Full text of "Report on the preparations for, and observations of the transit of Venus, as seen at Roorkee and Lahore, on December 8, 1874"

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REPORT

THE PREPARATIONS FOR, AND OBSERVATIONS OF

THE

TRANSIT OF VENUS,

AS SEEN AT ROORKEE AND LAHORE,

On December 8, 1874.

i 1 1 -^ .'A ',' -..' ■,)

^v-

Colonel J. F. TENNANT, R.E.,

CALCUTTA:
OFFICE OF THE SUPERINTENDENT OF GOVERNMENT PRINTING.

1877.

PREF/A-CE. \.v

'T^HE printing of these Reports has, be«n Jelay^<i by various causes
mostly beyond my control, among which was my transference to
Calcutta before it was revised.

It will be seen that I have departed in one point from Sir George
Airy's instructions. I have left all the equations with seconds of space
as a unit, instead of taking the last step, and making the unit a second of
observed time. In all the calculations connected with the observations
near contact, I have felt that except in the contacts themselves, the pro-
bable error of observed time was of no importance, and that that of space
alone affected the result ; in the case of the Alt-Azimuth Observations of
course it was the errors in space which were sought. Though I had
at one time intended also exhibiting these results here as proposed by
Sir G. Airy, I have finally left them, for uniformity, as they stand in my
papers.

The original memoranda of Observations, and one copy of the Com-
putations I shall, as early as possible, transmit to the Royal Observatory,
the other copy of Calculations I propose to retain.

J. F. TENNANT, Col, R.E.

H. M/s Mint, Calcutta, 1
May 21, 1877. J

REPORT

THE TRANSIT OF VENUS.

On December 8, 1874.

ERRATUM.

The reader is requested to correct the following errors which have been overlooked:-
Page 2, line 5 from bottom for " of " read " and "
II, line 5, for "flexture ' read ' flexure "
27, line \i,for " and find " read " which give "

.. ^.w««w AMOMuuMUi»| wmcn were atterwards to
be transferred to a Solar Observatory.

A copy of the British Association's resolution having come into my
hands, I was able to join their authority to that of the Astronomer Royal
in addressing the Secretary of State, which I did on September 12. The
result was a reference to India, and after a long time it was determined
to procure the instruments for the Transit of Venus and Solar Observation.
The Photoheliograph alone was first ordered ; but the delay had made
this too late, and it was only by Dr. De la Rue's kindly giving up an
instrument he had commissioned for his own use that it became possible
to provide for photogjraphic observation. The other special instruments
were not finally ordered till the early part of 1874. Their manufacture
was superintended by Colonel Strange, who placed them in the hands of

2 REPORT ON THE TRANSIT OF VENUS.

Messrs. Cooke of York. Probably instruments in hand were completed
for the occasion ; but the short period available for preparation makes the
fact that all were ultimately used in December, very creditable to those
concerned, and fully accounts for the small defects, which were unavoidable
under «uch circumstances of hurry.

When it was determined that I should be employed to superintend the
observations, my first care was to select positions for the observing sta-
tions. The Punjab is often cloudy in December, and thus it was desirable
not to go too far west. Then it was absolutely necessary that there
should be some facilities for altering and repairing instruments which one
could hardly expect to find perfect in all respects, and (not knowing what
were coming) I contemplated getting what I could, and making the necessary
adaptations. As there was no hope of any of the party being specially
trained at Greenwich, it was considered necessary to provide that the
instruments should (if received) be in position for some time, and no
temporary structures could possibly have stood the burning heat of an
Indian hot season, and the heavy rains which follow. Hoping, too, to use
such parts of the Observatory as could be moved in the proposed estab-
lishment for Solar Physics, I was led to contemplate a much more substan-
tial style of Observatory than that sent with the English expeditions, and
one which would require some workshop facilities for its completion.
I, therefore, selected Roorkee, a station to which I was called by other
duty, for my main station, and I resolved, if possible, to send an oflicer to
some station to the West to observe the last contacts of Venus with the
Sun, and to use for this purpose a 6-inch telescope by Mr. Simms which
Government had enabled me to purchase in 1871 for the Eclipse observa-
tions at Dodabetta. If the specially-provided instruments had not arrived,
I should, by giving up the western expedition, still have been in a position
to make some observations at Roorkee. These arrangements having
been approved by Government, I waited for information as to the forms
of the new instruments before proceeding to design cover for them, but
at last (without this) I was obliged, in November 1873, to submit plans of
estimates for the proposed Observatory, which were sanctioned. I did not,
however, begin building (though the revolving domes were ordered) till
February 1874, still waiting for such details as might guide me. Shortly
after I had begun, I heard that the instruments had at last been ordered,

PRELIMINARY j

but many points were still uncertain, and I was finally obliged to accom-
modate the clock and the Photoheliograph by placing large heavy stones
on the tops of the brick pillars I had prepared, which were too small.
Fortunately, with these exceptions, the arrangements proved convenient.

The general plan may be seen from the drawings. The piers for the
instruments are of brick-in-mortar as good as I could procure ; each is
capped by a stone to carry the instrument. The walls up to the level of
the plinth are of similar construction, as also those of the round rooms
containing the instruments used in observing the actual Transit. The
walls of the rectangular building above the plinth are of brick laid in
mud and plastered outside with mortar and inside with earth, which gives
the Transit room the pleasantest color for working in that I know of.
The roof of this is a flat brick arch, tied at intervals with wrought-iron
rods. I adopted this construction on the recommendation of the Super-
intendent of the local workshops, who also sent men to put it up. It is
cheap, and in this case proved weather-proof, which is not its usual
character. The floors are all of wood and rest entirely on the walls,
leaving a space of from one-half to three quarters of an inch all round
the pillars.

The construction of the rotating domes was the source of very
serious delay. The middle of September came before even one was
thoroughly out of hand in a barely workable condition, and it was only
on the 1st October that all three were ready. The domes consist of
wooden frames on cast-iron curbs covered with zinc and lined with
wadded cloth to keep them fairly cool ; they are each turned by a handle,
which causes a pinion to act on teeth cast in the upper curb. They have
given a great deal of trouble from the hurried way in which they were
finished, but I believe would be quite efiicient if taken down and properly
put up.

I have now to speak of my assistants. Captain Campbell, R.E.,
of the Great Trigonometrical Survey, was placed at my disposal by
Colonel Walker, the Superintendent: he had to go to England for a
short time in the end of 1873, and advantage was taken of this to employ
his services in examining and verifying the instruments : his knowledge
of them thus acquired was of essential service to me when they had
hurriedly to be put together, and I in every way benefited by his skill and

4 REPORT ON THE TRANSIT OF VENUS.

experience. Eariy in 1874, I had borrowed the services of three men of
Her Majesty's 55th Regiment, whom I purposed instructing in Photography,
and I had intended exercising the general superintendence of this depart-
ment myself, believing that all work having been reduced to a routine, and
the instruments having been tried and put in thorough order, I should
have no difficulty in arranging any small assistance. The great delays of
which I have spoken, however, made the success of this plan doubtful,
as it gradually became evident that, instead of each instrument succes-
sively being adjusted and studied, all would have to be taken in hand at
once. I was, therefore, glad to learn that my old friend. Captain Water-
house, would, with the permission of Colonel Thuillier, R.A., (the Sur-
veyor General) be able to join me ; and one of my trained Photographers
wishing to leave me at the last moment, I was glad to borrow one of the
nlen from Captain Waterhouse's office- The Roorkee party then stood
as follows : —

Captain Campbell, R.E.
„ Waterhouse.

Sergeant Harrold, R.E.

Lance Corporal George,

Private Fox,

and I had a small establishment of natives to aid in the various duties.

The work was divided as follows : —

Time determination Colonel Tennant.

Equatorial observations .... „ „

Alt-Azimuth Captain Campbell.

Photoheliograph ,, Waterhouse.

A day or two before the Transit, Captain Heaviside, R.E., came to
Roorkee and kindly undertook to look after the Chronograph during the
observations, winding up the used records strips and putting in new ones
if necessary : he was also to observe the last contacts with the Royal
Society's telescope, by Slater, which he had brought with him.

My western station at Lahore was taken by Captain George Strahan,
R.E., also by permission of Colonel Thuillier ; he was sole observer.

I shall in this Report deal with the Roorkee observations as made by
myself and Captain Campbell, and those made by Captain Strahan at

> H. M.'s 55th Regiment.

DESCRIPTION AND POSITION OF ROORKEE. e

Lahore, and I shall add a statement of the results deduced from Mr.
Hennessy's observations at Mussooreei which I have reduced for compa-
rison, though they were in no way under my superintendence.

