a president of the college had the
effrontery to tell a learned professor of Gottingen, when upon
a visit to this country a few years ago, that the licentiates of
the college were not proper physicians.
LORD KENYON. 329
George Baker, and the censors who examined
me, I had passed through those trials with more
than ordinary ease. In the interval, I had be-
come a member of the Royal Society, the cer-
tificate of my fitness for which was signed by
the late and present presidents of the college,
Sir George Baker, and Dr. Gisborne, and by
four others of the present fellows of that body.
During the same interval, I had endeavoured
to extend the boundaries of our knowledge in
various parts of natural philosophy; and two of
my attempts of this kind, certainly not the most
considerable, had been recorded in the printed
transactions of the Royal Society. As I had
thus demonstrated industry at least, in the cul-
tivation of sciences collateral to medicine, it is
not probable that I had been inattentive to the
study of my own profession, since my peace
of mind necessarily depended upon my under-
standing it. Nor had my opportunities of gain-
ing experience in it been very small ; for I had
been eight years a physician to an extensive
establishment for the relief of the sick poor,
and I had also been physician for some time to
another institution of the same kind, but still
more considerable. From all these circum-
stances, I think it will readily be allowed by
your Lordship, that it was not likely I had be-
come less learned since passing the trials of a
330 LETTER TO
licentiate, and that consequently there was a
s.trong presumption of my being sufficiently
learned to be admitted to undergo the addi-
tional tests of knowledge, if there be any such,
which the statutes of the college demand from
those who desire to be fellows. This will be
the more readily granted, when it is considered,
that though the college contains at present
many learned men, and will no doubt continue
to contain many such, as long as the inhabitants
of this country are sufficiently rich to reward
liberally the professional labours of physicians,
yet the degree of knowledge which is just suf-
ficient to enable any person to enter their body,
cannot be regarded, even by themselves, as very
high : For,
First, among the forty-three members who
have undergone the required examinations, how-
ever they may have differed in original talents,
industry, opportunities of studying their pro-
fession and in modesty, there is only one, whose
learning is said to have been declared insuf-
ficient upon his first application for admission :
And secondly, the three physicians, who to
my poor apprehension have appeared to have
the weakest understandings and the smallest ex-
tent of knowledge, of all those with whom I have
happened to converse, either in this or any other
country, are fellows of the college of London.
LORD KENYON. 331
I come now to the objections which might be
brought against me on account of my moral re-
putation.
How far my previous life had entitled me to
a reputation for good morals, it does not be-
come me to say ; and I am for many reasons
unwilling to exhibit the direct testimony of my
friends upon this part of my character. Such
a step, indeed, seems on the present occasion
altogether unnecessary, as I think I can easily
prove, that the majority of those who formed
the meeting of the college, when Dr. Pitcairn
proposed me for examination, did not conceive
me unfit to be received into their corporation,
by reason of my immorality.
It will, I suppose, be readily granted, that as
many, at least, as voted for receiving Dr. Pit-
cairn's proposal, entertained no objection to
me, on the ground which has just been men-
tioned. Now, the numbers on the different
sides of the question, when his proposal was
rejected, having been thirteen and ten, if I can
only show, that two of the thirteen had shortly
before manifested their satisfaction with my
character for morals, the object at present in
view must, in my opinion, be looked upon as
gained.
Dr. Gisborne, the president of the college,
who I venture to maintain voted for the rejec-
tion of Dr. Pitcairn's proposal, some years ago,
332 LETTER TO
as has already been mentioned, signed a certi-
ficate of my fitness for being a fellow of the
Royal Society. Now, as a good moral reputa-
tion is always esteemed there a necessary ingre-
dient of fitness, he must certainly have then
believed me to be possessed of that qualification.
That he professed a similar belief, only a few
months before Dr. Pitcairn proposed me, I can
assert upon the authority of Dr. James Robert-
son, a fellow of the college, at present with his
Majesty's forces in Minorca ; and I take upon
myself to say, that nothing happened in that
short interval, which ought to have lessened it.
Sir Lucas Pepys was another of the fellows
who voted for the rejection of Dr. Pitcairn's
proposition. In the beginning of the year 1797,
I appeared before the Board of Censors of the
college, to complain of irregular conduct in an
apothecary, who was also present to answer to
my charge. Sir Lucas Pepys, then sitting as
president of a court, the members of which are
sworn to do justice, addressed the delinquent in
a grave and solemn speech, in the course of
which he delivered these words : " Dr. Wells
is no mean person; he is well known to the
world both as a gentleman and a scholar."
Whether this opinion be just or not, is at pre-
sent no matter of inquiry. But in tenderness
to Sir Lucas Pepys, acting as a judge, under
the solemn obligation of an oath, it must be
LORD KENYON. 333
supposed that he really entertained it. In like
manner as when I spoke of Dr. Gisborne, I
venture here to affirm, that nothing occurred
in my conduct from that time to the 29th of
September in the same year, which should have
induced Sir Lucas Pepys to alter his opinion of
me as a gentleman.
I might proceed to show, my Lord, that other
fellows of the college refused to receive Dr.
Pitcairn's proposal, upon grounds that had no
connexion with my moral reputation. But, as
what] I have already said appears sufficient for
attaining the end I proposed, I quit with joy a
subject so distasteful, and betake myself to one
more congenial to your Lordship's feelings, the
consideration of the support which was given to
my fitness for being received at the college, by
the characters of him who made, and of him
who seconded the proposal for my being exa-
mined.
One of those gentlemen must already be well
known to your Lordship. I cannot, however,
refrain from saying respecting him, that the son
of the gallant Major John Pitcairn, who died
the glorious and enviable death of a soldier,
fighting for his country, and the adopted son
of the high-minded, upright, and generous Dr.
William Pitcairn, must have every title to the
334 LETTER TO
strictest honour, which inheritance, education,
and domestic example can bestow. But why
do I speak of titles, after his countrymen had
acknowledged his complete possession of that
most invaluable property, and had in conse-
quence, as well as from their high opinion of
his learning and skill, placed him at the head of
the profession of medicine, in the metropolis of
Great Britain*?
He who seconded the proposal, Dr. Matthew
Baillie, is more upon a level with myself, in re-
gard both to age, and length of residence in
London. Somewhat, therefore, of the obscurity
which involves almost every young physician,
may have hitherto concealed him from your
Lordship's notice. But that obscurity is fast
dissipating, and he must soon, my Lord, very
soon, appear to your view, with all the just
* Two circumstances must concur to place a physician at
the head of his profession in London ; 1. Great employment,
which alone, is certainly not sufficient for that purpose, as it
is often possessed by persons of no considerable ability. 2.
Respect from other physicians, indicated by their frequently
requesting his aid in their practice. This can arise only from
a high opinion of his honour and skill, of which qualities in
a physician, scarcely any but those of his own profession have
either opportunities or capacity to judge rightly. Dr. Piteairn,
from the death of Dr. Warren to his own unfortunate illness,
was indisputably the physician in London, in whom those
circumstances existed together in the greatest degree.
LORD KENYON. 335
proportions and accurate lineaments of a man
of integrity, learning, and great professional
skill.
Can it be conceived then, my Lord, that such
men were ignorant of the character of one, with
whom they had been acquainted for many years ;
or, that believing it to be unfit for mixture with
the college, they would yet pledge their own
honour upon its pureness ? None scarcely are
so depraved as to do wrong for its own sake ;
temptation is for the most part necessary to
induce the most abandoned villain to add to
his crimes. Supposing now for a moment, that
Dr. Pitcairn and Dr. Baillie were capable of
being actuated by unworthy motives, they could
not have possibly gained aught by proposing
me. They could not desire to get rid of im-
portunity, for what they did was of their own
free motion ; or to repay favours which had
been received by them, for on the score of good
offices I was already greatly in their debt. On
the other hand, they knew well, that what they
were doing was highly disagreeable to the go-
verning members of the corporation. These
men they were obliged to meet frequently,
either in ordinary society, or in the exercise of
their profession, or at the comitia of the college.
It was, therefore, of importance to their ease
and comfort at least not to offend them. Since,
336 LETTER TO
however, they did offend them, without deriving
the smallest advantage to themselves from their
conduct, they must necessarily have had the
firmest conviction of its rectitude ; and in this
conviction I find the strongest proof I can offer,
that in point both of learning and moral reputa-
tion, I was not unfit to be examined for admis-
sion into the College of Physicians of London.
I have now, my Lord, considered the two
grounds, upon which the college, consistently
with their charter, might possibly have regarded
me as unfit for admission into their body. But
perhaps it will be said, that they drew their
objections to me from a source different from
either of those which have been mentioned ;
that they believed me infected with the mad-
ness of the present times, and desirous of enter-
ing their corporation, for the purpose of assist-
ing more effectually to destroy it, along with
every other ancient establishment in this coun-
try. Such at least were the principles of con-
duct attributed by many of the fellows of the
college to those licentiates, who had engaged
in the scheme of opening the corporation to
every physician of learning and honourable
character. Even after Dr. Stanger's cause was
determined, when apparently no object to be
gained by calumny existed, one of the fellows,
Dr. Robert Bourne of Oxford, a gentleman, as
LORD KENYGN. 337
1 have since known, of great prudence, and of
the mildest manners, and who was then proba-
bly not acquainted with any one of those licen-
tiates, placed notwithstanding a revolutionary
spirit among the reasons which were assigned
by him, in a public oration, for their attempt
to gain admittance into the college. Nothing
can more strongly demonstrate the pains, which
had been taken to propagate such slander, than
its having been received, credited, and still fur-
ther spread by Dr. Bourne.
Opinions, leading to the overthrow of the
monarchical part of our constitution, have long
existed in this country, in a greater or less de-
gree; but since the termination of the grand
rebellion, they have been, till very lately, almost
entirely confined to a few speculative men, who
have shown little desire to gain proselytes, or
in any other way to attempt a completion of
their fanciful projects. -Neither therefore the
college of Physicians, nor, I believe, any other
of our corporations, ever formerly refused to
admit a person among them, merely on account
of his notions of government, provided he had
complied with the forms which were prescribed
by the laws of the country, or their own private
regulations. But the modern holders of repub-
lican principles, if indeed the workers of confu-
sion can be said to possess principles, and if
tends to the misery of the whole can be
LETTER TO
denominated republican, follow a far different
course. They labour with an apostolic zeal to
impress their tenets upon others. No fancy is
so wild as to be refused admittance into their
minds ; and whatever exists there is regarded
by them as a legitimate cause of action. To
employ the influence which they derive from
places of trust under an ancient government,
as a means of subverting it, is with them a
duty ; their great ambition is to show, that they
are ready to sacrifice friends, family, and coun-
try, to obtain their beloved object, the destruc-
tion of order. It appears*, therefore, highly
proper, that the guardians of the different public
establishments^ to whom any discretion is in this
respect allowed bylaw, should resist the entrance
of every person, who notoriously holds opinions
unfriendly to their existence. But though this
be granted, it surely ought not to follow, that
a vague surmise, an unauthorised suspicion of
disloyalty, should operate to the exclusion of
any one from a situation of honour or profit, to
which he is otherwise legally entitled. Envy
and malice in their native forms have considera-
ble influence over human affairs j if permitted
to assume the shape of patriotism, their power
must be irresistible.
Upon what ground the college charged the
licentiates with being disaffected to the consti-
tution of their country, I know riot. It was
LORD KENYON. 339
clearly no proof of their being so, that they
appealed to the Court of King's Bench, for a
rigid execution of a charter, which had issued
from the most tyrannic prince of the despotic
house of Tudor; or that they founded their
title, to what they prayed the court to grant,
upon -the interpretation given to that charter
by your Lordship's immediate predecessor,
Earl Mansfield, certainly no friend to levelling
principles, or to seditious combinations of men.
Perhaps the proof was derived from this cir-
cumstance, that no oiTe of the licentiates who
signed the address to the college, in which they
set forth their right to be examined for admis-
sion into the corporation, either enjoyed, or
expected to enjoy, any professional honour or
advantage directly connected with the present
government of the country. " Is it probable
that these men," the fellows of the college
might say, " who are attached by nothing spe-
cial to the existing constitution, can desire its;
continuance? Our own bosoms declare that
they cannot ; they must, therefore, be labour-*
ing to subvert it." But the pampered Rich
basely deserted his master in the hour of dis-
tress, while thousands of our countrymen, bound
to their sovereign by no other tie than their
allegiance as Englishmen, fought and died in
his defence. From whom did the expiring
340 LETTER TO
cause of royalty in France receive its last sup-
port ? Not from the pensioned courtiers of Ver-
sailles ; but from a Stoflet, and a Charette, men
before unknown, but whom the occasion that
called for their talents formed into heroes;
from the plain and simple inhabitants of Brit-
tany, actuated by no motives but what arose
from attachment to the ancient government of
.their country, and reverence for the religion of
their fathers.
Leaving, however, to more able advocates,
what further defence may be deemed proper
for the other licentiates, who have been charged
with disloyalty by the members of the college,
I shall now confine myself to a special vindica-
tion of my own character from so atrocious a
calumny. If, my Lord, I speak with warmth
upon this subject, I trust that I shall find an
excuse in the energy of your own feelings. He
that is wealthy may be robbed, without knowing
that he has experienced an injury. But the
poor man's all is often included in a single ob-
ject, which, though to other eyes worthless and
contemptible, may be to him the sole spring of
joy and hope. Any attack' upon it excites his
utmost powers of resistance ; its loss leaves him
without bond to the world, or interest in its
concerns. When we read of a rich man's de-
spoiling a poor neighbour of his only property,
LORD KENYON. 341
cc
one little ewe-lamb which lay in his bosom,
and was unto him as a daughter," our sympathy
with the sufferer is nearly as great, as if he had
been a monarch unjustly expelled from his do-
minions. I may well then be allowed to feel
acutely the attempt which has been made to
strip me of almost my only possession, to which
my title is founded upon paternal discipline and
personal suffering, and has been illustrated by
the whole tenor of my life.
I was born, my Lord, in Charlestown, in
South Carolina, but my parents were from Scot-
land. My father, who was a man of observation
and a scholar, though a tradesman, had carried
with him those opinions respecting the kingly
branch of the British constitution, which in the
former state of our parties constituted Toryism ;
and the resistance they met with in a country,
the inhabitants of which were, from their situa-
tion, always somewhat inclined to republicanism,
served only to strengthen them. These opinions
he early endeavoured to impress upon myself*
To remove, however, every fear of my being
infected, from my companions, with the factious
and disloyal principles, which had very generally
pervaded the British Colonies in America, from
the conclusion of the peace of Paris, in 1763,
and to give me at the same time an opportunity
342 LETTER TO
of receiving the elements of a sounder edu-
cation, in other respects, than Carolina could
afford,, he sent me while yet a boy to this
kingdom.
In one of his views he was not disappointed.