The whole of the photographs with all data for their reduction have
been transmitted to the Astronomer RovaLi to be dealt with like those
from other parties; and I have furnished all necessary information for
reducing them. I must leave the discussion of their results for the Astro-
nomer Royal, and have touched on this work solely to place on record
suggestions for future work.

DESCRIPTION AND POSITION OF ROORKEE.

Roorkee is a small station in the North- Western Provinces, about
20 miles easterly from the Railway Station at Saharanpore. It is near
the eastern edge of the high land included between the beds of the
Ganges and Jumna : immediately to the north-east is the bed of the
Solani, a torrent that issues from the Sewalik range and terminates in
the Ganges, and over which the Ganges canal is passed by a large aque-
duct to which Roorkee owes its existence, as, during the construction it
became the head-quarters of the Canal Department in the North-West-
ern Provinces, the site of extensive workshops, and of a College of Civil
Engineering : it has since been enlarged by becoming a small military
station. Over the bed of the Solani from N.N.W., nearly to the east, the
Sewalik ran^e and the Himmaleh are seen in clear weather ; some of the
great snow peaks rising high into the air. The geodesical position of
Roorkee, therefore, is not likely to agree with the astronomical determin-
ation.

In the cold season of i873<»74, Colonel Walker, R.E., Superintendent
of the Trigonometrical Survey, detached an officer of his department for
the purpose of connecting some points in Roorkee with the principal
triangulation of the survey that through them I might connect the observ-
atory with it. I found that connection by triangulation was impossible,

6 REPORT ON THE TRANSIT OK VENUS,

owing to trees ; but I made a traverse from Koorkee Great Trigonometri-
cal Survey Station, past the site of the Observatory and terminating in
the College dome. By this I find that the centre of my Transit pillar
is 4524*3 feet distant from Roorkee Station at an azimuth of 181* 13' 58" :
whence its position by the elements of the Great Trigonometrical Survey
are (when reduced to Madras taken at its present accepted value as an
origin of longitude) —

N. Lat. ... 29** 51' 5o*-95.

E. Long. ... 77" 5a' 49*'i = 5A. iiwi. Sis'j.

The Photoheliograph and Alt- Azimuth pillars are each 19 feet
south of the Transit pillar, and the Equatoreal 53 feet South. I have
deduced from these data for the geocentric latitude of the equatoreal
29"* 41' 57''*9, and have thought it sufficient to retain the same value for
the Alt- Azimuth.

In order to reduce the Alt-Azimuth observations, it is necessary to
have the astronomical latitude. This was accordingly determined by
Captain Campbell with the 36-inch theodolite by circum-meridian alti-
tudes of Nautical Almanac Stars with the following results : —

By North Stars. By South Stars,

y Ursae Majoris, ... 29** 51' SS'^'Ss 8 obs. a Hydrae ... 29* 51' 33*'73 4obs.
^ „ „ ... 34-316 „ a Leonis ... 35 '08 10

33 926 „ p „ ... 32-35 4

8 Crateris ... 33;50 6

* 99 99

9t
9t

Mean by North Stars 29* 51' 33*'-9S Mean by South Stars 29** 51' 33**67

General Mean 29* 51' 33**8i N.

I have used 29^ 51' ss^-S for the reduction.

Of course, the longitude is also affected by the attraction of the
mountain ranges. Provisionally I assume it in the calculations as being
5A. : iitn. : 31-005. east, but I have been enabled, by the co-operation
of the Telegraph Department and Mr. Pogson, Director of the Madras
Observatory, to determine directly, with sufficient accuracy for my purpose,
the difference of longitude between Roorkee and Madras. A line of

DESCRIPTION AND POSITION OF ROORKEE. W

wires having been laid by the Field Telegraph Department of the Sappers
from the Observatory to the Telegraph Office in the station, was there
joined on to the main line, so that signals could be sent directly from the
Observatory to Agra where they were translated on to Bombay, and then
again translated to Madras. The best plan would, of course, have been
to have recorded the signals at each end on a chronograph also used for
transits, but I found it impossible to make these arrangements and the
following was the procedure. The signals at Roorkee were made by
tapping an ordinary key for every loth second of the sidereal clock for
three minutes, and received by Mr. Pogson at Madras (who preferred his
ear to a chronograph) , and who used his mean time clock of which he
considered the error and rate to be well known. The signals at Madras
were made in the same way, but I arranged that the same pricker of
my chronograph which was used in transits should be worked by its own
battery when the current from Agra passed through a relay in the Observ-
atory ; they were thus automatically recorded. Two exchanges of signals
were made on each night, but some were missed from various causes,
though very few. The results are as follows : —

On May 31, 1875, I received from Madras in all 37 signals, one
being missed, and Mr. Pogson also received 37 signals, one being lost.
On June 2, I received 35 signals and Mr. Pogson 38^ and on June 5,
we each received the full number of 38.

The deduced longitude is : Roorkee { J^^^ ^i, 187S ... 9^- : 26^8..
West of Madras j^ ' "

Mean, Roorkee West of Madras gm. : 26' 185.

The transits at Madras having been taken by eye and ear, all have
been reduced by connecting the Observer's equations in the following way
through Captain Campbell when he passed through Madras.

It was found by experiments at Roorkee on two nights that Captain
Campbell observed transits earlier than I by 0*085., each using the chro-
nograph: we observed alternate Nautical Almanac Stars, but on the
second night he observed mine of the first night, and I his. Then on

8 REPORT ON THE TRANSIT OF VENUS.

a later night he observed a good many unknown stars, ^ both by eye and
ear and by chronograph, whence it was found that his chronograph transit
was later than that by eye and ear by 0*045., so that my chronograph
transit is 0*045. later than his transit by eye and ear. I have not the
data of the Madras comparison. A small correction + 0*045. has been
made to the times of Roorkee signals for the apparent retardation in record
(found by experiment) of signals made in the same way as those sent.

The longitude of Madras used is sensibly the same as that given
in Volume XVL, Royal Astronomical Society's Memoirs, by the late Mr.
Taylor, and indeed, I believe, no attempt has been made to determine
it since that time, 30 years ago. We have then —

Longitude of Madras 5A. : 20m. : 57-275. East of Greenwich.

Roorkee, West of Madras ... gm, : 26-185.

Longitude of Roorkee 5A. : iiwi. : ^rogs. East.

which value is probably as good as can be given till chronographic deter-
minations are resorted to throughout.

The time required for signals to pass on the three nights was 0*245.,
0*245., and 0'225, the distance being 1,931 miles with two translations.

, The second set (one-half) of signals from Roorkee was received at
Madras on each night by chronograph, as well as by ear, but the times
by chronograph differ systematically from those noted by Mr. Pogson by
ear, and as the transits were all taken by eye and ear I have preferred
adhering to Mr. Pogson's noted signals, thinking it more probable that he
referred signals and stars to the clock beat alike, than that the chronograph
recorded the signals in the same way that Mr. Pogson would have noted
transits.

When the longitude of some point in India has been determined by
the modem methods, it may possibly be worth while to ascertain again
the relative position of Roorkee, but it is hardly worth going further
till this work, most important in a scientific point of view, is carried out.

* The system of wires afterwards described b not what was used for this purpose. Finding the intervals too
great for convenient use, I bad a new set of wires placed in the instrument, whose distances from the middle one

• • • '

were approximately 33^., 165., 8s., and 4*., on each side. The four outer wires were observed by the eye and ear
msthodv and the five inner ones with the chronograph; • -

..RCX3RKEE. A

Indian longitude must eventually furnish the data for the Australian Colo-
nies and the large islands near the Equator, whence through China we
may hope to complete the whole circuit of the world and establish points
of reference everywhere. There is, of course, an alternative circuit
through Siberia, if the mere feat of surrounding the world were alone in
question, but India is an essential stepping stone to the British, Dutch
and Spanish Colonies, and I think that no time should be lost in estab-
lishing, beyond all doubt, the longitude of some point in the country with
all the accuracy attainable by modem appliances.

The height of my station was determined by connection with the
bench marks of the Ganges Canal which are connected with the lines of
level of the Great Trigonometrical Survey, and by them to the sea level
at Karachi. The floor of the Observatory has thus been found to be 876*2
feet above the sea. The effect of the mountain attraction, however,
would be to make this too small as an increment to the earth's radius
Vector. I have endeavored to allow for this by assuming the increment to
be 950 feet and the logarithm of p (—^l^^^ becomes 9-9996618,

of which five figures would be unaffected by any possible difference in
estimating the mountain masses.