For some time after I had returned to Carolina,
to pass a part of my youth under his immediate
care, a paper, called AN ASSOCIATION, having
been offered for signature to all the male in-
habitants of jCharlestown above sixteen years
of age, the subscribers to which bound them-
selves to obey implicitly certain authorities
unconnected with the former government of
the country, I was one of a very few who re-
fused to put their names to it. Those who had
now a legal controul over my conduct, my father
having shortly before fled from Chariest own to
avoid persecution, strongly urged my com-
pliance. They stated, among other things,
that many persons of the most undoubted
loyalty had signed the ASSOCIATION, and that
a continuance in my refusal would expose me
to the resentment of the populace. My answer
was, that men of established reputation might
conceive themselves entitled to a certain lati-
tude of conduct, to which I could not pretend,
who had yet a character to gain ; and that I
was therefore determined, whatever mightf be
LORD KENYON. 343
like event, that my entrance into manhood
should not be marked by what appeared to me
an act of treason and rebellion. I was conse-
quently obliged to leave Carolina, altogether
uncertain of the future means of subsistence;
but I found them here, in the exertions of a
father, who, to supply me with what was neces-
sary for the prosecution of my studies, sub-
mitted to privations ill befitting his age, and
former habits of life. I was in this way enabled
to take the degree of doctor of Physic, at Edin-
burgh, in 1780. Charlestown was now in the
possession of his Majesty's forces, and I returned
to it for the purpose of collecting the scattered
remains of my father's fortune. While there,
though exempted by my profession from mili-
tary calls, I made an offer of my personal ser-
vices to the commandant of the town, the pre-
sent lieutenant-general Nesbitt Balfour, and
was appointed by him an officer in a body of
volunteers, who, by performing a part of the
duty of the garrison, enabled a greater number
of the regular troops to take the field, than
could have done so, without such aid. When
Charlestown was abandoned by the king's
forces, I went to East Florida. Shortly after
my arrival there, apprehensions being enter-
tained for the safety of the province, I requested
permission from governor, now general Tonjn,
344 LETTER TO
to assist in its defence, and received from him,
in consequence, the command of a company of
volunteers, who were to serve without pay.
This company I raised, and kept together as
long as the fears continued, on account of which
it had been formed.
I have thus mentioned, my Lord, some of the
facts which I possess in proof, that my conduct
at least was not, formerly, disloyal. They hap-
pened at a time of life, from the age of eighteen
years to that of twenty-six, when actions are not
often discordant with internal feelings ; when
the veil of hypocrisy is seldom worn, and, if
ever assumed, is soon blown aside by the tem-
pests of passion, which so frequently arise in
that season of human existence. I shall, how*
ever, exhibit more direct testimony that my
conduct and principles were in unison. I shall
produce to your Lordship a profession of attach-
ment to my country and its constitution, which
was 'made by me in the midst of enemies, from
an unwholesome prison, and while threatened
with assassination on account of that attach-
ment. For, going to Charlestown, in 1783,
upon some family concerns, I was arrested there
and thrown into gaol, a few days after my arrival,
in violation of a flag of truce with which I had
entered the country. Such, at least, was the
opinion of governor Tonyn, who had given that
LORD KENYON. 345
flag ; for as soon as my arrest was known in
Florida, he sent a commissioner to Carolina,
Mr. Wyllie, the present chief justice of the
Bahama Islands, to demand my release. In the
mean time, a publication appeared respecting
me, signed by the gaoler in whose custody I
had been placed, which began thus ; " William
Charles Wells, a political sinner of the first
magnitude in this land, and now suffering but
a very small proportion of those pains and pe-
nalties which his high crimes and misdemeanours
have so justly deserved, in the common gaol
of this metropolis," &c. Nature had not formed,
nor had education trained me, to submit with
silence to oppression. By means of money, I
got a letter inserted in one of the Charlestown
newspapers, the following extracts from which
will show to your Lordship, whether my senti-
ments then partook of disloyalty.
Charlestown, in Gaol^ July 17, 1783.
" I left this place in August, 1775; pur-
posely to avoid signing a paper, at that time
handed about under the title of AN ASSOCIATION.
I returned to it in January, 1781, when in pos-
session of the British army, and left it again
with those troops in December, 1782. I am.
346 LETTER TO
I ever was, and I ever shall be, a subject of
Great Britain.
"In what respect, therefore, I can be a ' po-
litical sinner of the first magnitude in this land,'
and what are those ' high crimes and misde-
meanours' which I have committed, I cannot
well conceive. If indeed to wish well to my
country while contending with other powers,
and to be ready at all times to lay down my
life in support of its honour and interests, be
a crime, I cheerfully plead guilty to the charge."
" For a freeman to be deprived of his liberty,
and lodged in a common gaol ; to be kept con-
stantly locked up in a room, whose ceiling is in
that condition that the rain pervades it in every
shower, sometimes in such quantity that it must
be carried out in pails, and whose only window
looks to the north, a quarter of the heavens
from which the wind never blows when the
weather is most sultry, and which not being
glazed, obliges him to exclude the cheerful
light of day, at the same time that he shuts out
the storm * ; lastly, to be without the conversa-
tion of his friends, whom the dread of popular
* Thunder-storms occur almost daily in South Carolina,
in the months of July and August, and almost always pro-
ceed from the north or north-west.
LORD KENYON. 347
resentment prevents from visiting him*; if
these sufferings are but a small portion of what
* However unconnected it may appear with the subject of
this letter, I cannot forbear mentioning the conduct of two
of my friends in Carolina., Mr. John Harleston, and his wife,
Mrs. Elizabeth Harleston, persons of rank and fortune in that
country. I had received many civilities from them during
my stay in Charlestown, while it was a British garrison, and
had on my part, done them some small service. But small
as this was, it sunk deep into their noble natures, and con-
stituted a debt, unused as they were to receive obligations,
which seemed to them inextinguishable. On my return to
Charlestown, with the flag of truce, they insisted upon my
staying at their house j but it was during my imprisonment
that the energy of their friendship was chiefly conspicuous.
No one day of the three months which it lasted passed away,
without my receiving from them repeated instances of kind-
ness, such as I could have expected only from those, who
were bound to me by the closest ties of blood. This conduct
would at any time have merited my warmest gratitude 5 but
when I consider the circumstances under which it occurred,
my feelings altogether unman me. Mr. Harleston's estate
had been heavily amerced by the legislature of South Caro-
lina j and at that period, when the affairs of the state were
regulated by the narrow principles of a petty corporation.,
nothing could tend more to frustrate his hope, that the fine
would be taken off, than his showing attention to any one in
my situation. The reins of government also were then so
feebly held, that the populace almost daily wreaked their ven-
geance upon such as had fallen under their displeasure. One
night, during this anarchy, a mob surrounded Mr. Harleston's
house, threatening to destroy it on account of his behaviour
to me. He was from home $ but his wife, with the spirit and
dignity of a Roman matron, went out to the rioters, and told
34$ £ETtER TO
he is to bear, he can look forward to nothing
but DEATH, as the full expiation of his crimes.
Grant him but the choice of the mode, and he
will thank Heaven for the opportunity of de-
monstrating his attachment to his sovereign:
let but thousands witness that his last prayers
were for his country's prosperity, and it will
afford him more exquisite happiness in the ex-
treme moments of his life, than good men enjoy
when angels sing requiems to their departing
souls/*
them, that her husband and herself had done nothing towards
me but their duty, and that they should not be prevented
from continuing to perform it, by any menace whatsoever.
One of those persons is since dead ; the other still exists an
ornament to her sex. Excellent woman! enjoying in af-
fluence, in the midst of thy children, and their children, the
calm evening of a well spent life, and looking forward with
a firm hope, inspired by our holy religion, to another and a
better state, though thou seemest already to possess as much
of happiness, as is compatible with the infirmity of our pre-
sent natures ; it may yet afford thee some momentary satis-
faction to know, that neither distance of place, nor interven*
tion of time, hath lessened my sense of thine unspeakable
goodness -} and that, at this moment, my bosom heaves and
my eyes drop tears, while I reflect, that without thy tender
cares concerning me, when sick and in prison, and far re-
moved from those, whose duty it was to render me service
under such distress, I might long ago have been numbered
with the dead.
LORD KENYON. 349
The smallest drop of blood may become visible
on the surface of an animal body, and may serve
there some special and useful purpose; sent
back to the heart, it is mixed with such a mul-
titude of similar particles, that all marks of it
as an individual are lost. In like manner,
having returned from the frontiers of the Bri-
tish empire to its capital, I naturally sunk back
into the obscurity, which was suitable to my
condition in life, rendered now still more low
by the poverty, which had been brought upon
my family, by their adherence to a great public
cause. In more happy times, therefore, than
those which have since followed, I could scarcely
have expected an opportunity of demonstrating
a love for my country, otherwise than by a ready
obedience to its laws. In consequence, how-
ever, of the attempts which some men, incited
to deeds of parricide by the example of suc-
cessful crimes in a neighbouring state, have
made to overthrow our ancient constitution,
persons of every rank have within these few
years been called upon to declare their attach-
ment to it. I have gladly obeyed this call ;
and my name appears in the list of those inha-
bitants of London, who signed the declaration
at Merchant Taylors' Hall, in December, 1792 ;
and in that of the same description of persons
.who signed the declaration at Grocers' Hall, in
350 LETTER TO
December, 179-5. More lately, when profes-
sions alone were deemed insufficient for the
public safety, and a demand was made upon
the lovers of their country for their services as
its armed defenders, I obtained the honour of
being enrolled in a body of men, perhaps not
unknown to your Lordship, THE TEMPLE ASSO-
CIATION, and since I have belonged to it, my
exertions to fit myself by a knowledge of mili-
tary exercises, for the great object of its institu-
tion, have not been less than those of many-
members, younger than myself, and probably
not more engaged in other serious pursuits.
It may now appear to your Lordship, that I
have spoken of every possible personal objection
to my being examined for admission into the
college of Physicians. But as pretexts are
never wanting to those who wander from the
path of honour in search of them, I shall take
the liberty of mentioning still another ground,
which I have been told they affected to have,
for their refusing to inquire into my qualifica-
tions. For, Dr. Pitcairn informed me, in the
course of last summer, when it could not be
foreseen, that he would be unable in the ensuing
September to propose me a second time for exa-
mination, that, contrary to his former opinion,
LORD KENYON. 351
he now believed that his intended motion would
be opposed, on this among other accounts, as
he understood, that I had been active in the
late dispute between the fellows and licen-
tiates.
That an individual should lose his title to a
privilege which had been adjudged by a court
of law to belong to a body of men, of which he
was a member, merely because he had lent his
aid towards obtaining that adjudication, may be
perfectly consistent with the notions of right
entertained by the college of Physicians, but
is certainly not so with those of your Lordship.
For if any person had been pre-eminently active
in the dispute alluded to, it was surely Dr.
Stanger, who, by his applications to the Court
of King's Bench, had subjected the college to
considerable trouble, expence, and obloquy 5
and yet your Lordship expressly declared your
conviction of his fitness to become a fellow of
that corporation. My share in the dispute may
be described in a very few words. When it was
proposed to me by some licentiates, with whom
the scheme originated, to assist in endeavouring
to obtain admission into the college by process
of law, if it could not otherwise be gained with
honour, I immediately consented. I was after-
wards appointed one of five to draw up an ad-
dress to the college, and this address Dr. Cooke,
352 LETTER TO
Dr. Stanger, and myself, delivered to the pre-
sident. These were the only parts of my con-
duct, in that undertaking, which can be called
public, except this appellation should also be
given to the subscribing of a small sum of
money towards defraying its expence. My pri-
.vate conduct in it was studiously guarded ; for
as it very soon appeared to me, that the dispute
must be terminated by a court of law, I held
all private discussion of it with the fellows as
useless, and tending only to produce mutual
irritation of mind. I therefore constantly for-
bore to introduce it as a subject of conversation,
in the presence of a fellow. My reserve upon
this point was indeed so strict, that one of that
order, with whom I am more intimately con-
nected than with any other physician in London,
could not refrain from mentioning it to me, at
the same time that he compared my behaviour
in this respect with that of another licentiate of
his acquaintance, who made the dispute a topic
of conversation whenever they met. I mean,
however, only to state, not to extenuate my
conduct ; for had it been as active as that of
Dr. Stanger, I should for this very reason have
thought it entitled to considerable applause.
But I feel ashamed at occupying your Lord-
ship's attention with such trifles. Nothing in-
deed could have induced me to present them
LORD KENYON. 353
to your notice, but the desire of affording you
the most ample grounds for reconsidering the
opinion, which you publicly gave of the col-
lege of Physicians ; and trifles often furnish the
most sure, because the most unguarded, avenue
to a knowledge of the characters of men.
I HAVE thus, my Lord, endeavoured to prove>
that the college of Physicians have not, by
their conduct since the decision of the Court
of King's Bench, in the case of Dr. Stanger,
shown themselves worthy of the high praise,
which you were then pleased to bestow upon
them. But it appears to me, that if your Lord-
ship had minutely examined the materials of
which that body is composed, or had been well
acquainted with its previous proceedings, you
would not have regarded the honour and good
faith of its members, as sufficient barriers
against their acting unjustly towards the licen-
tiates, who should apply for admission into their
corporation.
In this country, the glory of whose legis-
lators has been to view men as they are found
to be by experience, the honour and good faith
of no person are, I believe, ever esteemed by
A A
354 LETTER TO
the law as adequate securities for his acting
justly, when he is tempted to act otherwise by
interest. The judges of our superior courts of
law are selected from a profession, the conduct
of whose members is more open to public in-
spection, and is consequently better known, than
that of the members of any other. No mistake,
therefore, can well occur with respect to the
characters they possessed before their elevation
to the Bench, more especially as few receive
that honour before they are past middle age;
and every one admits, that, in modern times at
least, they have been very generally, if not
always, chosen by the executive power with
the purest intentions. When they afterwards
appear to the world in the exercise of their pe-
culiar functions, the eyes of all men are fixed
upon them. Every part of their conduct is
scrutinized with the utmost care ; by some
whom education and habit have particularly
fitted for this purpose; by others, whose dearest
interests lead them to turn their whole atten-
tion to this single point, and whose disappointed
hopes naturally suggest some fault in those,
who have dissipated their gay dreams, and have
awakened them to poverty and disgrace. Yet
even these men, so formed to their stations, se-
parated by their retired life from many causes
of bias to human opinion, venerated by their
LORD KENYON. 35,>
country if they act uprightly, detested if they
furnish the least suspicion of a contrary con-
duct, possessing their places by the most certain
tenure to persons of honour, receiving for their
labours a fixed and ample reward, and solemnly
sworn to administer justice impartially, are still
supposed liable to be influenced by improper
considerations, and are therefore forbidden to
try a great class of causes, when these occur in
the counties where they were born, or at pre-
sent reside.
If a situation can be conceived in which in-
terest could furnish no temptation to the aban-
doning of duty, or none which might not be
easily resisted, this would surely occur, when we
were charged with the preservation of the life
of some one connected with us by the closest
ties of consanguinity, who from tender years
or imbecility of mind, might be unable to pro-
tect himself. On one side, good faith, honour,
humanity, the claims of blood, would urge us
to the faithful execution of our trust ; on the
other, public execration, eternal remorse, and
disgraceful death, would necessarily present
themselves as consequences of its breach. Yet
our Saxon ancestors, perhaps not less virtuous
than any other .nation in the world, whether
ancient or modern, building their law upon ex^
perience, and knowing hence how unfit men
A A 2
356 LETTER TO
are to resist repeated attacks of interest, where
there is the smallest chance that their yielding
to them will be concealed, refused to commit
an orphan, or person of insane mind, to the care
of the next heir, though he were the nearest
relation.
It would, I think, be difficult, if not impos-
sible, to point out, in any part of the world, a
large body of men, who are more likely, in
their collective capacity, to regulate their con-
duct by the principles of honour and good
faith, than the Commons of the Parliament of
Great Britain ; and yet not many years have
elapsed, since they confessed by their proceed-
ings, that they had often corruptly exercised
the power of determining contested elections to
their House, and by a noble act of general jus-
tice, deprived themselves of the means in future
of violating the rules of right in detail.
Distrust of the virtue of mankind, seems in-
deed to be a leading principle of the con-
stitution of our country. The supreme power
of the state is vested in no one person, or set of
persons , but is broken down into various parts,
which are distributed among different descrip-
tions of men. Each of these, from the original
laws of human nature, aims at its own aggran-
dizement, which the others labour equally to
oppose. From this contention arises the most
LORD KENYON. 357
lovely order ; our public happiness is thus bot-
tomed in our private infirmities, and the sta-
bility of our government is secured by the very
means, which to superficial observers appear
fraught with its destruction.
If therefore it cannot be inferred from the
common qualities of Englishmen, that the col-
iege of Physicians, when under no other con-
troul than that of honour and good faith, will
always act justly, it appears to me that, setting
aside actual experience, the only ground for ex-
pecting such conduct from them must be looked
for in the habits and principles, which physi-
cians acquire in the practice of their profession.
The probability of finding it there shall be my
next subject of inquiry. This perhaps will be
best conducted by considering, in the first
place, the state and estimation of medicine,
when exercised as a gainful art, in ages and
countries different from our own.
When men first begin in any country to prac-
tise the medical art for hire, their knowledge
of diseases, and of the proper modes of treating
them, is necessarily very small. To conceal,
therefore, their ignorance, they affect mystery,
and have recourse to various modes of decep-
jtion. Thus, in all rude nations, physicians
Jaave pretended to use supernatural means in
358 LETTER TO
the cure of diseases ; among those nations in-
deed, the different trades of conjurer and phy-
sician are commonly exercised by the same
person. But such a course of life must debase
the character, in every respect, of him who
follows it. No one can promise to himself,
that he will stop at any certain point in villany.
Temptation solicits him to proceed, and his
powers of resistance diminish as he advances ;
till at length he arrives where honesty and
truth seem no more than scare-crows, set up
by designing men to prevent the weak and
timid from pursuing their own good.
As the knowledge of diseases and their re-
, medies increases, the obtaining of it becomes
more difficult, and from the general progress
of improvement, there are now men who can
estimate the value of the acquisition. Phy-
sicians are therefore less tempted either to con-
ceal their methods of cure, or to pretend to
derive assistance from supernatural agents.
Hence medicine, considered as a gainful pro-
fession, has for the most part been less despised
in civilized, than in barbarous nations. It ap-
pears, however, to have been held in very little
estimation, even by the most polished nations
of antiquity, of which we have any tolerably
well authenticated accounts.
LORD KENYON. 359
In Egypt, a physician, who attempted to cure
a disease by means different from those which
were mentioned in the sacred books, forfeited
his own life, if his patient died. By the con-
fession of Hippocrates, medicine was regarded
by the Greeks as the lowest of the arts. The
oath which he exacted from his scholars, not to
commit some of the vilest crimes, and to keep
secret the knowledge which he should commu-
nicate to them, is a strong proof of the truth of
his observation. With the Greek comic writers,
" a son of Hippocrates/' was a term of derision.
So low indeed was the condition of physicians
in Greece, that Alexander the Great seems to
have been neither affected with remorse, nor
accused of cruelty, for crucifying Glaucus, the
physician of Hephasstion, though the death of
his favourite had been occasioned by his own
imprudence. Many learned men have shown
that, before Julius Caesar, the physicians in
Rome were, for the most part, if not alto-
gether, either freedmen or slaves. Afterwards,
medicine rose there somewhat in esteem, both
from the greater knowledge of its professors,
and the degradation of the former civil distinc-
tions in society ; but it was still attended with
so little respect, that even Galen was afraid
to prescribe some pepper in wine to Marcus
360 LETTER TO
Aurelius, for a pain in his stomach, because it
was too strong a remedy for an emperor.
It forms no argument against the justness of
this statement, either that kings and princes
anciently exercised the medical art, or that
physicians were sometimes held in considerable
estimation by the great. For, in the first place,
there are many arts which adorn those who
cultivate them for their own use or amusement,
or for the benefit of others, but which degrade
the persons that practise them for money. Our
country gentlemen are very desirous of knowing
the diseases of horses, and their remedies : but
the trade of a farrier is with us a very low one.
The talent of singing is much prized by females
of the highest rank ; yet how meanly are those
persons thought of, who gain by it their liveli-
hood ? And secondly, eunuchs, and other men
confessedly of the vilest condition, have not un-
frequently been entrusted with the management
of empires.
Physicians have, in modern Europe, obtained
a higher rank in society, than they possessed
among the ancients, principally however, as it
appears to me, by means entirely unconnected
with the exercise of their profession. For,
upon the revival of a taste for letters in our
western parts of the world, spme persons applied
LORD KENYON. 361
themselves to the study of the ancient writers
upon medicine, with the view of becoming
more successful practitioners of that art, than
those were, who had learned it in the ordinary
manner. But the same skill in languages, which
was necessary for this undertaking, fitted them
also for the acquisition of every other kind of
knowledge, which had been treated of by the
authors of Greece and Rome. They made use
of this advantage, and physicians became noted
for their proficiency in every branch of the
learning of antiquity. This erudition naturally
rendered those who possessed it respectable,
and, by an obvious association, raised their
profession in the esteem of the public. It
produced the same effect in another way. A
tedious and even expensive education was
henceforward deemed requisite for physicians,
which could be borne only by persons of some
fortune, and therefore, less likely to be guilty
of baseness and deceit, than men in the low
condition of the former practitioners of me-
dicine.
The operation of these causes was, in this
country, considerably assisted by the same cir-
cumstances, that have given our merchants and
manufacturers their present place in society ;
and by reason of this combination, its physicians
hold a much more elevated situation than those
362 LETTER TO
of any other considerable nation in the world.
When an English physician travels upon the
continent of Europe, he frequently finds that
his profession, if known, is a bar to his recep-
tion into good company, and therefore very ge-
nerally conceals it.
But, my Lord, though the physicians in this
country have been thus freed from, what may
almost be termed, the necessity, which formerly
existed for using improper means to gain em-
ployment, they are still often strongly tempted
to do wrong in the same pursuit. They are,
indeed so often, and so strongly tempted to do
so, and are at the same time, from the nature of
their profession, so little liable to be prevented
from yielding, by that great guardian of virtue,
public censure, that it seems to me beyond a
doubt, that the body of physicians here must
contain a greater proportion of persons, who
have made undue sacrifices to their rise in the
world, than several other classes of English-
men ; than, for instance, the body of barristers,
with which alone, indeed, it can properly be
compared. What knowledge I have of this sub-
ject, is derived from my residence in London ;
the observations, therefore, which I shall make
upon it are, in strictness, only applicable to the
state of physicians in the capital. Your Lord-
ship, however, will not suppose it my intention
LORD KENYON. 363
to insinuate, that I have not yielded to the
same temptations : Video meliora proboque ; de-
teriora sequor. A soldier may relate the defeats,
as well as the victories, in which he has borne a
share*
The young men, who apply to the study of
medicine in this country, are chiefly of small
original fortune, and the greater part of this is
commonly consumed in their education. Very
few physicians, therefore, when they come to
London to exercise their profession, which, if
they have graduated at either of the English
universities, they seldom do till they are nearly
thirty years old, have sufficient incomes for
living in the manner, which is thought here
becoming the rank of a gentleman. They are
consequently extremely desirous to supply this
deficiency in their private fortunes by the pro-
fits of practice, and their age strongly urges
them against every needless delay in attempting
to accomplish this end. Barristers, from enter-
ing more early into their profession, may witri
less inconvenience wait the gradual approach
of business. These too have frequently, soon
after they commence practice, opportunities of
appealing to the world, in the most honourable
manner, on their fitness to be employed. They
address themselves publicly to men well quali-
fied to judge of their abilities, and upon subjects
364 LETTER TO
of which almost every person understands as
much, as renders him capable of determining,
whether or not they have been rightly con-
ducted. If the exhibition of talents has been
considerable, it is soon very generally known,
and is in a short time followed by an increase
of employment, from the desire of many to
benefit themselves by their assistance. A phy-
sician has no such opportunity of showing the
knowledge which he possesses ; he possesses
indeed, on beginning practice, much less know-
ledge capable of being turned to immediate use,
than a barrister of the same standing, and equal
application. His art is founded upon experi-
ment and observation, and the rules for exer-
cising it are always modified by external cir-
cumstances, which can never be accurately
known, except by one long conversant with
diseases, as they actually occur. Skill in me-
dicine is therefore not to be acquired by reading
alone : whereas law, being a collection of the
opinions and ordinances of men, is necessarily
studied in books ; and hence a considerable
knowledge of it may be obtained by those, who
have seen little 'of its application to particular
cases. Besides, a young barrister does not
appear in the management of any case, until
a considerable time has been spent by him in
preparation^ but the first calls upon a young
LORD KENYON. 366
physician are frequently to oppose sudden at-
tacks of disease, which do not permit his think-
ing long, how this can best be done. For these
reasons, it seldom happens, that physicians
either merit much praise from their first efforts
Jto cure diseases, or quickly acquire a considera-
ble increase of practice from any single display
of great talents. They must consequently be
strongly disposed to adopt other means to raise
themselves to notice.
The present possession of practice being a
considerable recommendation of physicians to
further employment, every young physician
finds an advantage in having it thought, that
his business is greater than it actually is ; and
should he endeavour to impress the public with
such an opinion, the privacy with which the
medical profession is for the most part exer-
cised, prevents any flagrant discovery, that it is
not well founded. Many of them are therefore
induced, notwithstanding the smallness of their
incomes, to imitate the exterior expence of their
seniors, hoping that the world will hence believe,
that they enjoy a corresponding degree of em-
ployment. The business of a. barrister being,
on the contrary, chiefly conducted in open
courts, any attempt to make it appear greater,
than it is in reality, would soon expose him
to ridicule and disgrace* He lives, therefore.,
366 LETTER TO
except his private fortune be large, for man}'
years in Chambers, and goes to Westminster
Hall in a hackney-coach ; whereas a physician,
sometimes immediately upon coming to town,
very commonly only a year or two after, occu-
pies a whole house, and visits patients in his
own chariot. But this expence, though its
object should be ultimately attained, reacts in
the mean time upon the cause which gave rise
to it, and augments in him the necessity for
professional gains.
The female sex, it is well known, have great
influence on the extent of practice which phy-
sicians possess. But, for many reasons, they
are averse to communicate their own complaints
to any one who is unmarried, and they naturally
recommend to others the person whom they
consult themselves. Physicians, therefore, very
generally marry soon after they commence
practice. As they are then far from being
wealthy, if they marry women in other respects
equal to themselves, they seldom receive for-
tunes with them. In this case, the calls for
money increase, for some time at least, more
rapidly than the beneficial effects of their new
situation ; and hence, actions, which were
formerly regarded as contemptible, will now
perhaps seem even praiseworthy, from afford-
ing subsistence to the objects of their most
LORD KENYON. 367
tender affections. If, on the other hand, they
marry rich women, these are commonly unequal
to them in some material circumstance, in age,
education, habits, or personal appearance. But
a sacrifice to interest, in so momentous a con-
cern, is surely no pledge, that they will not
make others of less importance, in the exercise
of their profession. Barristers are much less
exposed to this cause of ill conduct in the pur-
suit of employment. Marriage gives to them
no advantage in it ; and hence, they generally
either enter into that state later in life than
physicians, or remain single to the end of it.
What I have said, my Lord, seems sufficient
to show, that the physicians of London are often
placed in situations, in which temptations to do
mean things for money are known by experience
to act forcibly. But collections of men appear
to be more or less virtuous, nearly in proportion
to the number and greatness of the enticements
to vice, with which they are surrounded. The
principles of honour may, indeed, become more
firmly fixed in the bosoms of some few indivi-
duals of uncommon make, from the very at-
tempts which are made to loosen their hold ;
but- though gold is purified and brightened by
fire, common metals are by the same agent
turned into dross. According to the model of
prayer, which has been given to us by the divine
368 LETTER TO
author of our religion, we are not to petition for
strength to resist temptation ; man's presump-
tuous confidence in his own powers might have
been heightened by such a permission : but we
are humbly to beg our heavenly father not to
lead us into it, hereby confessing our insuf-
ficiency for the contest, whenever it shall occur.
I do not, however, my Lord, wish to convey
an opinion, that physicians become dishonest in
the situations which I have described ; my de-
sign is fully answered, if I have rendered it pro-
bable, by stating the difficulties in which they
are frequently involved, that their temptations
to lay aside the character of men of high ho-
nour, are sometimes too great for resistance*
I now add, that proofs of their actually yielding
to those temptations are furnished by what we
daily hear of their needless visits to sick persons,
their rapacity with respect to fees, and their ser-
vility to apothecaries*. When these, or similar
* The present division of medical practice in this country,
between physicians and apothecaries, did not commence in
London, until some time after the separation of the latter
from the grocers, in l6l7» and was not firmly established,
before the great plague in 1 665, during which, by far the
greater part of the physicians having fled into the country,
the apothecaries were left with almost the entire care of the
sick. These facts were at least advanced in a controversy,
which existed about the end of the last century, respecting
the title of apothecaries to practise medicine, and were not
LORD KENYON. 369
practices have been adopted, they are not often
afterwards abandoned, because the circumstances
then contradicted. To support them, it may be mentioned,
that according to a publication from the college, dated 1698,
the number of apothecaries in London and Westminster,
sixty years before, was not 100, but was then above 800 ;
and that in 1701, they were said to be nearly IOOO, partners
included. At the date of their charter, in 1617, the number
was 1 14 5 so that it must have decreased for the first 20 years
after their separation. This division, however, seems to
have begun more early in some other parts of the kingdom ;
for a physician of Salisbury speaks of it in P566 as being
lately introduced there. Its origin may, I think, be placed
in the greatness of the fees/ which English physicians have
always been accustomed to receive. I find many notices of
an angel, or ten shillings, being the usual fee to them, from
16(55, to the beginning of the present century ; and in 1670,
Dr. Goddard, a fellow of the college, and Gresham Professor
of Physic, asserted, that the fees then given were according
to the ordinary and accustomed rates, time out of mind in
England. Many persons, therefore, who wished to receive
benefit from medicine, but unable or unwilling to fee phy-
sicians so largely, and at the same time too proud to solicit
their gratuitous aid, would naturally apply to those, who
offered both advice and medicines at a cheap rate. This also
seems the chief reason, and not the greater credulity of the
people, why empirics formerly abounded here, more than in
any other country in Europe. For, since the complete esta-
blishment of apothecaries, as medical practitioners, the num-
ber of empirics has been considerably lessened ; the descrip-
tions of men, who on account of cheapness used to resort to
the latter, now applying to the former, for the cure of their
complaints. The existence then of a lower order of practi-
tioners of medicine appears necessary in this country ; and
B B
370 LETTER TO
which gave them origin have ceased. The
pride and delicacy of a gentleman, if once
the attempts of the college to destroy it were as absurd and
unjust, as they were fruitless.