ROORKEE STATION.

Determination of Local Time.

My first care was to secure the means of determining this essential.
Having occasion to go to Calcutta in December 1873, I examined the
stock of the Government there, and selected a Portable Transit of 30
inches focus which seemed little tiaed, and directed that it should be put in
order for me. It reached me a couple of months later, and as soon as I
liad any cover for it I lost no time ia testing it To my disappointment the
results were hopelessly wild^ It was only after weeks of trouble during

jQ REPORT ON THE TRANSIT OF VENUS.

the hottest weather in experiment and rectification of defects that I got
it to work at all ; then, with great caution in touching it, it would give
fairly acicordant results, though the position was in a constant state of
change with temperature. The transit made for the purpose of these
observations only reached me on November 24, and it was immediately
put in position. It was impossible, however, to use it with any satisfac*
tion till various changes were made, and at that late period I was very
unwilling to undertake these, but a natural reluctance to revert to an
unstable instrument and the advantages [urged by Captain CAMPBELLJ.of
reflection observation of the wires if I could get them visible, induced me
to enter on this work ; the necessary alterations in the illumination of
the wires and adaptation of the eyepieces were made, and it was hoped
that the difficulty of seeing the wires by reflection was got oven

This instrument (by Messrs. Cooke and Sons) is, I understand, a
combined design of Mr. Cooke, Colonel Strange, and Professor Magnaghi.
A strong central cube carries the frusta of the cones which form the Tran-
sit axis on two of its sides, and the tube of the Telescope is fastened to
other two. The object glass has an aperture of 3 inches, and a focal
length of about 34$ ; the eye end is furnished with two micrometers at
right angles to each other; one carried the system of Transit wires, and
the other a single wire to be used in conjunction with a delicate level for
determining latitude by means of differences of Zenith distance. The
Transit axis is 18^^ inches long, and terminates in (what are apparently)
steel pivots resting in segmental hearings. Both pivots are perforated,
but the light passing through one alone falls on the wires after its reflection
from a small metal surface in the centre of the cube. There is only one
setting circle near the eye end of the telescope which is counterpoised
on the other side ; its verniers read to half a minute and are con-
nected with two levels, one of which is the fine one for use with the
vertical micrometer. The pillars carrying the bearings of the Transit
axis are cast in one mass with a weighty base plate. Pillars and plate
turn on a second plate to which they can be firmly bolted in any position,
and a slow motion for accurate relative adjustment is supplied. The
lower portion is supported on 3 foot screws of Colonel Strange's pattern,
and is furnished with a few divisions to enable the direction of the Transit
axis to be changed by a right angle so as to allow Transits to be taken in

ROORKEE. X I

the prime vertical. There is a very convenient reversing apparatus which
enables the coUimation to be determined with a single collimator. The
inclination of the Transit axis to the horizon is determined by viewing
the wires by reflection from Mercury, Levels were supplied to be attach-
ed to the central cube for the purpose of watching the flexture of the
Transit axis ; I have not used these except for roughly levelling on occa-
sion ; I have no description of their adjustments ; have failed to find
any convenient mode of making them ; and believe that they are perfectly
unnecessary.

As a Transit, the performance of this instrument is excellent : I believe
time can be determined with it as well as in any permanent Observatory.
In its other capacities I have no experience of it ; but I am disposed to
think that it would be better without the arrangement for differences of
Zenith distance whose micrometer is inconvenient when using a high
power diagonal eyepiece ; a very much better arrangement, I think, would
be to have only a single micrometer which could be turned round so as to
have the moveable wires either vertical or horizontal.

The clock is by Messrs. Cooke and Sons : it does not seem to have
any peculiarity. It marks a chronograph by means of a wheel (on the
same axis as the scape wheel) of which the teeth pass a spring just
before the scape wheel is stopped by the pallets; one tooth, as usual,
being cut away to leave out the mark at the even minute. Three jour-
neymen clocks or dials were also worked by the same clock, the contacts
for this purpose were made by a pin on the pendulum which, in the mid-
dle of its beat, depressed an agate stud and thus pressed two springs into
contact. This has not been found satisfactory. It worked (after a
great deal of trouble) for some time, but the contacts became burnt, and
I have reason to believe that it affected the rate of the clock very
irregfularly.

All time signals from every source were recorded on a chronograph
specially made by Messrs. Cooke and Sons. My plans for observing the
Transit of Venus originally involved a large number of Transits, both
with an alt-azimuth and equatoreal. It seemed to me impossible that
writing down eye and ear observations could meet the want, and I was,
therefore, desirous of having a chronograph which should record all the
times of observation. As regards the Photoheliograph, anything which

12 REPORT ON THE TRANSIT OF VENUS.

should give the time of observation, even to a quarter of a second, would
answer, and as regarded any of the other instruments it seemed to me
that if the probable error of a reading were as small as that of an observ*
ation in the old way, there would be a gain, and looking to the large pro-
bable error of eye and ear, and even chronograph* recorded observations
of Transits, it seemed that extreme accuracy of motion might be dispensed
with ; and that, even limited as the time for preparation would be, a suffi-
ciently good apparatus could be made to record the observations of several
instruments on different pieces of paper. Messrs. Cooke and Sons have,
under Colonel Strange's instructions, made an instrument which, I
believe, is adequate to any astronomical work, and which furnishes separate
records of four instruments at a very moderate cost. The mode of gov-
erning the motion is by a clock precisely similar to that which is used by
the Messrs. Cooke and Sons for their equatoreals with success: it must be
evident to all, I think, that an arrangement which can keep a star bear-
ably on a wire must be quite adequate to produce a movement equable
enough for ordinary chronographic work, and I believe it does so. The
bevel wheel on the clock, which usually drives the shaft leading to the
tangent screw of the polar axis, is here made to drive a long horizontal
shaft, which again drives a transverse shaft at each jend of the table.
The ends of the transverse shafts have wheels fastened on them, which
draw out the fillets of paper on which the record is made. Each fillet
has to pass under two prickers, one for the clock, and the other for the
observer. The instrument has, on the whole, answered well : the princi-
pal trouble has been from the fillets becoming displaced. This is, I
believe, principally due to the size in the paper, which becomes hard in
dry weather, and does not readily bend round the rollers, hence the paper
occasionally slipa off the guides and stops the clock. Suitable guides
will prevent this, and I prevented any mischief from this cause for a
long time, though unfortunately I did not know of the defect when the
chronograph was most wanted. There is, of course, a correction, differ-
ing for each record, to reduce the mark of the observer's pricker to that
clock, but it is very Miall, and I have not thought the refinement of using
it necessary. . ,... ... ...

The whole of the Transits for the determination of time were made
by me. The plsicea of the stars employed were mostly taken from the

ROORKEE.

Nautical Almanac for 1874, but I have used some others, of which I
annex a list, with their adopted places for 1874*0, and the authorities for
the places —

Stars' names.

Mean iR.

1874-0.

Authority.

Bradley, 3147

5,
50*20

Greenwich 7 y. Cat. for 1864.

yy 144^ *** ***

19*48

Ditto and Results for 1868, 1869,
and 1870.

Groombridge, 2053

08*67

Greenwich 7 y. Cat. for 1864.

3834

03-38

Ditto.

c Eridani ••• ••• •••

59-68

Nautical Almanac, 1875.

17. Tauri ... •••

2374

Gr. 7 year, Catalogue for 1864.

l*he intervals of the wires were determined in terms of the micro-
meter screw on November 25, 1874 ; by making five intersections with
each of the cross of a Collimator, the following results were obtained : —

Wires.

Mean Reading on Cross.

Distance from D.

rev,
2*0128

rev,
13*8788

8*9550

6*9366

12*4x60

3*4756

15*8916

19*3468

3'455«

! F

22*8000

4-9084

29*7792

13-8876

In the position of the eye-end held up to November 29, the value of a
revolution was 3'972735. as determined by Transit of j Cephei, B. A, C.

REPORT ON THE TRANSIT OF VENUS.

80261 8215, 651, 896, and 879 over the wires A and G. Hence the equa*
torial intervals adopted to that day, inclusive, are —

AtoD. BtoD. CtoD. E to D. F to D. G to D.

s. s, s, s, s, s.