When the division first took place, one of its effects was
probably not foreseen. For apothecaries coming at length
to be employed by many persons who were sufficiently rich
to fee a physician ; when the assistance of one was desired
by these in dangerous disorders, the choice of the individual
was frequently left to the apothecary,, he being supposed
better qualified to make it, than the sick person or his family.
From this time, therefore, the friendship of apothecaries be-
came highly useful to physicians, and was often sought for,
and requited by them, in the most disgraceful manner. I
might bring many proofs of these points from authors of the
last century; but! shall content myself with one, the authen-
ticity of which is beyond doubt, as it is found in an account
of the proceedings of the college, in establishing a dispensary
for the relief of the sick poor, which was published by them-
selves, in 1697- They there say; " Several amongst them
[the apothecaries] set themselves by all the art and industry
they were capabk of to frustrate the whole design ; and find-
ing no method so promising, as to stir up a party among
ourselves, to oppose our proceedings, they fell to intriguing
with several of our own members, ivho were too easily lured
off to seme the apothecaries' interest, for their own private ad-
vantage. And from this cause, as we have too much reason
to believe, have chiefly sprung the unhappy differences,
which are still fomented among us. But notwithstanding
all the discouragements we met with from those of our own
members, who contrary to all the obligations of honour and
conscience,, constantly discovered to our adversaries what-
soever passed in the college relating to this design, and ex*,
posed to them the names of such as were promoters thereof,.
LORD KENYON. 371
surrendered, are scarcely, I fear, ever fully re-
gained. No one, however, who does not com-
pletely possess them, is surely fit to constitute a
part of the sanctuary of honour and good faith.
\
that they might be kept out, as far as in them lay, from all
patients where they should be proposed, and themselves
brought in," &c. The college of Physicians, therefore, a
hundred years ago, were surely not the sanctuary of honour
and good faith ; since one part of them were then declared
by their colleagues to have violated every obligation of ho-
nour and conscience in pursuit of their private interest j while
those, who had thus erected themselves into censors of mo-
rals, openly confessed, that they were afraid to have ft known
they were doing a right thing, lest they should not be called
in by apothecaries to see their patients. Physicians, in ge-
neral, have in the course of the present century become more
prudent, and, I believe, more honourable j but it is, not-
withstanding, very notorious, that many of them at present
cultivate the acquaintance of apothecaries, in ways very
disreputable to gentlemen. Barristers may be tempted,
though, I think in a less degree, for reasons already men-
tioned, to act similarly towards attornies ; but the restraints
upon their yielding, are much greater. Their frequent meet-
ings in courts, and upon circuits, afford many opportunities
of discovering defaulters, and of inflicting punishments,
which few are hardy enough to disregard ; whereas phy-
sicians, having little necessary intercourse with each other,
are consequently in a great measure without the salutary fear
of the reprehension of their equals. In what estimation
would a barrister be held, who should give frequent and
costly dinners to attornies ? But it is said, and I believe
truly, that physicians of great eminence have derived much
of their practice from giving such dinners to apothecaries.
B B 2
372 LETTER TO
But there are various circumstances in the
practice of medicine, unconnected with its
profits, which tend to injure the character of
those who follow it. An action at law remains
at rest, except it be urged forward by human
force, and its termination is induced by means,
which we can easily comprehend. The value,*
therefore, of the talents employed by any one
in conducting it, may be tolerably well appre-
ciated, and the fame which hence arises to him,
is almost always proportioned to his merit. It
is far otherwise in medicine. Diseases proceed
by their own energy, and terminate sponta-
neously, for the most part, in health. Such a
termination, however, of a dangerous disease, if
a physician has been concerned in its manage-
ment, is very commonly attributed to his skill.
He may at first blush at undeserved praise. At
length, from frequent repetitions of it, he often
fancies himself really capable of producing the
eifects, which he hears attributed to his agency.
Again ; should a barrister have any natural ten-
dency to overrate his talents, the frequent mor-
tifications he must experience, in his daily con-
tests with others of his own class, before public
assemblies of men, will soon teach him to value
them more justly. The same corrective is not
applied to physicians. In the exercise of their
profession, they appear always as dictators of
LORD KENYON. 373
rules to others, and the feeling of self-import-
ance, which this situation excites, in time often
diffuses itself over every part of their conduct.
Men too form insensibly an estimate of their
own worth, from secretly comparing themselves
with those whom they see most commonly. But
well-employed physicians spend much of their
time in the company of persons weakened in
mind by disease, and of the female attendants
of sick rooms ; it ought not then to seem strange,
if, like schoolmasters from conversing chiefly
with children, they should acquire an opinion
of their own talents, much higher than what
they merit.
I shall take notice of only one other source
of injury to the character of physicians. Those
among them of the greatest learning and ex-
perience know well, that the most unexpected
changes sometimes take place in diseases, and
are best acquainted with the difficulty of re-
ferring to their proper causes, the various events
that occur in so complicated a structure as the
human body. It might therefore be thought,
that such men would always be modest, cautious,
and even timid, in the practice of their art. But
this is not the conduct which recommends a
physician most. It suggests to a sick person,
what indeed may be true, that a doubt exists
respecting the nature of his complaints, than
374 LETTER TO
which nothing can be more distressing to him.
He often, therefore, applies to one, who ac-
knowledges no difficulty in the treatment of
diseases, who pretends to see clearly what is
hidden from human beings, and who speaks of
uncertain events, as if they were entirely under
his command. In this way, the sick man is
gratified, but too frequently at the expence to
the physician of one of the most valuable parts
of the character of a gentleman, and faithful
observer of nature. The exquisite painting by
Moliere of the vanity, affectation, and pedantry
of the French physicians of his time, exhibits
a resemblance to the general character even of
English physicians of the present day, which is
sufficiently strong .to make it probable, that those
qualities are, in a greater or less degree, almost
inseparably connected with the exercise of the
medical profession. But he in whom they exist,
though he should have the most upright inten-
tions, will often decide as unjustly, when his
own interest or consequence in the world is
concerned, as if he had been actuated by the
vilest motives. Before men, who are not go-
verned by others, can do what is right, they
must first clearly perceive it, which nothing cer-
tainly more effectually prevents, in whatever
has relation to themselves, than a false or ex-
travagant opinion of their own worth.
LORD KENYON. 375
Many of our physicians have no doubt re-
ceived little injury from the causes of the cor-
ruption of character, to which they have been
exposed ; and some few may have escaped their
influence altogether. One of these few, Dr.
William Heberden, I must conclude to have
been well known to your Lordship, from the
eulogy which you pronounced upon him, dur-
ing the trial of Dr. Stanger's cause. He was
probably, indeed, the only physician with whom
you were intimately acquainted, and hence,
from the natural error of attributing to a whole
species the properties of its only individual we
have seen, you might imagine, that he possessed
his many virtues in common with the rest of his
class. But Dr. Heberden, my Lord, stands, in
a manner, alone in his profession. No other
person, I believe, either in this or any other
country, has ever exercised the art of medicine
with the same dignity, or has contributed so
much to raise it in the estimation of mankind.
A contemplation of his excellencies therefore
can afford little help towards obtaining a just
notion of the general worth of physicians. In
speaking of a mole-hill, we would not employ
terms that had relation to the immensity of a
mountain.
Were I, my Lord, possessed of talents ade-
quate to the undertaking, I should here en-
deavour to describe at full length the character
376 LETTER TO
,of that illustrious man. In this attempt, I should
first mark his various and extensive learning, his
modesty in the use of it, and his philosophical
distrust of human opinions in science, however
sanctioned by time, or the authority of great
names. I should then exhibit him in the exer-
cise of his profession, without envy or jealousy j
too proud to court employment, yet underva-
luing his services after they were performed ;
unwearied, even when a veteran in his art, in
ascertaining the minutest circumstances of the
sick, who placed themselves under his care,
taking nothing in their situation for granted,
that might be learned by inquiry, and trusting
nothing of importance that concerned them to
his memory. To demonstrate his greatness of
mind, I should next mention his repeatedly de-
clining to accept those offices of honour and
profit at the British court, which are regarded
by other physicians as objects of their highest
ambition, and are therefore sought by them with
the utmost assiduity. I should afterwards take
notice of his simple yet dignified manners, his
piety to God, his love for his country, and his
exemplary discharge of the duties of all the
private relations in which he stood to society;
and I should conclude by observing, that his
whole life had been regulated by the most ex-
quisite prudence, by means of which his other
virtues were rendered more conspicuous and
LORD KENYON. 377
useful, and whatever failings, he might as a
human being possess, were either shaded or
altogether concealed. After my description
was finished, I should think it proper to say,
that I had never been acquainted with Dr.
Heberden, and consequently could neither be
dazzled by the splendour of his virtues, from
approaching them too nearly, nor influenced in
my opinion concerning them, by benefits he had
already conferred upon me ; and that standing,
as he does, upon the verge of this state of exist-
ence, ready to wing his flight to another of
glory, his ear must now be closed to the voice
of flattery, had he ever listened to that siren,
or were I base enough to solicit her aid, in the
foolish expectation of receiving from him some
future reward.
I think, my Lord, it has now been shown,
that physicians, considered singly, cannot by
reason of the discipline of their profession,
claim exemption from the moral infirmities, to
which the other inhabitants of this country are
subject. Is it then to be supposed, that a body
of them will always be governed by the strictest
rules of justice ? Is it, my Lord, at all consistent
with the experience we have of human actions
to expect, that those, who may have individually
LETTER TO
yielded to temptations of interest, will, when
exposed in a collected state to similar tempta-
tions, continue long to deserve the title of the
sanctuary of honour and good faith?
But perhaps it will be said. here : " Granting
that the college of Physicians, like other men,
are open to the influence of motives, which
pervert or corrupt the judgment, it is yet im-
possible not to believe, that their general con-
duct is agreeable to the common maxims of
prudence. Their reputations must surely be
dear to them ; these therefore they will not
hazard, without the prospect of some advantage
to compensate the risk. But with respect to
the admission of licentiates into their body, the
circumstance which has given birth to the whole
of this discussion, what interest have they in
acting unjustly? Unless then it shall be clearly
established, that they have such an interest, the
attempts which have been made by the author
of this letter to depreciate their character, must
be regarded as the offspring of spleen or disap-
pointed ambition, to bestow upon them no
harsher appellation." Anticipating, my Lord,
these observations, I proceed to reply to them.
In doing this, I shall be led to the last purpose
of my address, namely, to present to your Lord-
ship's view, several proceedings of the college,
previous to the decision of the Court of King's
LORD KENYON. 379
Bench in Dr. Stanger's case, which, if known
or minutely considered by you, might have pos-
sibly induced an opinion respecting the in-
tegrity of their corporate conduct, far different
from what you then so warmly expressed.
In the first place, it will be scarcely denied
by any one, in the least acquainted with me-
dicine as a practical art in London, that phy-
sicians conceive it of much importance to be
fellows of the college. This indeed seems suf-
ficiently proved, both by the eagerness with
which admission into the fellowship has been
sought by some of our most celebrated phy-
sicians, Hunter, Fothergill, and Fordyce, not
to mention other and later names, and by the
obstinacy with which their endeavours to gain
it have been resisted, by those already in pos-
session of the corporation. It will not diminish
the force of this argument to assert, that the
object in dispute was altogether unworthy of
the exertions, to which it gave rise. Men do
not always estimate the value of things, either
according to the profit they produce, or by the
rules which may possibly guide the opinions of
superior beings. What more trite, and, at first
sight, more just subject of ridicule is there, than
the vehement desire which many exhibit, for the
possession of a piece of ribbon of a particular
380 LETTER TO
colour ? Yet this desire exists with persons of the
first talents, fortune, and rank in this country :
te Let school-taught pride dissemble all it can,
<f These little things are great to little man."
Though it be unnecessary, therefore, to pro-
ceed further in proving the value of a fellow-
ship of the college, I shall, notwithstanding,
briefly mention some of the advantages, which
accrue to physicians from possessing it.
There are various offices, lectureships, and
appointments in the college, which are attended
with profit, and are filled by fellows alone. The
emoluments of these, though not considerable,
are still of sufficient magnitude to render them
objects of desire to physicians in the first years
of their residence in London ; and hence, as I
have been informed, they are frequently given
to the younger fellows, with the view of assist-
ing them during that difficult period.
The chief advantages, however, which a phy-
sician enjoys from a fellowship of the college,
are in consequence of his being often placed by
it, in very conspicuous and honourable situa-
tions. Soon after receiving it, he becomes an
examiner of the fitness of other physicians to be
fellows or licentiates ; a visitor of the shops of
apothecaries, for the purpose of inspecting the
LORD KENYON. 381
quality of their medicines; and a commissioner,
under an act of the legislature, for licensing
houses for the reception of lunatics. By these
means, though he may be a very young phy-
sician, he nevertheless appears to the world as
a man of rank in his profession. Such a cir-
cumstance to the greater part of persons must
be highly gratifying, without regard to its con-
sequences. But in medicine, the slightest sign
of distinction is frequently a source of profit to
the possessor ; for as men, in general, have not
sufficient knowledge or discernment to choose
their physicians on the ground of merit, they
commonly take those who exhibit marks of
public approbation and confidence. A fellow-
ship, therefore, by bestowing such marks, is
often greatly conducive to the advancement of
the interests of a physician. It is far indeed
from always happening, that fellows of the col-
lege rise to eminence, as practitioners of me-
dicine 5 but the fact is undoubted, that they
rise to it more frequently and more quickly,
than licentiates in every respect equal to them-
selves, except as to the relation in which they
stand to the college'.
But it is evident that these, and all other ad-
vantages of a fellowship, will be more or less
amply enjoyed by individuals, according as
few or many are entitled to partake of them.
382 LETTER TO
Whether any body of men would be able to resist
such a temptation to restrain the increase of
their number, I know not. It is certain, at
least, that the college have not been so, but have
often adopted measures for this purpose, which
are declared, by persons of the highest au-
thority, to have been contrary to the laws of
our country. " Licences," said Lord Mans-
field, while delivering a judicial opinion upon
the conduct of that corporation, " probably
took their rise from that illegal by-law, now at
an end, which restrained the number of fellows
to twenty. This was arbitrary and unjustifiable ;
they were obliged to admit all such as came
within the terms of their charter."