+ 5513673 + 2755724 + 13*80762 - 1372658 - 2744521 ^ 55I7I69

On November 29, I became convinced that the plane of the image
of the wires seen after reflection was not that of the wires seen directly ;
this was corrected next day, and on December 14, I observed Transits
of B. A. C. 7167, 8074, 8124, 8217, 8334, 39, 86, 194, and 280 for wire
intervals. These were deduced in the usual way to be as follows : —

A to D. B to D. C to D. E to D. F to D. G to D.

s. s. s, s. s, s,

+ 55-2710 + 27-6021 + 13*5739 - I35988 - 274800 — 55IOI6

The observations, however, being less accordant than I could have
wished, I have deduced the value of a revolution of the screw by compar-
ing the sum of these intervals with the sum of the distances by screw,
whence i Rev. = 3*968255., and I have used the intervals (calculated from
micrometer intervals), which follow in preference to the last —

A to D. B to D. C to D. E to D. F to D. G to D.

s, s. s. s. . s, s,

+ 5507455 + 27-52616 + 1379205 - I37IIIO - 2741426 - 5510947

The position of the optical axis of the telescope has been deter-
mined by taking five readings of the cross of the Collimator in each
position of the instrument. From December i, this has been done
both before and after observing. Up to November 30, the inclination
of the Transit axis was occasionally found to be best got by superposing
the wire ^n its ill-defined image: after that date it has always been
placed so as to touch its image alternately on each side, and the inclina-
tion observed both before and after taking the night's Transits.

The instrumental corrections have been computed as follows: —

CoUimation correction for centre wire = reading of micrometer for
optical axis — reading for Transits — 0'00455. (for diurnal aberration.)

Level correction = reading for optical axis — reading at coincidence
of direct and reflected wires.

ROORKEE.

The azimuth correction has been determined nightly by stars above
and below the pole, except on November 28, when a north and south
stBr were used, and on November 29, when two south stars are com-
bined with a north one, and each night's Transits was reduced by the
aisimuth as determined on that night. It being now evident from the
Collimator readings that the changes of azimuth of the instrument after
December 2 are rather the result of errors of determination than of
instrumental changes, I proceeded to reduce all the clock errors to what
would have been obtained on the hypothesis that the azimuth remained
after this day constant at its mean value up to December 11, and the last
column shows the clock error so adjusted.

The table on the following page shows the information necessary
for deducing the clock'$ errors and the mean corrections as reduced.

It is to be understood that I mean by ** clock correction" the time
which is algebraically additive to the clock time, in order to get sidereal
time, and the clock rate is the increase of clock correction in 24 hours.
The following table shows the clock time of mean of observations, the
correction and rate for each day from December 3 to December 11: —

Date.

Clock Time, Mean
Observation.

Clock Correction

Rate.

h, m.

December 3

• • •

2 181

— 201

• •

2 13-8

— 2-35

— 0-34

* • *

2 34-8

— 278 1

' 1

— 0-43

• ••

2 143

-3*53

- 0-38

• • •

2 13-8

- 3-87 i

— 0-34 !

• ••

2 524

- 392

— 0-05

■ • •

2 524

- 3*94

— 0'p2

.«.

2 42-8

— 414

— 0-20 1

I have assumed that on December 8 at 14A. 05W. by the clock, the
correction was — 3'885., and the rate — o'o85.

REPORT ON THE TRANSIT OF VENUS.

•s

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M

PHOTOHELIOGRAPH.

THE PHOTOHELIOGRAPH.

I had intended, as I have before said, to take the work with this
instrument under my own supervision. I believed that I should have had
ample opportunities for experiment and for instructing assistants, as
during the Transit of Venus the work would be merely of routine. I
did, in fact, instruct three men of Her Majesty's 55th in all the ordinary
operations of wet collodion photography, but the instrument was late,
and cover for it later, nor did the special chemicals arrive early enough.
I had, therefore, no means of experimenting on dry plates, and it was
evident that I should be so much pressed with the arrangements gene-
rally, and that unless I abandoned other work, I should have great
difficulty in arranging for the Photoheliograph. It was, therefore, very
acceptable to learn that my old Assistant Captain Waterhouse, would
be again able with the Surveyor General's permission to join me and
relieve me of this instrument, and that he could bring a man who was
used to photographic operations to aid him. Captain Waterhouse, even
before joining me, entered very zealously into the question of dry plates,
and made hundreds of experiments in Calcutta, where he succeeded well
with them.

As soon as I had cover, I placed the instrument approximately in posi-
tion, about which there was no difficulty ; but when I sought to complete
its adjustment, I found that I could only use the sun, as stars and planets
were invisible in the Camera. After some trouble I got a small telescope
fixed onto the leaden counterpoise, and I was thus able to use the stars
for adjustment and get the polar axis into position with reasonable facility
and sufficient accuracy. Having done this, I made the instrument over
to Captain Waterhouse, who was quite competent for the photographic
business. He has furnished me with a detailed report of his operations,
a copy of which has been sent to the Astronomer Royal, together with
all the photographs, it being intended that the measurement and reduc-
tions shall be made precisely as for the British Government's expeditions.

REPORT ON THE TRANSIT OF VENUS.

I shall, therefore, here only speak of the instrument and such points as
nearly a year's experience of working it have brought to my notice.

As to processes for Photographing. ^In the first place all dry pro-
cesses failed in giving satisfactory results at Roorkee. Years ago
when stationed there, I had met with similar results, and had become
doubtful whether it was desirable to adopt such uncertain procedures, but
I hoped that the difficulty would yield to systematic experiment. It is
evident, however, that want of knowledge cannot account for Captain
Waterhousb's difficulties with processes he had proved in Calcutta, and
the only solution which occurred to us was that particles of lime from the
walls must be floating about in the dust inevitable in a dry climate. The
great difficulty we had before experienced with Mr. De la Rue*s procedure
with a simply iodized collodion made us both feel more confidence in
bromoiodized collodion developed with an iron and sugar solution which
(as we found in 1871), gives nearly as fine a deposit as pyro-gallic acid.
I have since found that this was unfortunately the worst developer we could
have used. The fact is that there is a superabundance of light and a deve-
loper whose great feature is the bringing out of slight impressions of light
without solarizing the brighter parts is just what is not wanted. I have
succeeded best with a pyro-gallic solution greatly restrained by citric acid,
and next to this with iron similarly restrained. The best results have
been with some collodion made after a formula I used in preparing for
Captain Waterhouse, and some are quite equal to the best dry plates ;*
but I have used various mixtures of Thomas' collodions with fair success,
though I am disposed to think his collodion for iron development, as good
as any of these if used alone.

The instrument seems to me to vibrate too readily, but its weight
would probably be a good deal increased if this were rectified entirely.
The great defect appears to be want of flatness of field : greatly as the
enlarging lens has been improved, it seems to me to share the defect in
this respect of all photographic lenses. It is possib]|^ to get fair defini-
tion over the central part of the sun, but the outer portion of the disc
it seems quite impossible to define, so that the picture of a spot shall be

* My latest ezperienoe has shown me that in using a highly hromiaed ooUodkm for this instrument it is very
necessary to be careful that the plates do not stay too long in the sensitizing solution. With this precaution, I bcKcTc.
good piotaret of Ae ton showing oil the detail the imtmrncnt can giiv are easfly obtained.

PHOTOHEUOGRAPH.

reasonably sharp. If this cannot be remedied with a lens of so short a
focus, then possibly it might be desirable to increase the whole size of the
instrument, and use a longer focal length in the enlarging lens, and fail-
ing this, I fear, the long focus lenses used by the American parties will
have a great advantage.

If the Photoheliograph is to be employed at stations near the con-
tacts in 1882, I would point out that this defect is of the very greatest
importance. At these places I would use some form of Dr. Janssbn's
plateholder revolving in five minutes, and giving a picture every five
seconds, and thus record cusp distances from which the actual contacts
could be deduced, and also measures of the planet's distance from the
limb. The distances to be measured being comparatively small, the
scale might, I think, be inferred with sufficient accuracy from pictures of
the whole solar disc when the planet is on it, but to make this arrange-
ment hopeful, I conceive that the greatest sharpness of definition and free-
dom from distortion is essential.

In any case the moments of exposure should be automatically
registered. This was done at Roorkee with the square plates in the
following way : blades forming a pair of shears were attached to the two
parts of a signal key of the pattern sent out for use with the chronograph,
these being used to cut the cord, on severing which a slit flashed across
the field and gave the exposure ; the same pressure on the key thus cut
the thread and made the contact which marked on the chronograph. The
manner in which the exposures on the Janssen's plates were made to
record themselves is shown in Mr. De la Rue's account of the dark
slide in Royal Astronomical Society's Monthly Notices, Vol. XXXIV,
p. 347. I do not think the above arrangement for recording exposures of
the square plates perfect, for it was hurriedly got up and not very well
executed ; but well made it would be quite efficient in giving a better
record than could be got from a chronometer

20 REPORT ON THE TRANSIT OF VENU9.

EQUATOREAL INSTRUMENT.