The effect, which was once derived from re-
straining by-laws, is now produced by means
less odious in appearance, but not less sure in
operation. Though a. degree of doctor in me-
dicine, from Oxford or Cambridge, has been
demanded by the college, almost from its
foundation, as a qualification for a fellowship ;
yet, for a considerable time, it was occasionally
dispensed with, and when it was not, physicians,
who had graduated elsewhere, could for a small
sum of money, readily procure such a degree
from those universities, by incorporation *. But,
* The degrees, which students of Oxford and Cambridge
receive from their own universities, are conferred by creation*
LORD KENYON. 383
towards the end of the last century, laws were
passed by our universities, at the desire, it is
said, of the college, to prevent in future the
incorporation in them of physicians, who had
graduated in any place out of England, except
Dublin; and since then, the college have never,
I believe, admitted any one to an examination
for a fellowship, who did not possess an English
degree of doctor in medicine. The consequence
has been, that the number of members, which in
1677 was sixty-five*, without including twenty
honorary fellows, a class no longer existing, is
now only forty-eight t, notwithstanding the vast
increase, which the capital has in the mean
time received, in point both of population and
riches. But all surprise at this diminution of
the number of members will cease, when it is
known, how greatly that of licentiates has during
the same interval been augmented. In 1667,
but when a graduate from a different university is admitted
in either of them, ad eundem gradum, this is called incorpora-
tion.
* Fifty-three fellows and twelve candidates, who are both,
in the language of the college, named college?. The term
candidate is used in a very different sense by the college from
what is commonly given to it ; with them it means a person
who has passed all the examinations which are required for
a fellowship, but who is not actually in possession of it. I
have for this reason very seldom employed it.
f Forty-five fellows and three candidates.
384 LETTER TO
there were only ten persons in that class; while
the present college list contains one hundred
and five, the far greater part of whom would
have been admitted as fellows, if the English
universities had not repealed their former laws
for granting degrees by incorporation.
The system of admission which has produced
these effects, is that which the college, after
being repeatedly admonished of its narrowness
and injustice by Lord Mansfield, professed to
amend, by the two by-laws already so often
spoken of. That they have an interest, how-
ever, directly contrary to the pretended object
of the new laws, is clear from the tardiness
alone with which these were brought forward.
Lord Mansfield began in 1767 to censure the
old laws of admission, yet the new were not
made before 1778*. The succeeding history
of one of the latter demonstrates the existence
* The college, during the trials of Dr. Stanger's case,
seemed to have been much ashamed of the dates of these
laws. They were not mentioned in Mr. Roberts's affidavit,
and when asked for by the judges, the counsel of the college
appeared ignorant of them. If the omission had not been
by design, they would surely have been inserted in Dr. Gis-
borne's affidavit in answer to Dr. Stanger's second applica-
tion ; but upon this subject he was equally silent with Mr.
Roberts. At length, after repeated questions from the judges
during the second trial also, it was extracted from Mr. Dam-
pier, that the new laws were made in 1778.
LORD KENYON. 385
of the same interest still more strongly. This
at first authorized the introduction, by favour,
of two licentiates every year into the college.
But it was quickly after enacted, that only one
should be annually proposed for introduction ;
and again, that no proposition of this kind
should be made oftener than once in two years.
Such are the changes which the letter of the
law has undergone. If we look to its execu-
tion, it may MOW be regarded as abrogated;
since no licentiate has been proposed under it
for six years past.
But, though the college have thus shown,
that they possess a strong interest in preventing
the increase of their number, from the introduc-
tion of licentiates by favour, it is yet easy to
prove, that they must have a much more power-
ful one, in resisting the entrance of persons of
that class, through the means of examination.
Licentiates made fellows in the former way will
naturally adopt the maxims of their patrons,
with respect to the management of the corpora-
tion ; and even if they should not, they can
never be sufficiently numerous to form in it a
party of any consequence. On the other hand,
licentiates admitted to be fellows of the college,
after an examination of their fitness, would be
free to act in all its concerns, according to their
own views of what was right. They might
c c
386 LETTER TO
consequently dispute both the justice and ex-
pediency of acknowledging in the graduates of
Oxford and Cambridge, any title to be received
into the corporation, which does not depend
upon their learning and good character ; and
their own number might in a few years become
so great, as to exceed that of all the other re-
sident fellows. Can we now even imagine,
that the present fellows of the college, all of
them, except five persons who have been ad-
mitted through favour, physicians from Oxford
and Cambridge, are not generally hostile to a
measure, which, if executed, must immediately
diminish some of their own advantages, and
may hereafter deprive the members of the En-
glish universities of the chief rule in a corpora-
tion, which has long been regarded by them as
their own ?
I have thus, my Lord, replied, and I hope
satisfactorily, to the question concerning the
interest, which the college have in acting un-
justly towards those licentiates, who may apply
to them to be examined for fellowships ; and,
while doing this, I have proved by indubitable
testimony, that even before the decision of Dr.
Stanger's case, they had not always shaped
their conduct by the rules of honour and good
faith. It may therefore be thought, that my
address to your Lordship ought now to close,
LORD KENYON. 387
since its various objects have been attained.
But^ as in my opinion, it deserves to be still
further considered, whether an accurate know-
ledge or estimation of some preceding acts of
the college might not possibly have produced a
doubt in your Lordship's mind, on the pro-
priety of surrendering to them the sole deter-
mination of claims, which they have various
and manifest temptations to determine unjustly,
I shall venture to trespass a little longer upon
your Lordship's patience, by offering a few
additional observations upon this part of my
subject.
The first I shall make is derived from a cir-
cumstance in the general conduct of the col-
lege, of which your Lordship took notice, when
you delivered your opinion upon Dr. Stanger's
second application. On that occasion your
Lordship said : "By what fatality it is, that
almost since this charter has been granted, this
learned body has somehow or other lived in a
course of litigation, I know not ; one is rather
surprised, when one considers, that the several
members of this body, including the licentiates,
the commonalty of this corporation, are very
learned men : and as much as it is not generally
the fruits of learning, at least not the best fruits
of learning, to get into litigation, one cannot
tell how those learned gentlemen have fallen
c c 2
LETTER TO
into so much litigation." The fact here men-
tioned, though highly important, may not to
many, however, appear so surprising as it did
to your Lordship. Learned occupations, by
withholding their followers, for the most part,
from the busy paths of life, necessarily exempt
them from many occasions of dispute, to which
other persons are exposed; but few are more
ready, than literary men, to embrace such oc-
casions of dispute as are presented to them.
In whatever regards the fruits of their mental
labours, this is universally acknowledged to be
true ; the title of genus irritabik, though more
especially given to poets, is found to be ap-
plicable, in a greater or less degree, to every
description of authors. Some of the malevolent
passions, indeed, frequently become in learned
men more than ordinarily strong, from want
of that restraint upon their excitement which
society imposes. Perhaps too, from a well-
known law of human nature, their moral feel-
ings may be less correct than those of many other
men, in consequence of the great and frequent
exercise, which is given to the powers of their
understandings. Physicians, therefore, as men
of learning, have their causes of dissension with
each other ; as men seeking wealth by their
learning, or affectation of learning, they have
many more. The great bulk of mankind being
LORD KENYON. 389
unable to judge of the truth of their dogmas,
or the propriety of their practices, it is very
natural, that a number o£ them should jointly
endeavour to persuade their sovereign, that
they are the only fit persons to take care of
the health of his subjects ; while in truth, the
great object of their combination is to establish
a monopoly of medical employment in their own
favour. This I believe to be the real origin
of our college of Physicians, notwithstanding
the praises which have been lavished upon its
founders. Its charter was granted in the age
of monopolies, when men of much higher rank,
and greater private respectability than phy-
sicians, were eager to obtain them. Some sur-
geons procured, about the same time, a mono-
poly of their profession in London ; but being
less wary than the physicians, or the operations
of their art being more subject to the examina-
tion of the external senses, they were shortly
after declared by an act of Parliament to have
abused their trust most grossly. Though the
college have not experienced a similar disgrace,
the defence of their monopoly has yet involved
them in that constant course of litigation, which
has so much excited your Lordship's surprise.
But had your Lordship advanced a single step
further in this subject, it would certainly, I
think, have occurred to you, that the members
390 LETTER TO
of a body, which for nearly three hundred years
had been almost constantly engaged in law-suits,
were not very fit persons to be entrusted with
the power of deciding on the claims of those,
whom it was their interest to depress. The
frequent appearance of men in our courts of
law, whether as plaintiffs or defendants, is not,
I believe, generally held such a proof of their
virtue, that they are hence thought capable of
exertions of self-denial, to which others of a
more retired life are acknowledged to be un-
equal.
Possibly another source of doubt, respecting
the fitness of the college to execute with fidelity
so difficult a trust, without the inspection or
controul of some superior power, would have
been furnished to your Lordship, by a com-
parison of the circumstances, which precede and
attend the admission among them of the two
descriptions of men, who are entitled to apply
for it. A physician of Oxford or Cambridge,
who possesses a desire to enter the corporation,
has no obstacle to fear to its completion, from
any general prejudice against him in the minds
of those who are already members. He has,
on the contrary, reason to expect, that he will
be received by the body at large with pleasure,
both because he comes from one of their own
universities, and has completed there the course
LORD KENYON.
391
of study, which they regard as by far the most
proper to form a physician, and because his
admission will tend to prevent the necessity of
their adopting persons of a different education,
to render their number sufficient for the cus-
tomary rotation of corporate offices. Nor can
any of the members well entertain a personal
dislike to him, as he has scarcely yet begun
to contend with them for employment. Under
these circumstances he applies to the college,
at any time he finds convenient, for an examina-
tion of his qualifications, which is immediately
granted as a matter of course. The examina-
tion is delegated to the president and the four
censors, who are all chosen to their offices for
only a year, and, to use the language of the
college, " are strictly sworn to do justice." It
is divided by them into three parts, each of
which is .held at one of their separate meet-
ings*, and their decision upon his fitness is
seldom or never formed, until he has been sub-
jected to all the parts. Should the decision be
in his favour, at the next general meeting of
* I know that the president and censors may hold the
examination, if they please, at the general meetings of the
college ; but no instance of their doing so has, I believe, oc-
curred for many years^ and if they were to hold it at those
meetings, none^ except themselves would have a title to de-
termine on the fitness of .the person examined.
392 LETTER TO
the college he is proposed for admission. A
ballot is then taken, and if a majority of the
votes be in support of the proposal, he becomes
a member of the corporation, with the title of
candidate. The whole of these proceedings, in-
cluding the original application, are sometimes
finished in a week or two, and always in less
than three months. After he has been a can-
didate for twelve months, without further ex-
amination, and almost without further cere-
mony, he is received into the order of fellows.
If he has come to London shortly after obtain-
ing a doctor's degree, his admission into the
fellowship almost always takes place, either be-
fore or about the thirtieth year of his age.
I turn now, my Lord, to the licentiate who is
engaged in a similar attempt. Though the col-
lege, from deference to the authority of Lord
Mansfield, have apparently ceased to view an
English degree, as an indispensable part of the
title of a physician to be examined for a fel-
lowship, the prejudices* and interests, which
* Some notion may be formed of the extent of these pre-
judices, from the undermentioned circumstances in the con-
duct of Sir Lucas Pepys, as physician general to the army.
I possess indeed a still more flagrant example of their in-
fluence ; but I prefer the present, as being of a public nature.
Suspicions having arisen in the beginning of the present
war, that the dreadful mortality of our troops in the West
LORD KENYON. 393
dictated their former laws of admission, still
exist with undiminished force. Whenever,
Indies, had, in part at least, been owing to their want of
proper medical aid, it necessarily became an object of great
national concern, that the immense armament, which was
preparing, in 1795, to be sent to those countries under the
command of Sir Ralph Abercrombie, should be provided
with able physicians. In this state of things, Dr. William
Wright of Edinburgh was mentioned to a person in power,
as being well acquainted with the diseases of the West In-
dies j in consequence of which, a gentleman, connected with
administration, authorised a common friend to make him the
offer of being a physician to the armament. Having signified
his willingness to accept this appointment, he was desired
to remain in Edinburgh, until his services should be required.
It is proper to say somewhat here concerning the fitness of
Dr. Wright, for the situation to which he was designed. He
was a fellow of the college of Physicians of Edinburgh j and had
formerly served his Majesty seventeen years, chiefly in the
West Indies. He had, besides, practised medicine in Ja-
maica, while unconnected with the army, for thirteen years,
during great part of which time he was Physician General to
the militia of the island. His talents had not, in the mean
while, been confined to the cultivation of the practical part
of his profession. Having included natural history among
the objects of his study, he had, during his residence in
Jamaica, explored almost the whole of it, in his attempts to
extend the limits of that science, and had in consequence
made many important discoveries of plants, some of which
had been published in the Philosophical Transactions of
London and Edinburgh, and various other works. By these
means, he had become well known to many of the learned
in different parts of the world, and had been admitted a
member of the Royal Societies of London and Edinburgh,
394 LETTER TO
therefore, a licentiate applies for an examina-
tion, a contest in reality arises between the
and several other bodies of literary men. In short, if private
worth, patient industry, diversified knowledge, great general
skill in medicine, and long experience of those diseases in
particular, which attack Europeans in the West Indies, were
qualities to be desired in a physician to his Majesty's forces
there, the fitness of Dr. Wright to be one was most eminent.
To return to my narrative ; in September Dr. Wright
came to London, expecting to receive the promised appoint-
ment immediately upon his arrival ; but he was told at the
Army Medical Board, that, by a rule of Sir Lucas Pepys, it
could not be given to him, unless he hud a licence to practise
medicine from the college of Physicians of London. He declared
his readiness to submit to the forms necessary for obtaining
one ; but these could not be completed before the end of
/December; and the armament it was intended he should
accompany was almost upon the point of sailing. Sir Lucas
Pepys was therefore strongly urged by several persons to
suspend his rule j among others, by two of his own friends,
who told him, that Dr. Wright would certainly be appointed,
whether he recommended him or not. His answer was, he
would never recommend Dr. Wright, and was sure the King
•would not sign his commission. But it was quickly seen, that
he had grossly overrated his consequence. It was indeed
not to be supposed, that a rule of a court physician, whose
connexion with the army had commenced only a year or
two before, by his being placed at once at the head of its
medical department, would long prevent the execution of u
measure, deemed by the ablest judges highly beneficial to
the military service of our country. In October, by the in-
fluence chiefly of Sir Ralph Abercrombie, Dr. Wright was
appointed a physician to the armament, and shortly after
went with it to the West Indies.
LORD KENYON. 395
graduates of Oxford and Cambridge, and those
of the Scotch and foreign universities. But
The only possible ground, upon which Sir Lucas Pepys
could consistently with his duty to the public have formed
his rule, appears to be, that he regarded an examination of
medical ability by men whom he knew, and upon whose
report he could therefore implicitly rely, as a necessary test
of the fitness of those, who were to be entrusted with the
important charge of watching over the health of his Ma-
jesty's troops. But if this be supposed the principle. of his
rule, what must be said of his recommending, notwithstand-
ing, several persons to be physicians to the army, who had
never undergone such an examination ? Perhaps they were
evidently so superior in ability to Dr. Wright, as to justify
even a breach of principle in their favour : — No j they were
young men, who had not yet completed their academical,
education, and who probably had never had the entire ma-
nagement of a dangerous disease committed to their care.
They were, however, Bachelors of Physic from Cambridge.
The degree of Bachelor of Physic is now given at Oxford,
the eighth year after matriculation j about thirty years ago
it was not given till the tenth, but even then, so little know-
ledge of medicine was thought requisite for it, that he who
received it was only said to be admitted, to read the aphorisms
of Hippocrates. At Cambridge, the same degree may be
obtained as soon as the fifth year after entrance is com-
pleted. The candidate first keeps an act ; which consists in
defending two questions, one chosen by himself, the other
by the professor of medicine; but the latter is given when
asked for, however long this may be before the defence is to
be made. The statutes of the university require also, that
the candidate should oppose another candidate for a degree in
Physic; but this is now dispensed with for twenty shillings.
396 LETTER TO
who are appointed to decide it? graduates of
Oxford and Cambridge. The members of the
These ceremonies then have not the least resemblance to an
examination', and no person, I believe., is ever rejected at
them for want of medical learning. It is on the contrary,
well known, that students at Cambridge, to save time, often
take the degree of Bachelor of Medicine, when they have
scarcely entered upon the study of their intended profession,
meaning no doubt to apply to it with great diligence, during
ihcjive years which must afterwards pass away, before they
can receive a doctor's degree. Yet, in the sight of Sir Lucas
Pepys, a Cambridge bachelor of Physic appears fit, without
further trial, to be a physician to his Majesty's forces in the
West Indies, while a man, so gifted and adorned as Dr.
Wright, appears unfit, and is therefore sent by him to be
examined by the college of Physicians of London ! Such are
the grounds upon which the physicians of Scotch and foreign
universities must build their expectations of justice from the
college, when they apply for admission into the fellow-
ship. If it be said, that no conclusion from the conduct of
an individual ought to be applied to the whole body; my
answer is, that the conduct of that individual must, in its
principle at least, be approved by the body at large, since he
is marked by their opinion to succeed Dr. Gisborne, in the
presidency of the corporation.
It may be gratifying to many to know, that by his Ma-
jesty's command, orders were last year issued from the War-
Office, to regulate, in future, the appointment of physicians
to the army j and that, in consequence, it is now no longer
necessary that*- they have licences from the London college,
or degrees from the English universities. Those, who
formerly nominated physicians to the land forces, were
allowed to form their own rules, and a like indulgence was
LORD KENYON. 397
college being thus both parties and judges in
the cause, it will doubtless be thought, that
from respect to their own characters, they have
attempted by every means in their power to
lessen the invidiousness, and even danger of
their situation. Have they truly done so ? No,
no, my Lord. They have, on the contrary,
invented a mode of trial, which places their
adversaries in the most difficult and humiliating
circumstances, and lays themselves open to the
influence of some of the basest passions of the
human mind.
In the first place, before a licentiate is ad-
missible to the examination he desires, it is
demanded by the college that he be of seven
years standing, and upwards of thirty-six years
of age. But a rivalship for seven years with
his judges, for employment, may have excited
considerable animosity against him in the minds
for some years enjoyed by Sir Lucas Pepys. When this was
taken away; some persons thought, that after such a dis-
grace, as they termed it, he would feel himself obliged as a
man of spirit, to resign his office, as he could in no other
way demonstrate the purity, if not the wisdom, of his inten-
tions in framing the rules which had been annulled. Fortu-
nately, however, he has been influenced by no such extrava-
gant notions of personal dignity ; but from unbounded zeal
for his sovereign's glory, and a most tender regard for the
welfare of our gallant soldiers, in eoery part of the world,
still remains Physician General to the army.
398 LETTER TO
of some of them ; and the disgrace of being
rejected at an examination must prove highly
injurious, not only to the reputation, but to the
fortune also of a physician, who has passed his
thirty-sixth year. Such a disgrace may even
more readily befal him than a younger man.
For many things which he formerly learned,
and the knowledge of which is required at the
college examinations, are now unknown to him,
from never having experienced their use in the
exercise of his profession ; and his present oc-
cupations may afford little leisure for regaining
them.
But secondly, the application for his examina-
tion can be made upon only one day in the year,
and it must not even then come directly from
himself; he must find some fellow of the col-
lege to make it for him. As the number of
resident fellows, however, is under thirty, it
may surely happen, that they shall all agree to
regard it as a point of honour not to propose
a licentiate for examination.
Let it now be granted, that a fellow has pro-
posed him ; in this case your Lordship, during
the trial of Dr. Stanger's cause, seemed to think,
from your acquaintance with the pure and ho-
nqurable conduct of the benchers of the inns of
court in similar situations, that admission into
the college must follow of course. But, in
LORD KENYON. 399
truth, he has only gained a title to have a vote
taken by the secret method of ballot at the
present meeting of the corporation, whether
his qualifications for a fellowship shall hereafter
be examined. If a bare majority be against his
being examined, the proceedings are stopped,
and cannot be begun again for a twelvemonth.
I need not, however, point out to your Lord-
ship, how much more likely it is, that a ma-
jority of votes, secretly taken, should appear
against a licentiate before an examination, than
that an English graduate should be rejected by
a similar mode of voting, after he has been exa-
mined and approved by the president and cen-
sors, this being the only time at which tiie latter
is subjected to a general ballot, before admission
into the college.
The examination, which may have been al-
lowed to the licentiate in consequence of the
ballot, is of the same kind as that which an
English graduate undergoes; but the first part
of it is not held till three months after the
grant, and the same space of time is interposed
between its first and second parts, and between
the second and third. In this way, if he is not
in the mean time rejected, he is to be tortured
for nine months with doubt and anxiety respect-
ing its event. All its parts too are held, not at
the private meetings of the president and cen-
sors, as in the case of an English graduate, but
400 LETTER TO
at the public meetings of the corporation ; and
should he, from natural timidity, or from that
embarrassment which every man must feel,
upon personally submitting his talents to the
scrutiny of those, whom he believes to be un-
friendly to his views, appear ignorant of any of
the subjects proposed, no opportunity is allowed
to him, as to an English graduate, of compen-
sating such a seeming deficiency by any after-
exhibition of knowledge. For the majority of
a general meeting must declare their approba-
tion of the first part of his examination, before
he can be admitted to the second ; and of the
second, before he can be admitted to the third.
If every part of his examination has been ap-
proved, and he has thus obtained four majorities
of general meetings of the corporation in his
favour, all of them declared by ballot, three
months afterwards, that is, twelve months after
being proposed for examination, he may be
proposed at another general meeting for admis-
sion, and if the majority is found by a fifth,
ballot to consent, he is then to be received into
the college as a fellow.
These conditions of a licentiate's entry into
the college are contained, I confess, in a by-
law, which your Lordship pronounced to be,
not only free from blemish, but possessed of
such virtue, as to render sound an older by-
law, emphatically declared by you to have had
LORD KENYON. 401
in it the seed of death, before it received this new
infusion of health. I am much inclined, how-
ever, by what has been already mentioned, to
suppose, that your Lordship's opinion was de-
rived from a very cursory view of the subject
to which it relates, and I embrace this conclu-
sion more strqngly, when I consider a further
point of difference between the by-law in ques-
tion, and that for the admission of physicians
from Oxford and Cambridge, the simplest notice
of which must excite disgust and indignation in
every bosom, the least animated by a love of
justice.
The persons, who decide on the examination
of an English graduate, are those to whom it
is committed, the president and censors. The
examination of a licentiate is also committed to
the president and censors, but not its decision.
When this is given, they vote as individuals
only, in a meeting consisting frequently, I be-
lieve commonly, of more than twenty members,
none of whom, except themselves, are under
any other than the ordinary obligations of men
to good conduct, or are even required to be
present at the examination, whose event they
are to determine. But if these obligations have
been esteemed insufficient to ensure justice from
English graduates to one of their own class,
and it has therefore been thought necessary to
D D
402 LETTER TO
delegate the decision upon his merits to five
persons, who are solemnly sworn to the faithful
discharge of their duty, what notion are we to
entertain of the design of the college in com-
mitting the decision upon the merits of a licen-
tiate to the discretion of a general meeting? We
are taught, my Lord, by the slightest experience
in the affairs of the world, to seek for the mo-
tives of men in their actions, when these are at
variance with their words. No credit was ever
given by the Romans to the declarations of
clemency, with which Domitian used to preface
his cruelties, or by ourselves to the robbers and
murderers of France, when they pretended,
that their conduct towards foreign nations arose
from a disinterested desire to give liberty and
happiness to mankind. When, therefore, I ob-
serve, that the college of Physicians have per-
mitted themselves to decide upon the examina-
tions of licentiates, without the restraint of an
oath, at the same time that they strictly swear
those to do justice, who are to decide upon the
examinations of the graduates of Oxford and
Cambridge, I hold myself fully authorised to
infer, notwithstanding any protestation to the
contrary, that their design in establishing this
difference was, to allow room in the former set
pf examinations, if any such should ever take
place, for the operation of principles, the most
LORD KENYON. 403
remote that can be conceived from honour and
good faith.
It will perhaps be expected, that I should
illustrate what I have said upon the theory of
this by-law, by an appeal to the facts which
have relation to it. But scarcely any such
exist. During the nineteen years which inter-
vened between the framing of the law, and the
decision of the Court of King's Bench in the
case of Dr. Stanger, the licentiates had been so
intimidated both by its intrinsic difficulties, and
by the threats of fellows of the college, that no
person who applied under it should ever obtain
what he desired, that only one of them, Dr.
James Sims, had endeavoured to profit by it.
He was regularly proposed for examination by
Dr. Burges, whose motion, however, the college
refused even to consider, on the ground that no
one had seconded it. With what justice or de-
cency this was done, I learn from your Lord-
ship. " He is not to wait to be seconded,5*
your Lordship said, in Dr. Stanger's case, while
speaking of a licentiate in the situation of Dr.
Sims, " the by-law does not require that."
These circumstances respecting Dr. Sims were
mentioned to the court by Mr. Christian, one
of Dr. Stanger's counsel, but, I suppose, in a
manner too unimpressive to fix them in your
Lordship's mind. For had they been present
D D 2
404 LETTER TO
to it, when your decision was given, you would
necessarily have entertained some suspicion,
that they, who had openly violated one part of
a law, were not to be restrained by honour and
good faith from violating any other part of it,
when their conduct should be screened by a
ballot.
The last act of the college, to which I shall
solicit your Lordship's attention, seems alone
sufficient to have demonstrated their total un-
fitness to decide between themselves and other
men, when the only guard against their doing
wrong should consist in their feelings o'f what
is right, Some of the circumstances, indeed,
which I am going to relate, occurred in your
Lordship's presence, in the course of Dr.
Stanger's cause ; and I am not ignorant, that
you then considered them as unconnected with
any serious intention, on the part of the college.
Admitting, however, for a moment, this to have
been the case, surely the system of morality,
which permits its followers to accuse a gentle-
man, by way of joke, of a most disgraceful crime
before the Lord Chief Justice of England, ought
to have no place in the sanctuary of honour and
good faith. But not to dwell longer upon this
argument, I shall, I think, soon convince your
Lordship, that the charge to which I have
alluded was deliberately formed, and seriously
LORD KENYON. 405
urged by the college, with the horrible design
of destroying the character of an innocent per-
son, because he was bold enough to oppose their
injustice.
When a physician is admitted by the college
into the class of licentiates, he gives his promise
or faith, that he will observe their statutes, or
readily pay the fines which shall be imposed
upon him for disobedience*. Sir William
Blackstone, who, I believe, is not generally
reckoned a loose moralist, holds it established,
that, when a penalty is annexed to the non-
compliance with laws, " which enjoin only
positive duties, and forbid only such things as
are not mala in se, but mala prohibita merely,
without any intermixture of moral guilt — the
alternative is offered to every man, *• either
abstain from this, or submit to such a penalty ;'
and [that] his conscience will be clear, which-
ever side of the alternative he thinks proper to
embrace." Possibly some doubt may be enter-
tained of the justness of this doctrine when
applied to laws, which affect all persons equally,
and are made by those who are to be controlled
* The president says to him — dabis fidem, te observaturum
statuta collegii, aut mXiltas tibi contri facienti irrogandas
prompte persoluturum, omniaque in arte medica pro viribus
facturum in honorem collegii, et reipublicse utilitatem — to
which he assents.
406 LETTER TO
by them. But, however this maybe, it is at
least certain, that no doubt can exist, whether
a licentiate is entitled to take either side he
pleases of the alternative, which is offered to
him by the college themselves, not by implica-
tion, but by the most direct and explicit expres-
sion, with respect to the observance of statutes,
made always without his consent, and sometimes
tvith the avowed design of placing him beneath
men, whom the laws of their common country
declare to be no more than his equals. He will
even merit no blame from them, as lawgivers,
by disobeying such of their statutes as forbid
what is evil in itself, provided he immediately
pays the fines which are demanded from him.
iThe blame, which he here incurs, depends upon
his having broken the laws of some far higher
power, those of God or his country. But I fear
I render this subject confused, by holding it up
too long to view. Luminous objects are best
discerned by a single glance of the eye ; if we
suffer our sight to dwell upon them, their very
brightness soon causes them to appear indistinct.
The degree of obedience, which is due by a
licentiate to the laws of the college, being then
so evident, no one can imagine, that it was ever
unknown to the many learned and well-informed
men, who are members of that body. The in-
tention, therefore, of those men, in acting even
LORD KEN YON. 407
for the shortest time, as if it were unknown to
them, could not have been honourable ; but as
they persisted in this conduct for nearly three
years, they must necessarily have been serious.
Shortness of duration is essential to every kind
of joke, whether verbal or practical.
About the middle of 1794, a rumour became
prevalent among medical men in London, that
the college viewed, as a breach of faith to them,
the attempt of certain licentiates to render the
corporate distinctions of their profession ac-
cessible to every physician of sound morals and
learning ; but it was thought by those licentiates
too absurd to be credited. " We know," said
they, " of no statute of the college, by which
we are forbidden to endeavour to gain admission
into it. If there be any such, let it be pointed
out, and let the fine be demanded, which is an-
nexed to our disobedience. Were indeed such
a statute to exist, it would be not only tyran-
nical, but contrary to the laws of our country,
and therefore without force. At all events, to
desire the removal of a grievance can never be
justly held a breach of our promise to the col-
lege. For to what purpose has the Court of
King's Bench been charged with the inspection
and controul of corporations, if applications to
it against the oppression of by-laws can, by
other by-laws, be legally declared violations of
408 LETTER TO
faith in those who seek for relief ?" But they
soon discovered their mistake in supposing that
the rumour must be false, because it seemed to
them absurd ; for in October of the same year,
the accusation which it contained was publicly
brought against them by Dr. John Latham, one
of the fellows of the college. " We are at-
tacked*/5 said Dr. Latham in his Harvei'an
oration, " by ferocious, daring, and obstinate
enemies, regardless of the faith which they have
pledged for the observance of our statutes.
I might complain at greater length of the injury
which they have rashly done us, but liberality
forbids me to say more/'
Flagitious conduct, my Lord, ought, in my
poor opinion, never to pass uncalled by its
proper name. If vice be not termed vice, if
baseness and dishonour be suffered to come
forth into the world, without the mark of in-
famy, we remove one of the most powerful
checks upon the evil inclinations of man, and
* " Hostis — aggreditur, ferox, audax, pertinax, posthabita
fide de observandis [collegii] statutis. — Verum enimvero
tametsi mihi esset occasio querendi prolixius de facta nobis
temere injuria, vetat amplius disserere liberalitas." These
quotations are from the printed copy. The author of this
letter did not hear Dr. Latham deliver his oration, but from
the reports of others he has reason to believe, that the whole
of the abuse, which was then thrown upon the associated
licentiates, has not been printed.