This was in my own charge. It is by Messrs. Cooke and Sons of
York, of 6 inches aperture and about 82 inches focal length, and being, I
believe, of their usual pattern requires no particular description. The
main instrument with the usual fittings (except a parallel wire micrometer)
and a double image micrometer by Mr. Simms reached me towards the
end of October. Its shelter was ready, being a circular building 15 feet
in internal diameter covered by a revolving roof: an opening 30 inches
wide with shutters gives the means of examining any part of the sky from
the zenith to the horizon.

No time was lost in putting this instrument in position, an operation
in which I was greatly assisted by Captain Campbell. Having adjusted
it so that the polar axis pointed within a few seconds of the true pole, I
at once set to work on the double image micrometer, an instrument of
which I had no previous experience. I had asked for means of determin-
ing differences of right ascension and declination ; these did not come
with the instrument and it was doubtful if they would arrive in time (when
they did arrive in the middle of December I found that it was impossible
to use them) and I was thus obliged to take to an instrument which I had
never used, and which I had never heard was even likely to come. Fortu-
nately for the purposes of such work as the Transit of Venus it is singu-
larly easy to manage, and the only trouble is the determination of scale,
a process which I think it would not be difficult to facilitate very greatly.

The transit instrument, determination of micrometer scale, and
measures of double stars for practice, kept me constantly employed in the
evenings. The afternoons, after Captain Strahan's arrival on November
I, were devoted, whenever possible, to observations of the model of the
Transit of Venus in order to determine our relative equations. It was
impossible to use this model as at Greenwich, for the tube of the Cooke
Equatoreal is only complete when it is mounted and there was no means
of putting another telescope near it. I was obliged therefore to place

EQUATOREAL INSTRUMENT.

Captain Strahan's (Simms') Telescope, whose tube admitted of it, on a
temporary support outside the observatory and we had to adjust two
heliostats at some distance behind the model, so that one threw the sun
light into each telescope. The model and heliostats were placed on
a building (the Magistrate's office) across the Meerut road, and the
result was far from satisfactory. In any case a ray of light passing for
some 140 yards not very far from the surface of the dry earth would be
greatly disturbed in India, but to this was added the dust from traffic on
the road, and the interruptions caused by clouds, and the losses of observa-
tions from one telescope having a good image, while the other was badly
illuminated. Owing, too, to the distance of the telescopes it was abso-
lutely necessary to interchange the observers, as there was a sensible
equation due to the positions of the telescopes.

Captain Campbell's equation was determined at Greenwich by refer-
ence to Father Sidqreave and Captain Browne. It has been found here
that I observe such a phenomenon as we saw in the model earlier than
Captain Campbell by 0*775., and that Captain Strahan again observed
it earlier than I by 0*345., or 1*115. earlier than Captain Campbell. Also
by direct comparison between Captains Campbell and Strahan, it was
found that the latter observed earlier by 1*085. : showing, I think, that the
comparisons fairly represented our estimates. But we saw no phenomenon
of the same kind during the Transit itself, and I question if the com-
parisons are applicable : I have therefore not used them.

Of course, my first object was to observe the contacts as accurately
as possible. The first external contact could not be observed, the Sun
being too low, but soon afterwards I began measuring cusp distances.

Dial Time.

Micrometer
Reading.

Assumed

Distance observed.

Refraction.

True
Distance.

Zero.

Micrometer.

Space.

h. fn. s,
12 26 40
27 40
31 18

3« 58

36 03

37 40
39 20

12 40 58

rev.
14-278

6*404

6-289

13-987

13-582

7-396

7-704

12*122

1
t

rev.

( \

rev,

3958
3-916

4*031

3667

3*262

2*924
2*616

1-802

63-575
62-901

64748

58-901

52*396

46-967

42*020

28*945

•••
•••
•• •
•••

0739
0*623

0-434
0-268

m
• ••

.a.

• ••

53-135
47-590
42-454
29-213

22 REPOBT ON THE TRANSIT OF VENUS.

Only the last four observations have been used for deducing the
contact internally, the others being taken at too great a distance from the
contact. The micrometer measures have been converted into space by
using the value of a revolution determined from a large number of transits
of stars not far from the pole, which gave it 16-0625' ± 0-0132', and the
refractions are computed by the formula of Professor Chauvenet's work.

The contact of limbs was observed at 12A. 42m. 305. by dial.
This dial was set by the Transit clock before observing, and was found
to agree in the second after the Transit was over. At a convenient time
during the Transit I marked seconds from the dial on the chronograph,
which -showed that the difference was only about o-025., the delay in
marking and hearing compensating for the small amount by which the dial
beat from its construction preceded that of the clock : I have considered
the two practically identical. I had means for marking the time on the
chronograph, but I could not use the chronograph key and the micro-
meter at the same time, and in this case I had not time to get the key,
having continued observing cusp distances rather late. For this reason
too I observed the contact with the double image micrometer which, with
the first lens in use, gave a magnifying power of 128-4 : the shade of
dark glass (a wedge achromatized) was rather deep in order to reduce
the haze round Venus, and the points of the cusps seemedvery fine. The
contact took place without any black drop or distortion : at the moment
it occurred I was endeavouring to get hold of the tappet or key,* and in
doing so, lost count, but I looked up at once at the dial (and allowed for
the lost time), when I was sure that the critical phenomenon had really
past without any of the peculiarities I had been led to expect.f This
estimated interval was three seconds, and I think that all causes of error
would be included in ascribing a probable error of two seconds to the
observation of time.

^ I may mention to save question, that my eye never left the telescope^ and that this was the canae of my fail-
ing to get the key, as I was obliged to feel for it only.

t Bfr. Stone has written a paper (Royal Astronomical Society's Monthly Notices, December 1876) which leads
me to say that the time was not occupied from any uncertainty from penumbra, ftc., but in scrutinizing to be sure
that there was a complete break without any phenomenon such as I had expected to see.

EQUATOREAL INSTRUMENT.

The contact having passed, I proceeded to observe distances of the
Planet's limb from that of the Sun with the micrometer, as in the
following table : —

Time
by Dial.

Micrometer
Reading.

Assumed
Zero.

Observed

Distances.

Remarks.

' Micrometer.

Space.

h, m, s.

rev.

12 47 05

49 «2

15-051
5*446

\ j

4*694
4-91 1

75*397
78-883

V Distant Limb of $ .

50 30

9*289

1-068

17J55

Ailer this are near limbs.

51 26

11-546

1-189

19-098

•53 04

11-741

■w^

1*384

23-331

54 20

55 14

8-878
8-786

1*479
1-571

23-756

25-234 ;

56 39

12-071

1-714

27-531 !

57 40

12*272

1-915

30-760

58 32

8-396

1-961

3i'499

59 22

8-274

2-083

33*458

12 60 51

12*537

/ \

2- 1 80

35-016

13 04 06

12-967

\ /

2-631

42*260

Focus slightly modified.

5 20

7*551

2-785

44734

6 08

7-446

2-890

46-421

7 45

13*384

3-048

48-959

8 40

13*493

3*157

50709

9 49

7-161

3*175

50-998

10 54

11 58

7*o6o
13-868

/ 1

3*276
3*532

52-621
56-733

13 10

13924

3-588

57-632

14 24

6-616

3*720

59*753

15 07

6-447

3*889

62-467

16 22

14-258

3-922

62*997

17 23

14*352

4-016

64-507

19 54

6-181

/ \

4-155

66-740

REPORT ON THE TRANSIT OF VENUS,

Some time was now spent in discussing the appearances at the contact
and in sending a telegram to Captain Strahan at Lahore, warning him
that probably he would not see the phenomena which had been seen in
the model.

I then proceeded to take measures of the diameter of Venus. I had
intended to measure only horizontal diameters, so as to have them free
from refraction, and also to avoid, as far as might be, the tremor from the
air which was rapidly warming ; but after half a dozen measures, I gave
this up, for the position of the micrometer required constant rectification
and I found that this caused slight changes of zero. I took to measures
of diameters in declination, as the direction of these was fairly near the
horizontal : and 28 of these were measured half on each side of zero : I
then measured 20 diameters in right ascension for a set, and afterwards
measured another set of 20 measures. The results of these measures
were communicated to the Royal Astronomical Society and published in
their Monthly Notices, Vol. XXXV, p. 345 : it is not necessary to give
the individual observations here. When this work was completed, Venus
was getting near enough to the Sun's limb to allow the distances of the
limbs to be measured, and I proceeded to make the measures which are
given in the following table : —

Time
by Dial.