LORD KENYON. 409
indirectly discourage the practice of virtue.
If, therefore, the titles of reproach used by Dr.
Latham had been merited, it would have been
gallant, it would have been praiseworthy in
him to have bestowed them. But to whom
were they applied ? To fourteen persons of his
own profession, all of whom, except one, were
at least equal to himself in every quality and
accomplishment, which physicians are required
to possess. And upon what occasion ? Because
these men had, in a temperate, and even re-
spectful address to the college, set forth their
claims to admission into the fellowship, and
had requested to know, whether they would be
allowed to prove their fitness for what they de-
sired, by undergoing the examinations which
are prescribed for the graduates of Oxford and
Cambridge. This was the only measure they
had hitherto taken for obtaining their object.
Your Lordship will now assuredly conceive,
that such expressions were heard with disgust
by the other members of the college. I firmly
believe, my Lord, that they were heard with
great disgust by some of its members. But the
body at large hastened to adopt them, by so-
liciting their author to print his oration. Happy,
however, would it have been for Dr. Latham,
if their zeal to injure the moral characters of
those, whom they denominated their enemies, had
410 LETTER TO
not blinded them to the danger, to which they
were about to expose the literary reputation of
one of their dearest friends ; if they had not by
their own praises so fanned his desire for general
applause, as to occasion his giving a work to the
world, which sets at defiance every principle of
taste in composition, and exhibits more than a
schoolboy's ignorance of the common language
of the learned.
The next public indication of the plan of the
college to defame the associated licentiates, (for
I purposely avoid mentioning any private proof
of it) was furnished in April 1796, by Sir George
Baker's swearing before your Lordship, that
Dr. Stanger, upon being made a licentiate, had
given his faith, or promise, that he would obey
their statutes. It now became clearly evident
to those, who had watched the conduct of the
college, that they meant to urge this, among
other arguments against the claim of that gen-
tleman, that he was unworthy of admission into
their body, from having, by his present applica-
tion to the court, forfeited all title to confidence
in his future declarations. No notice, indeed,
was taken of this part of Sir George Baker's
affidavit, in the pleadings which immediately
followed ; but Mr. Erskine was the only one of
their counsel, who completed his speech upon
that occasion, and there are strong grounds for
LORD KENYON. 411
concluding, (with which, however, I shall not
trouble your Lordship,) that his omitting to
bring it forward was highly disagreeable to his
employers.
In January, 1797, the circumstance of Dr.
Stanger' s having given his faith to observe the
statutes of the college was a second time sworn
to by their president, and in the trial which
took place in May, Mr. Erskine did not again
neglect to touch upon it. But the whole of this
part of his speech seemed to denote a struggle
between the ingenuous feelings of a gentleman,
and the desire of an advocate to gratify his
clients. " I do not mean to say any thing
offensive to Dr. Stanger ; he will understand
that I am using the words of Lord Mansfield.
— I have done justice to this gentleman, who,
I have no doubt, is a learned man, and a person
of honour and character in his profession."
These were expressions employed by Mr.
Erskine, while speaking of the engagement
under consideration. But as the only possible
view of the college, in producing it to the
court, must have been to pretend that it had
been violated, to call Dr. Stanger " a person of
honour" was directly in opposition to their de-
sign, and plainly demonstrated the aversion of
their principal advocate to lend his aid towards
its completion.
412 LEWER TO
The two advocates of the college, who spoke
next, were silent upon the subject of Dr. Stan-
ger's engagement. But their deficiency on this
point was fully supplied by the youngest counsel,
Mr. Warren. He was the son of one of his em-
ployers, and consequently possessed the most
ample opportunities of being acquainted with
their real motives and views, and as he had
evidently been retained in the present cause,
for reasons unconnected with his general fame,
he must have been strongly disposed to requite
the favour he had received, by doing what he
knew would be most agreeable to them. He
therefore did not inform the court, as Mr.
Erskine had done, that he was not instructed
to make any insinuation against the character
of Dr. Stanger, but boldly and explicitly
charged that gentleman, with " a violation of
something, less formal, but not less sacred, than
an oath." The court now exerted their au-
thority, and prevented his proceeding further
in this strain*. But, my Lord, had the dagger,
* My authority for saying, that Mr. Warren was in-
terrupted in this part of his speech, is the following con-
versation between Lord Kenyon and Mr. Christian, one of
Dr. Stanger's counsel, which took place two days after,
while the latter was replying to the arguments against the
issuing of the mandamus.
Mr. Christian. " An argument was pressed the other day
LORD KEN YON. 413
which he drew from beneath a robe, intended
to give dignity to the assertor of innocence and
right, been even suffered to reach its destined
object with all the force that his arm could im-
part, it would have still struck harmless upon
the armour of honourable reputation, to the
which I was sorry to hear, because it might wound the feel-
ings of a very honourable mind ; it was said that Dr. Stanger
had pledged his faith to observe the statutes."
Lord Kenyan. " That was put an end to immediately as
it was mentioned."
Mr. Christian. " It seemed to be pressed and relied upon
as a serious argument."
Lord Kenyan. " Certainly not."
I must, however, confess, that I see no mark of any such
interruption, in Mr. Gurney's report of Mr. Warren's speech.
I presume, therefore, that the Court's disapprobation of the
shameful attack upon Dr. Stanger's character must have
been expressed by some gesture or look from the Bench,
which, though sufficiently intelligible to Mr. Warren, might
easily pass unobserved by a writer intent upon his papers.
How far his Lordship himself thought the honour of that
physician affected by his application to the court, may be
known from the following passage in his speech at the close
of the trial. " It is fit that I should put the mind of Dr.
Stanger, in case it is in an uneasy situation, in a perfect
state of repose with regard to one thing. Undoubtedly his
moral character is not at all tainted by the application that
is now made. I have not the honour of knowing him ; I
have heard nothing but to his advantage when I have heard
him spoken of, and I dare say all the eulogy, which his
warmest friends could bestow upon him, his character both
as a moral and professional man deserves."
414 LETTER TO
confusion of every hope conceived by the cold-
blooded, corporate cruelty, which had urged
him to the deed.
I cannot forbear making one observation
more upon this atrocious attempt of the college.
Though a licentiate is obliged to give his faith,
that he will observe their statutes, he is never
furnished with any opportunity of learning what
they are. The last printed edition of them is
dated in 1765, and is now so scarce, that many,
I believe I may justly say most, of the licen-
tiates have never seen a copy of it. The code
too, since 1765, has undergone very consider-
able alterations, none of which, as far as I
know, have ever been communicated to the
licentiates. In 1796, Dr. Stanger swore before
the Court of King's Bench, that to the best of
his knowledge and belief, no person could be
admitted into the order of candidates, who did
not enjoy, by birth, all the privileges of a
British subject ; and yet it was afterwards de-
clared by the counsel of the college, that the
statute requiring this condition had been re-
pealed upwards of twenty years. Dr. Stanger
swore also, that he had shortly before applied
to the president and register of the college, for
some information respecting their laws, but that
both those officers had refused to give it to
him. Caligula, among other acts of tyranny,
LORD KENYON. 415
caused several of his edicts to be written in
very small letters, and afterwards fixed in situa-
tions of difficult access, in order that those
who were to be affected by them might offend
through ignorance. His ultimate object, how-
ever, was only to procure the pecuniary fines
which were imposed upon the want of obe-
dience ; when these were obtained, he readily
acquitted the transgressors of all further blame*
Men calling themselves Britons likewise con-
ceal their laws, but, with a refinement in cruelty
beyond the conception even of a Roman tyrant,
declare persons to be infamous, who do not ob-
serve them.
I have now, my Lord, finished my journey
through the dreary waste, which I undertook to
explore. In my progress, no spot of verdure
has been found, upon which the weaned eye
might repose, and scarcely an object of terror
has occurred, to break the flat uniformity of
the scene, one wide expanse of pitiful fraud,
and paltry chicane. My labour has been in-
glorious ; but should it furnish your Lordship
with a more accurate knowledge of the ground
I have passed over, than that which you for-
merly possessed, I shall esteem it most amply
repaid.
That the conduct which I have described
416 LETTER TO
should have been exhibited by men, many, per-
haps all, of whom discharge with propriety the
duties of their private stations in society, is
one of those facts relative to the human cha-
racter, which, however difficult to be explained,
are still unquestionably true. There is a certain
gallantry in doing a wrong thing for the sake
of another, which in some degree lessens the
deformity of the action. The odiousness of
such an action is still further diminished, should
it tend to the benefit of many. If it promises
to promote the interests or happiness of a whole
nation, its name, if not its nature, is often
changed ; and what in private life would have
been denominated vicious, may now be re-
garded not only as pardonable, but even as me-
ritorious. Besides-, the members of corpora-
tions commonly imagine, that they have a right
to do every thing which has been done by their
predecessors, notwithstanding the circumstances
may have long ceased to exist, under which
their ancient rules were established. Again ;
the actions of most persons, when they are not
under the dread of general laws, seem to be
chiefly regulated by the praise and blame of
those by whom they are » immediately sur-
rounded. The peasantry upon our coasts, who
in the ordinary situations of life do not appear
LORD KENYON. 417
to be more depraved than other men, have
often been known to commit, in bodies, the
most detestable' cruelties upon shipwrecked ma-
riners ; and the vilest malefactors often meet
death at the. gallows with the greatest firmness,
if strengthened by the presence and approba-
tion of their former companions. If to such
considerations we add, that no one is personally
answerable for the acts of a corporation, and
that these often proceed from a bare majority,
or a number even less than a majority of its
members, we may possibly obtain from the
whole an explanation, why the public conduct
of the college of Physicians is frequently so
very different from what any one might expect,
who has looked only to the private characters
of some of those .who compose it. But what-
ever opinion may be formed concerning the
grounds of explanation which I have offered,
the fact, to which they are meant to apply,
still rests upon the basis of testimony, and is
laterally supported by innumerable other facts
of the same kind. "All men," said an author,
whose wisdom and eloquence have produced a
change in the state of human affairs scarcely
inferior to any, that has ever been effected by
the arms of a conqueror, but who most un-
fortunately does not live to witness the grati-
tude of the world, for his noble, energetic, and
E E
418 LETTER TO
invigorating exhortations to resistance against
its common and most dangerous enemy, when
almost every one was benumbed by despair,
and sought only to prolong a miserable exist-
ence by base submission ; " all men," said Mr.
Burke, " possessed of an uncontrolled discre-
tionary power, leading to the aggrandizement
and profit of their own body, have always
abused it ; and I see no particular sanctity
in our own times, that is at all likely, by a
miraculous operation, to over-rule the course
of nature." I have thought proper to add thus
much, to free myself from the suspicion of being
actuated, in what I have written, by private re-
sentments against individual members of the
college. If such feelings had ever been pro-
duced in me, it would have become my duty,
and I trust I should have had strength to per-
form it, either to stifle them as unworthy of
life, or to make known their existence, in a
more direct way than the present, to those who
had given them birth.
A more difficult task, my Lord, remains for
me to perform — that of again apologising to
you for this letter. When I began it, my only
view was to acquaint your Lordship with the
event of an application to the college of Phy-
sicians, which had been occasioned by your
advice. But, after I had proceeded some way
LORD KENYON. 419
in accomplishing this design, I thought it might
be both curious and useful to show, that their
rejection of the application was not incon-
sistent, either with the principles which it might
have been supposed would influence a body of
physicians in their situation, or with the actual
tenour of their ^conduct, prior to the decision
of the Court of King's Bench in the case of Dr.
Stanger. I saw, indeed, that such an attempt
would be an indirect attack upon the propriety
of that decision, not as connected with the in-
tentions of those who gave it -9 the honour, and
integrity, and uprightness of English judges, like
axioms in science, are always beyond doubt j
but as far as it was founded in considerations,
the strength, or weakness of which many per-
sons had better opportunities of knowing than
your Lordship or brethren. I imagined, there-
fore, that in making the attempt, I should only
act similarly to one, who applies to a court of
justice for a new trial of his cause, in 'con-
sequence of obtaining new evidence to support
it, or who appeals from the jurisdiction of one
court to that of another; and hence I con-
cluded with some confidence, that the plan of
my letter would be regarded by your Lordship
as blameless. But now that it is finished, I
greatly fear, that the execution will not be
420 LETTER TO
esteemed altogether so ; that, on the contrary,
the liberties of expression in which I have
sometimes indulged may appear to your Lord-
ship, if indeed you should ever bestow a mo-
ment's thought upon the subject, as not a little
reprehensible.
The plainness and freedom of speech, my
Lord, which so remarkably distinguish English-
men, have always seemed to me, not only to
be essentially connected with the existence of
their thrice happy and unparalleled form of go-
vernment, but even to give rise, in great mea-
sure, to some of their characteristic virtues;
among others, to their humanity. I mean not
the humanity which is dictated by policy, or
that which originates in a morbid sensibility in-
capable of bearing the sight of distress ; but
the humanity which is so firmly ingrafted upon
the wild stock of our populace, that the greatest
storms cannot tear it away; the humanity which
withholds our mobs, in their most guilty ex-
cesses, and while maddened by strong liquors,
from the spilling of blood. Hatred and re-
venge spring up in concealment, and must be
nourished by long and painful meditation upon
injuries received, before they can attain any
vigour. But Englishmen, by loudly and fear-
lessly declaring their wrongs as soon as they
LORD KENYON. 421
feel themselves aggrieved, prevent the very
beginnings of those baleful passions, and thus
preserve their hearts always in a condition to
obey the great command of their Maker, to
venerate his image in man. Our climate, my
Lord, may be rude and boisterous, but still it
is free from the hurricanes, which desolate
countries possessing skies, for the most part,
calm and serene. Under the influence of these
opinions, I have long been accustomed to give
free expression to my sentiments upon the con-
duct of other men, and experience of the benefit
hence derived to the health of my mind has
contributed to establish the practice. If, there-
fore, I should be regarded by your Lordship
as having employed too great liberty of speech
in this address, I humbly request that you will
ascribe my fault, either to error of principle,
or inveteracy of habit, but in no degree to any
deficiency of respect for your high station and
character.
I retire at length, my Lord, from your pre-
sence, and at the same time relinquish my
struggle with the college of Physicians. I con-
sider myself now as a veteran in the contest,
and therefore as entitled to repose ;
Spectatum satis, et jam donatum rude.
To those, however, who still combat on the
ACCOUNT
OF A FEMALE
OF THE WHITE RACE OF MANKIND,
PART OF WHOSE SKIN
RESEMBLES THAT OF A NEGRO, &c.
INSTANCES of the absence of the black co-
lour, in the whole or part of the skin in persons
of the negro race, are not very uncommon ; bui
there is, I believe, no one upon record of an
individual of the white race having any part of
the body, covered with a skin similar to that of
a negro. The following account, therefore, of
such an instance, will, perhaps, be acceptable
to the philosophical public. I have been ena-
bled to form it by the permission of Dr. Turner,
one of my colleagues at St. Thomas's Hospital,
into which the person, whose case I am to de-
scribe, was lately admitted by him, on account
of some bodily ailment.