Micrometer
Reading.

Assumed
Zero.

Obsirvbd Distanck.

Remarks.

Micrometer.

Space.

h. M, s,

15 57 01

58 30

59 30

16 00 32

2 15

3 40

5 32

7 07

8 04

9 02
9 46

10 33

11 20

12 22

13 01

14 II

rev,

13-956
6736

6-835
13*664
13*466

7-308

7'493
12-973

12-842

7916

7-907

"•535
12-520

8-276

8-372

12*176

rev,

\ 1

8
b (

/ \

rev,
3656

3-564
3-465

3-364
3-166
2-992
2-807
2-673
2-542
2-384

2-393

2-235
2-220

2-024

1-928

1-876

m
58725

57*247
55-657
54*034
50-854
48-059

45-087

42-935
40-831

38-293

38-438
35-900

35-659
32-511

30-969
30-133

EQUATOREAL INSTRUMENT.

I was obliged to stop early, in order to see that the chronograph strip
for my instrument was all right, to change the eye-piece, and prepare for
warning the photographers of the approach of the internal contact.

For observing, the internal contact, I used a Huyghenian eye-piece
magnifying 204*5 times, with a smoke colored glass giving a yellow image
of the Sun, which I had found to suit with the model, and to give a singu-
larly clear contrast in the different parts of a sun spot. There was no
black drop or distortion ; the image was, indeed, dancing from the state of
the air, but the cusps were quite fine, and I doijbt if their thickness or
that of the last relic of the solar limb exceeded the small amount due to
the diffraction of the object glass. The contact was observed at 16A. 28m.
405. by the dial, and I also marked on the chronograph for greater
exactness, but unfortunately one of the strips of paper had, in Captain
Heaviside's short absence to observe with the Royal Society's Slater
telescope, stopped the apparatus as I have before explained.

I then proceeded at once to change the eye-piece and observe mea-
sures of the chord joining the cusps. Unfortunately, changing the eye-
piece and focussing takes time, and thus the most valuable cusp measures
are lost. This circumstance, and also the few measures which are possi-
ble, while the distance is rapidly changing, are what have led me to
believe that many good photographs would be very desirabl

Dial Time.

Micrometer
Reading.

Assumed
Zero.

DiSTANCB

OBSBRVBD.

True

BficTometer.

Space.

Distance.

Am Wm S,

rev.

reo.

16 3a 14

13-028

1 2-648
3-894

42-534

•'•

42-534

33 13

13*274

46-485

34 24

I3'576

3" 1 96

51-336

36 u

13900

•••

• ••

• •4

37 18

6713

•••

• ••

«•

38 u

6*603

•••

• a.

• •1

39 08

6-504

} S

•••

• ••

• •

40 00

6-415

• . •

•••

• . <

41 28

6-403

•••

a. <

42 17

H-393

• • •

•••

••«

43 01

14-367

•••

• ••

• •<

44 04

$'343

•••

• •1

45 02

6-390

•«•

• ••

..«

45 54

i4'276

*• •

• «.

• •1

46 42

14*073

•••

»••

• •(

47 30

6-560

•••

»••

• •<

26 • REPORT ON THE TRANSIT OF VENUS,

The report of the chronograph stoppage caused a slight confusion,
which made me measure all the early chords on the same side. Unfor-
tunately too the later measures are too far from the contacts to be useful ;
but I have used them for getting an approximation to zero which is tested
by the cusp measures. I have not thought it necessary to use refractions
where the direction was so nearly horizontal and the space so small.

The last contact was noted at i6h. 55m. 445. by the dial, and pricked
at the same precise second on the chronograph. I have marked in my
note book that "the qjror may be a second" for the impression of the
Planet was very clear to the last, but on consideration I have preferred
assigning a probable error of a second to the internal contact, and one
of two seconds to the external. Notwithstanding atmospheric tremor,
the vision was excellent, and the power and dark glass were as for inter-
nal contact.

As regards the measures: the best were certainly the diameters;
next, the distances from limb, and lastly, the cusps : the great difficulty
arose from the continual dancing of the images, parallel and perpen-
dicularly to the line of separation, and, of course, this mainly affected the
measures between points. I imagine that this difficulty would apply
principally to a double image micrometer of this sort where each image
is made by half the object glass. Where the two images are formed by a
double refracting prism, they should be relatively steady, and if a satis-
factory micrometer of this sort were to be had, it would be better for
cusp measures, and probably for measures generally.

REDUCTION OF EQUATOREAL OBSERVATIONS.

The first step in this process was, of course, to get good interpola-
tion formulae. For this purpose the places and log distances of the
Sun and Venus were taken from the Nautical Almanac for each Green-
wich noon from December 5 to December 12, each co-ordinate being
represented by a series A + Bt + Ct* + Dt* + Et*, where the origin

REDUCTION OF EGLUATOREAL OBSERVATIONS.

of t was taken at 8.5d. and the unit was a mean solar day. The values
of the co-efficients were then determined by the method of mean squares,
which seemed as little laborious as any other procedure,* The formulae
obtained were then transformed for convenience into others where the
origin of time was I2h. of my clock, and the unit one of its seconds.

On comparing the places from these formulae with those given in the
Astronomer Royal's Tables, I found that the Sun's Right Ascensions were
too small by 2*^03. I have repeated the deduction of the formulae without
finding any sensible difference from my first values, but the error must lie
with me, for I have also computed the value from the Solar Tables directly,
and find it close to the Greenwich value as does interpolation with second
differences only. The other quantities I have found to agree with the
Astronomer Royal's values.

I have, therefore, corrected all my equations (as found in the com-
putations) for this error, and they will be given here so as to be compar-
able with those deduced under Sir G. Airy's supervision at Greenwich.

The general principles of the remaining calculations are those laid
down by the Astronomer Royal in his paper in Vol. XXXV, p. 277 of
the Monthly Notices, of which he kindly furnished me with an early
copy. In the formulae I shall use the following symbols : —

n is the mean Equatoreal Horizontal Parallax of the Sun.

p is the Horizontal Parallax of the body at the time and place.

iK and S are the Tabular Right Ascension and Declination.

r the assumed sidereal time^ and the Tabular Hour Angle.

L is the Longitude East, ^ the Astronomical, and ^' the Geocentric Latitude.

Accented letters denote apparent values.

The arbitrary corrections to the quantities are denoted by the sign a,
while the changes by change of epoch are represented by differential
co-efficients with respect to t.

Thus AiR is the correction to the Right Ascension for errors of

Tables, while -^ is the motion in Right Ascension in one second of time.
The local sidereal time is thus T + At, and the complete value of is —

= 15T + 15 At - iR - AiR - (At - AL). ^.

^ This prooedare is very laborious and appears needless. My reason for adopting it was as follows : — The
Astronomer Royal placed great stress on places being computed " with the greatest precision " to o"*oi, and I saw no
other way of readily getting general accordance to this ezten^ M. Leyerrier's Tables give petturbadoos, &c., only to
this amount, and tbe places of Venus would vary more than o''*io according to the mode of deduction.

28 REPORT ON THE TRANSIT OF VENUS.

The Parallax in Right Ascension has been calculated from the formulae P^ =
/ cos. ^' sin. sec. S. Hence its complete value is P^ + cos. ^' sin. sec. S A/ -

P^cot. [is At -AiR-(Ar -AL) ^]. And

apparent ^ = A + P^ + cos. ^' sec. 8 sin. A/ + AiR (i + P^ cot sin. i'')

+ At [(i + P^ cot. sin. i*) ~ - ^5 P^i ^^^ ® sin. i^

- AL (I + P^ cot sin. O ^^.

The Parallax in declination is computed from the formulae —
tan. y = tan. ♦' sec. 0. Pp = -/ sin. (y - 1) cosec. y.

To get the complete value we have —

Sec' y. Ay =s tan. <f' sec. tan 0. A = tan. y tan. 0. A 0.

Ay = sin. y cos. y tan. [1$ At - AiR - (At - AL). ^].

= sin. y cos. y tan. [(15 - -^) At - AiR + AL ^J^].

sin. /y + Ay-8-A8- (At-AL) ^\

Complete Par. in Dec. = - (^+ A^) sin. ^' > r—, =r-;^ ^

sm. (y + tfy).

sm. y sm. y

jk «:.* a' sin. 8 y. *,:«,#!_ ji «:^ ^' cos. (y — I)

— / sm. $ -; — s — Ay. sm. I. + p sm. 6 — r-^^^ — '-
sm.» y ^ sin. y

[A8 + (At-AL)^] sin.!/

Substituting now the value of a y, omitting the term involving -^
and also the last term of the complete value which is very small : we have —

Complete Par.in Dec. = Pd + ^ Pd + / sin. ^' sin. i *5!L® (15. dr - dIR) sin. i.'

f tan.y.

in which A^ may also be neglected.

Thus apptNPD = Tab. NPD-P,.- Mp - [^ - is/sin.*'sin. 8 ^^^i^siai^AT

/ ^ ""at tan. y "*

A/ has in every case been converted by the proper factor into All.

REDUCTION OF EQUATOREAL OBSERVATIONS. gO

The contacts and measures of distance of Limbs have been con-
sidered as variations of the same thing.

Taking Diff. M = appt. M Q - appt. M $

and Diff. NPD = appt. NPD - appt. NPD ? by the Tables we have—

tan. 4r = £)]]J7^p5^/ sin. appt NPD © sin. appt. NPD ?
^ ^ Diff. M

Diff. NPD
and D' = appt. Distance of Centres = Diff. NPD sec. ^,

If we call the sum of the terms which are added to the difference of
Right Ascension as computed from the Tables ViR; vNPD the cor-
responding quantity in NPD; vD' the consequent correction to D', and
^ and v^' that to 4'-

^^ ^ . . A Diff NPD. vJR- Diff m\ NPD.
Then we have sec. tjf v v = A. (Diff NPD^^

and V D' = - ^^X Diff NPD v 4'+sec. ^ v NPD
= - A sin. 4^ V -^ + COS. 4^ V NPD
and thus have a complete value of the distance of centres.

The true semi-diameter of the Sun S has been derived from the
mean value used in the Nautical Almanac; or 961 '82' = [2*9830938].
Tne apparent semi-diameter —

S' = S. sj"- ®' sin- NPD; j^^ y^^^ conveniently got
sin. sin. NPD . **

by taking log. S' = log S + Par. in JR change of log, sin. Q for lo^

. Par in NPD change log, sin. NPD for 10.^

For Venus I have used as a mean semi-diameter, the value deter-
mined by Mr. Stone, 8*472,'' which is near the value I have found by my
own measures. It is unnecessary in this case to consider the difference
of the true and apparent semi-diameters.

In the case of internal contacts, we have, of course —

D' + vD = S' + AS - (<r + A<r) ; <r being the semi-diameter of Venus.
For external contact D' + vD' = S' - AS + <r + A<r

and if it be the measured distance of limbs —

D' + vD' 5= S' + AS- (<r + A<r) -»f.

REPORT ON THE TRANSIT OF VENUS.

The micrometer-measured distances of limbs have been taken in
groups of four, the mean of the distances of the group being taken to corre-
spond to the mean of the times of observation. By this means all uncer-
tainty as to the slight changes of the micrometer zero have been avoided,
and in the first group at Ingress I have eliminated the semi-diameter of
Venus,

The following tables show the data used :— •

Near Ingress of Venus.

Mean Time by
DiaL

Mian.

Corrected
Distance.

Remarks.

' Observed
j Distance.

Refraction.

m. t.

•

49 3575

47-633

0-014

47-65

Centre of 9 from Limb of .

54 49*25

24*688

0-007

24-70

Distance near Limbs.

59 06*25

32-683

o-oo8

32-69

Ditto ditto.

05 4975

45*594

o-oii

45'6i

Ditto ditto.

10 20-25

52765

0-014

5278

Ditto ditto.

14 4575

60-712

0-015

60-73

Ditto ditto.

■

Nfap

. Egress.

58 53-25

56-416

0-044

56^46

Distance near Limbs.

04 38-50

46734

0-036

46-77

Ditto ditto.

09 21*25

38-366

0-029

38-40

Ditto ditto.

12 43'5o

32-318

0-025

32"34

Ditto ditto.

But the measures of cusp distances have been differently treated.
The rapidity and irregularity of the relative motion of the cusps render
it impossible to group them, and I have availed myself of the facility with
which each would give a value of the contact time, in order to eliminate
sensible error in the zero, or rather to show that it does not exist*

REDUCTION OF EQUATOREAL OBSERVATIONS.

Ar c

Ao* Ar
0* "~ 0*

- Sin. Q.

sec. Q

Ao-
- tan.

fan P

Let 2C be the measured distance of the cusps,

c c

take = sin. Q — q— = sin. R.

and, internal contacts alone being necessary to be considered, we should
have, if the data were all accurate —

Dist. of Limbs =s X = (l - cos. Q) <r - (i -cos. R) S' = 2 [o- sin.^ Q-S sin.«4 R]
also AX = (I - COS.Q) Ar - (i - cos.R) AS + <r sin. Q A Q - S sin. R A R

and cos. Q A Q as
orAQ =

similarly AR = -^^ sec R - -^^tan. R.

Hence AX = (tan. Q - tan. R) Ar + (i - sec. Q) A<r - (i - sec. R) AS'
and the complete value of X is X + AX.

In order to deduce the time of contact from the distance of limbs, we
have first to compute the equations for the observed time of contact where
the computed difference of centres contains a term in at., whose co-effi-
cient is --^y whence we easily deduce ^jy, but we cannot consider that

the motion is quite uniform. To get j^ we proceed as follows :—

D' = ;r« -ny = sin. NPD' © sin. NPD' % (Diff. iR)« + (Diff. NPD)«.
^,d\y dx dy

^^^ ^^r +-^-rft7

Id \y Y rfVD' _ / <£r Y / dy Y since the motions in Si and Dec.

\ dt ) d\} "" \ rft / "*" \ rft / may be considered constant.

He-^=^{(4)*-(4-)'-m"}

and

d« t «/t»

dD'

s —

(^)'

Then T (time of measure) =T^ (time of contact) ± (X -1- AX) ~y^

+ i(X + AX)»^,

the upper sign being used at Ingress. And, if we neglect AX^ we have

dr ,^t fit d't)

To = T-X5^-J (X)« j^ - A X \j^ + X j^J'

REPORT ON THE TRANSIT OF VENUS.

LAHORE STATION.

Lahore, the Capital of the Punjab, was chosen as a secondary station
mainly because it was easily accessible by rail. To have gone further west
would have entailed carriage by carts, which would have taken a consid-
erable time, if the neighbourhood of Peshawar had been chosen ; and,
moreover, the further west we proceeded the greater seemed the chance of
the whole observations being frustrated by clouds.

The station chosen by Captain Strahan was in the compound (or
enclosure) of a house occupied by Dr. Calthrop, commonly known as
Mr. Elphinstone's house, and the property of His Highness the Maha-
rajah of Kashmir: the house is about 500 yards, N. N. W. from Govern-
ment House, and the data given by Captain Strahan will ensure the
identification of the places of the pillars (even if they be destroyed) as
long as the house stands, and with a little trouble at any time. The
equatoreal pillar was connected by a traverse with a temporary station
on a mound, called by Captain Strahan Donald Town Station, which,
again, was connected with three stations (secondary) of the Great Trigono-
metrical Survey, and with the site of a fourth which had been destroyed.
As the three known points are all spires of buildings, angles could not be
taken at them, but Majdng Station (the destroyed one) is. Captain
Strahan has informed me, on a mound so limited that it could not have
been more than two or three feet away from the original position. I have
verified the position of Donald Town Station as follows. Captain
Strahan's observed angles give the means of deducing the positions both
of his station of Donald Town and also the point he assumed to be
Majdng from the three undoubted points of the Great Trigonometrical
Survey, and consequently the distance and azimuth of Donald Town from
Majdng Station admits of verification. I find —

1st — ^Assuming that Captain Strahan's is the true Majdng Station ;

Donald Town from Majdng Station is 6517*1 feet at an azimuth at
Majfeng of 239' 49' OS'.

REDUCTION OF EQUATOREAL OBSERVATIONS, ^^

2nd, — Interpolating Majang Station, and thence deducing Donald Town ;

Donald Town to Majdng Station is 651 81 feet at the same azimuth.

^rd. — Interpolating Donald Town and thence deducing Majdng Station ;

Donald Town to Majang Station is 6520-1 feet, azimuth at Maj&ng
239' 48' 57".

There can thus be no sensible uncertainty as to the place of Donald
Town.