426 AN ACCOUNT OF, &c.
Hannah West, the subject of this account,
was born in a village in Sussex, about three
miles distant from the sea, and is now in the
twenty-thirfl year of her age. Both of her
parents were natives of the same county. Her
father was a footman in a gentleman's family,
and died while she was very young. She can-
not, therefore, remember his appearance ; but
she has never heard, that it was in any way ex-
traordinary. Her mother is still alive, and has
black hair and hazel eyes, but a fair skin, with-
out any stain or mark upon it. West was the
only child of her father ; but her mother, having
married a second time, has had eleven other
children. Nine of these are living, all of whom
are without any blackness of the skin. Her
mother, she says, received a fright, while preg-
nant with her, by accidentally treading on a live
lobster ; and to this was attributed the blackness
of part of her skin, which was observed at her
birth.
West is somewhat above the middle size, is
rather of a full habit, and till she came to Lon-
don from Sussex, which was about four months
ago, always enjoyed very good health. The
hair of her head is of a light brown colour, and
is very soft ; her eyes of a faint blue ; her nose
prominent and a little aquiline ; her lips thin ;
the skin of the greater portion of the uncovered
AN ACCOUNT OF, &c. 427
parts of her body very white ; in short, her ap-
pearance is in every respect, except the one
which has been mentioned, that of a very fair
female of the white race of mankind.
The parts covered by the black skin are, the
left shoulder, arm, fore-arm, and hand. All
these parts, however, are not universally black ;
for on the outside of the fore-arm, a little below
the elbow, a stripe of white skin commences,
about two inches in breadth, and differing in
no circumstance from the skin of the other arm,
which, proceeding upwards, gradually bends
under the arm, and at the arm-pit joins with the
white skin of the trunk of the body. The black
skin, wherever it is contiguous to the white,
terminates rather abruptly, so that its boundary
may always be distinctly traced.
The colour of the black skin is not every
where uniformly dark. Thus, the skin of the
back of the hand, and of the wrist, is marked
by fine lines of a reddish black, which cross
one another at right angles, while the small
rectangular spaces bounded by these lines are
entirely black. Part of the cuticle of the hand
having been removed by exciting a blister, the
reddish lines were found to be the summits of
very thin folds of the true skin, which were raised
above its general level, and were less thickly
covered with the black rete mucosum than the
428 AN ACCOUNT OF, &c.
more depressed parts. Their reddish colour
was, no doubt, occasioned by the external air, as
the skin of the other hand was red from that
cause. All the other parts of the black skin
are fully as dark, as I found on making the
comparison, as the corresponding parts of a
dark negro, and are much darker than those of
many negroes. One part, indeed, of her skin
is considerably darker than the corresponding
part in any negro whom I have seen ; for the
palm of her hand and inside of her fingers are
black, whereas these parts in a negro are only
of a tawny hue.
A considerable part of the black skin is as
smooth to the touch, as the skin of the white
arm ; but the cuticular lines in the black arm,
appeared everywhere stronger to the sight, than
similar lines in the arm of a black man, whose
skin I examined at the same time. In the
greater part, however, of West's black skin,
those lines sink deeper beneath its general sur-
face, than the lines of any other human skin
that I have seen, which was not evidently dis-
eased. These depressions are extremely narrow,
and proceed chiefly in one direction, obliquely
upwards and inwards from the outer part of the
arm. On removing a small portion of the cuti-
cle, they were found to be occasioned by the
sinking down of that membrane between very
AN ACCOUNT OF, &c. 429
narrow and slightly elevated folds of the true
skin, nearly contiguous to one another, which
held the direction mentioned.
A great part of the black shoulder exhibits
a singular appearance; for, near to the back
bone, the skin, over an extent of six inches in
length and two in breadth, resembles a thick
coat of pitch, or black paint, which by drying
had split into a great number of small square
portions. The fissures in the skin are about a
line in depth. Mr. James Wilson, teacher of
anatomy, and fellow of the Royal Society, who
saw this person once along with me, pulled
away a little of this black matter, upon which
several narrow processes of the skin, perpendi-
cular to the plane of the part, became visible.
Winslow says, that the cuticle of a negro is
black, and that the contrary supposition arose
from its tenuity and transparency, in like man-
ner as a thin film of black horn appears almost
colourless. I have found by my own observa-
tions, that this opinion of Winslow is just;
and I found also, that the cuticle of West's
black skin is likewise dark. I may add, that
the nails of her black fingers are darker than
those of the white, and darker also than those
of a negro's hand.
Sir Everard Home, who likewise saw this
person once along with me, thought that the
430 AN ACCOUNT OF, &c.
black arm smelt more strongly than the white.
I made the experiment immediately after him,
and thought so too, But on repeating it several
times with more attention, I could perceive no
difference. It seems to me, indeed, from a
similar experiment made on the arm of a dark
negro, whose appearance did not lead me to
suppose, that he had been very careful with
respect to the cleanliness of his person, either
that all negroes do not possess a strong smell,
or that this does not proceed from all parts of
their skin, since I could perceive no difference
between the odour of his arm, and that of the
white arm of West.
. On the black fore-arm are about a dozen small
hard substances, the largest of which are of the
size of a common pea. Some of them are very
black ; others are less black, and one or two
are of a reddish black colour. I thought, at
first, that they consisted of thickened cuticle,
but I found afterwards, that they readily bled
upon being punctured with a needle.
The upper and outer part of the black arm
has a number of very black hairs growing from
it, some of which are three quarters of an inch
long. The inner part of the arm, which is
equally black, is free from hairs.
The black arm is as firm to the touch, and as
fleshy as the white j and according to the young
AN ACCOUNT OF, &c. 431
woman's own report, there is no difference in
their strength or feelings of any kind.
The last circumstance which I shall mention
concerning her is, that no change has taken
place within her remembrance, either in the
degree or extent of the blackness of her skin.
Two inferences may, I think, be made from
what has been related respecting Hannah West.
The first is, that the blackness of the skin in
negroes is no proof of their forming a different
species of men from the white race.
When a white man is much exposed to the
action of the sun, his skin becomes more or less
brown, and as the intensity of this colour, after
equal degrees of exposure, is generally propor-
tional to the heat of the climate, it has hence
been supposed, that the colour of negroes is
derived from a very great degree of the same
cause. But this conclusion seems to me very
faulty. For, setting aside that a white man,
rendered brown by the sun's rays, begets as
white children as those of another of the same
race, the colour of whose skin had never been
altered, it appears to me probable, from ob-
servations lately made on two negroes, that the
action of the sun tends rather to diminish than
augment the colour of their race. Both of
those persons were born in European settle-
ments, and had been accustomed to have their
432 AN ACCOUNT OF, &c.
bodies clothed, yet, in both, the trunk, arms,
and lower extremities, were considerably darker
than the face, and in one, were somewhat darker
than the hands. But admitting this observa-
tion to be of no force, still it must be granted,
in consequence of what has been said upon the
state of part of West's skin, — that great heat is
not indispensably necessary to render the human
colour black ; which is the second conclusion to
be drawn from the account which has been given
of her.
On considering the difference of colour be-
tween Europeans and Africans, a view has oc-
curred to me of this subject, which has not been
given by any author, whose works have fallen
into my hands. I shall, therefore, venture to
mention it here, though at the hazard of its
being thought rather fanciful than just.
There is no circumstance, perhaps, in which
these two races differ so much, as in their capa-
city to bear, with impunity, the action of the
causes of many diseases. The fatality to Eu-
ropeans of the climate of the middle parts of
Africa, which are, however, inhabited by ne-
groes without injury to their health, is well
known. Let it then be supposed, that any
number of Europeans were to be sent to that
AN ACCOUNT OF, &c. 433
country, and that they were to subsist them-
selves by their bodily labour ; it seems certain,
that the whole colony would soon become
extinct. On the other hand, the greater liabi-
lity of negroes in Europe to be attacked with
fatal diseases is equally well established. If,
therefore, a colony of the former race were
brought to Europe, and forced to labour in the
open air for their subsistence, many of them
would quickly die, and the remainder, from
their inability to make great bodily exertions
in cold weather, and their being frequently
diseased, would be prevented from working an
equal number of days in the year with the
whites. The consequence would be, that with-
out taking farther into account the unfriend-
liness of the climate to them, their gains would
be inadequate to the maintenance of themselves
and their families. They would thence become
feeble, and be rendered still more incapable of
supporting life by their labour. In the mean
time, their children would die from want, or
diseases induced by deficient or improper nou-
rishment, and in this way, a colony of the negro
race in a cold country would quickly cease to
exist.
This difference in the capacity of the two
races to resist the operation of the causes of
many diseases, I assume as a fact, though I am
F F
434 AN ACCOUNT OF, &c.
utterly unable to explain it. I do not, however,
suppose, that their different susceptibility of
diseases depends, properly, on their difference
of colour. On the contrary, I think it proba-
ble, that this is only a sign of some difference
in them, which, though strongly manifested by
its effects in life, is yet too subtle to be disco-
vered by an anatomist after death 5 in like
manner as a human body, which is incapable of
receiving the small-pox, differs in no observable
thing from another, which is still liable to be
affected with that disease.
Regarding then as certain, that the negro
race are better fitted to resist the attacks of the
diseases of hot climates than the white, it is
reasonable to infer, that those, who only ap-
proach the black race, will be likewise better
fitted to do so, than others who are entirely
white. This is, in fact, found to be true, with
regard to the mixture of the two races ; since
mulattoes are much more healthy in hot cli-
mates than whites. But amongst men, as well
as among other animals, varieties of a greater
or less magnitude are constantly occurring. In
a civilized country, which has been long peo-
pled, those varieties, for the most part, quickly
disappear, from the intermarriages of different
families. Thus, if a very tall man be produced,
he very commonly marries a woman much less
AN ACCOUNT OF, &c. 435
than himself, and their progeny scarcely differs
in size from their countrymen. In districts,
however, of very small extent, and having little
intercourse with other countries, an accidental
difference in the appearance of the inhabitants
will often descend to their late posterity. The
clan of the Macras, for instance, possess both
sides of Loch-Duich in Scotland ; but those who
inhabit one side of the loch are called the black
Macras, and the others the white, from a dif-
ference which has always been observed in their
complexions. Again, those who attend to the
improvement of domestic animals, when they
find individuals possessing, in a greater degree
than common, the qualities they desire, couple
a male and female of these together, then take
the best of their offspring as a new stock, and
in this way proceed, till they approach as near
the point in view, as the nature of things will
permit. But, what is here done by art, seems to
be done, with equal efficacy, though more slowly,
by nature, in the formation of varieties of man-
kind, fitted for the country which they inhabit.
Of the accidental varieties of man, which would
occur among the first few and scattered inha-
bitants of the middle regions of Africa, some
one would be better fitted than the others to
bear the diseases of the country. This race
would consequently multiply, while the others
436 AN ACCOUNT OF, &c,
would decrease, not only from their inability to
sustain the attacks of disease, but from their in-
capacity of contending with their more vigorous
neighbours. The colour of this vigorous race
I take for granted, from what has been already
said, would be dark. But the same disposition
to form varieties still existing, a darker and a
darker race would in the course of time occur,
and as the darkest would be the best fitted for
the climate, this would at length become the
most prevalent, if not the only race, in the par-
ticular country in which it had originated.
In like manner, that part of the original stock
of the human race, which proceeded to the colder
regions of the earth, would in process of time
become white, if they were not originally so,
from persons of this colour being better fitted
to resist the diseases of such climates, than
others of a dark skin.
The cause which I have stated, as likely to
have influence on the colour of the human race,
would necessarily operate chiefly during its in-
fancy, when a few wandering savages, from
ignorance and improvidence, must have found
it difficult to subsist throughout the various
seasons of the year, even in countries the most
favourable to their health. But, when men
have acquired the knowledge of agriculture,
and other arts, and in consequence adopt a
AN ACCOUNT OF, &c. 437
more refined mode of life, it has been found,
that an adherence to their ancient customs and
practices will preserve them long as a distinct
race from the original inhabitants of the coun-
try to which they had emigrated. Examples
of this kind are frequent in the islands in the
eastern seas in the torrid zone, where the in-
habitants of the sea-coast, evidently strangers,
are in some degree polished, and of a brown
colour, while the ancient natives, who live in
the interior parts, are savage and black. Simi-
lar facts occur in respect to other species of
animals. It seems certain, for instance, that
fine woolled sheep, like the Spanish, never both
arose and sustained their breed in the northern
parts of Europe ; yet, by care, this feeble race,
after being formed in Spain, has been propa-
gated and preserved in very cold countries.
Thus the late Mr. Dryander, the learned libra-
rian of the Royal Society, informed me, that
the breed of fine woolled Spanish sheep had
been kept perfect in Sweden during a very long
term of years, I think he said a century. I£
then, my memory be accurate upon this point,
we have here an example of a variety of animals,
much more liable to be affected by external
circumstances than the human race, being pre-
served without change, in a country very dif-
ferent from their own, by assimilating their new
438 AN ACCOUNT OF, &c.
state as much as possible to their old, during at
least fifty generations, that is, during a period
equivalent to 1500 years in the history of man.
Hitherto, while speaking of the external ap-
pearance of negroes, I have taken notice only
of their colour. I shall now say a few words
upon their woolly hair, and, according to our
notions of beauty, the deformity of their features.
There are several facts which seem to show,
that these circumstances are somehow connected
with their low state of civilization.
First j the black inhabitants of the Indian Pe-
ninsula within the Ganges, who, compared with
the African negroes, may be regarded as a po-
lished people, have hair and features much less
dissimilar to the European.
Secondly ; Woolly heads, and deformed fea-
tures, appear again, as we proceed further to
the east, among the savage inhabitants of New
Guinea, and the adjacent islands, at the distance
nearly of half of the circumference of our globe
from Africa, and consequently without the
smallest probability of any communication
having ever existed between the two countries.
Lastly ; it appears probable from the reliques
of ancient art, that the early inhabitants of Egypt
were of the negro race. If, then, the negroes
of Africa were ever to be civilized, their woolly
hair and deformed features would, perhaps, in
AN ACCOUNT OF, &c. 439
a long series of years, like those of the Egyp-
tians, be changed. On the other hand, their
present external appearance may possibly be
regarded not only as a sign, but as a cause of
their degraded condition, by preventing, in
some unknown way, the proper developement
of their mental faculties ; for the African ne-
groes have in all ages been slaves ; and the
negroes in the eastern seas are in no instance,
I believe, masters of their handsomer neigh-
bours, but are in many places in entire subjec-
tion to them, though the latter be frequently
less numerous.
It will no doubt be objected, to what I have
advanced respecting the difference of colour
between Europeans and Africans, that the
Indian inhabitants of the greater part of the
immense continent of America have skins
nearly of one hue. Plausible reasons may, I
think, be given for this fact, consistently with
what has been said* upon the colour of the two
former races; but I forbear trespassing any
longer upon the time of the reader, in discuss-
ing a subject which admits only of conjectural
reasoning.
THE END.
LONDON :
T. DAVISON, LOMBARD-STREET! WHITEFRlARS.
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