From the mean of the last two values of the data and the Great
Trigonometrical Survey data for Majdng Station, the position of Donald
Town is deduced as —

N. Latitude 31** 33' 37*84^

E. Longitude 74** 22' 39*45^

and since the traverse shows the equatoreal to have been distant
from Donald Town Station 157 feet at an azimuth of 173** i/, we
have for the position of the equatoreal by the Great Trigonometrical
Survey —

N. Latitude 31'' 33' 38-51'

E. Longitude 74** 22' 29*00''

The last quantity has to be decreased by 3' 01 '8^ to reduce the
Great Trigonometrical Survey longitude to that which would be got by
using, as a normal value, the present accepted longitude of Madras.
Thus I get-
Geocentric Latitude ... 31* 23' 23*5' N.

Longitude ... 74' 19' 27*2' E. = 4&. 57^. 17-81^. E.

The height I have assumed from the best data I can get as 920 feet above
the normal Ellipsoid of the earth, and hence p i=: 9*9996209.

Captain Strahan determined his time with. a transit, instrument qf
the Russian pattern belonging to the Great Trigonometrical Survey of

REPORT ON THE TRANSIT OF VENUS.

India. It is made by Messrs. Cooke and Sons, and the vision is through
one end of the axis, the converging rays from the object glass being
reflected down the axis. The aperture was about three inches. Captain
Strahan reports the instrument very convenient to work with, but some-
thing led him to believe that the coUimation correction was not constant,
and he made a series of determinations throughout a day, of which the
result seems to be that the position of the optical axis on the micrometer
frame for the wires depends on the temperature. It may well be so, as it
is hardly possible to conceive any mode of attaching a reflector in the
middle of the axis by which the elasticity of metal shall not be brought
into play, but while Captain Stbahan's observations fully establish the
variability of the reading of the optical axis, they can, I think, hardly be
considered as exhatisting the subject and proving temperature to be the
sole cause. The level correction is determined by the application of a
level to the pivots ; these have been assumed to be equal, as it was found
that there was no considerable inequality; it would be an improve-
ment to observe the wires by reflection. A 7-inch theodolite, by
Messrs. Cooke and Sons, was enclosed in a blackened wooden box
and used as a collimator. The Transit pier was 18 feet east of the
equatoreal pillar.

Captain Strahan was furnished with two Chronometers, a Solar and
a Sidereal. The Solar Chronometer was kept unmoved, and in as nearly
a constant temperature as possible, in order to furnish a check on the
Sidereal Instrument which was used in the Observatory where it was
exposed to considerable changes of temperature, but the comparisons
made three times a day and the rates from Transits unite to show that
the Sidereal Chronometer was quite trustworthy, and I have consequently
neglected the check one.

The following table shows the determinations of Chronometer cor-
rection to Sidereal time, and will be sufiicient. I do not enter here into
detail as to the determination of instrumental errors, because the only
one as to which any doubt could exist is that of collimation, and in such
an instrument it would be difiicult to give the means of re-determining
the error without drawings showing the parts of the instrument, or detail
descriptions such as I have not now the means of making ; but the deter-

REDUCTION OF EftUATOREAL OBSERVATIONS.

minations have been carefully deduced and verified by Captain Campbell
and myself: —

V —

Date.

Chronometer
Time.

Correction.

Change
between Observ-
ations.

Rate per day.

h» m.

December i

I 21*2

+ 16-87

• ••

•••

„ 2

I 29*2

2392

+ 7-05

+ 7401

I 29*2

31-48

7-56

„ 4

I 13-8

39*42

7-94

8-03

I 44-4

4776

774

7-58

I 28-8

54-53

7-37

7-45

I 44*2

62*17

764

7-56

4 217

71-04

8*87

8-00

I 34-9

78-00

6-96

rBr

„ lO

J 45-5

85-33

7-33

7-a8

The individual observations are very good ; and the effect of the
chronometer rate is seen in looking over the individual determinations of
the correction by the separate stars. On December 16, an attempt was
made to determine the difference of Captain Strahan's habit of observing
from mine, each of us taking a set of Transits of the same stars with the
instruments and time^keepers, &c., as when working separately ; but on
examining the comparisons of the chronometers and clock, it was found
impossible to clear up satisfactorily an error in recording the minutes of
comparisons between the solar and sidereal time-keepers. I am sorry
therefore that I cannot give the result ; but it is evident that Captain
Strahan's peculiarity of noting time must have affected his Transit
Observations just as they did those of the contacts.

For observing the contacts of the Transit of Venus, Captain
Strahan was supplied with a 6-inch telescope by Mr. Simms, which I
had procured for the Total Eclipse at Dodabetta in 1871. For this I

REPORT ON THE TRANSIT OF VENUS.

had had an equatoreal mounting made at the Roorkee Workshops, which,
though very rough and very heavy, was steady. A roughly-graduated
circle (for cutting the divisions on which I am indebted to the Mathemat-
ical Instrument Department in Calcutta), read by two Verniers, gave the
means of adjusting the polar axis to its proper position. A handle with
a Hook's joint enabled the observer at the eye-piece to keep an object in
the field, and another gave slow motion in declination : each handle
actuating an endless screw working in a racked circle.

Some days before the Transit > experiments were made as to the best
vision obtainable of spots on the Sun : it was found that an eye-piece
magnifying about 125 times and applied with a plane solar reflector to
the whole aperture gave the best results. Mr. Simms had sent an eye-
piece supplied with means for correcting the dispersion at low altitudes
and which was of higher power ; this I had intended to be used, but the
telescope stand was not ready till it had to be packed up, and we had
no opportunity of trying the eye-pieces in position at Roorkee. On
trying this, it was found that when the solar reflector was interposed, it
would not come to focus ; it was then too late to remedy what could
easily have been corrected if known in time.

Captain Strahan after stating the conclusions arrived at as to suit-
able arrangements for vision, says of the observations : —

'* Having assured myself on these points, and knowing that my
** attention would not be distracted by having any measurements to make,
** I awaited the phenomenon without much anxiety. The weather on
" the morning of the gth was most favorable. Ingress so far up-country
*' as Lahore was not visible : and for about an hour after sunrise, the
*' limbs of. both the Sun and Venus were trembling considerably ; but as
*' the Sun got higher, the definition became better, till, about 15 minutes
*' before cofitact, the edges of the two bodies were as hardly and sharply
** defined as the most sanguine observer could have wished.

*' Your telegfram had prepared me for the absence of the black drop,
** but not entirely for the appearance eventually seen. As the planet
" moved towards the Sun's limb, she appeared to push away his edge
" before her, the cause of which became evident in a few seconds ; the
" planet's edge was, in fact, encircled by a ring of light nearly as bright

REDUCTION OF EQUATOREAL OBSERVATIONS.

•' as the Sun, which prevented any contact, properly so called, from taking
•* place at all. The monient I have assumed for internal contact, and
** recorded as such, is No. 2 in the Sketch (not reproduced), when the
** Sun's light, unbroken by the ring of light, would just have grazed the
'* limb of Venus: This appears in my note book as 'slight darkness
** between the limbs/ I have considerable confidence in the accuracy of
'' this observation, as the limbs were beautifully steady and sharp, and no
" distortion apparent. There was no vestige of anything that could be
** called a black drop or ligament. The following* is a verbatim copy of
** the hurried notes made at the time" (the times given are chronome-
ter times) : —

** Slight darkness between the limbs at i6A. 13m. 22s. ; some uncer-
** tainty about the minutes ;t seconds correct, no black drop; the darkness
** enveloped more of the limbs without increasing in depth (of shade) ; no
** dark ligament occurred at all. After contact the limb of Venus outside
** distinctly visible, owing to bright line of light round enveloping two-
** thirds of it. The part absent (nearly) being on the east side. At 16A.
** igw. the light was round three-quarters of the Umb of Venus, confirmed
** by two spectators ; cusps absolutely and perfectly sharp. At 16A. 23 w.
** the edge of light diminished to half the circumference of the western
" limb. The part of the planet outside the Sun was palpably darker*
''than the sky; dense black background being purplish. Its shape in
** no way distorted, magnified or diminished. At 16A. 26m. the edge of
** light was fading; at 16A. 31W. 155. gone, traces occasionally seen again ;
'* at 16A. 34m. 0555. edge again distinctly visible ; at 16A. 37W. it extended
** round two-fifths of the circumference on the same part of the limb as
** before; at 16A. 38W. 155. visible as a glimpse only. External contact
** and total disappearance at 16A. 41W. 095."

'' An elaborate discussion of these observations would, perhaps, be
^* out of place in this Report ; but I am, nevertheless, tempted to offer a
*' few remarks on appearances so unexpected. There can be little doubt
^' that they point to the existence of an atmosphere round the planet, and
'' there is certainly no a ^ion improbability in such an explanation. It
'^ is difficult to account for the position of the strongest part of the ring

* Captain Strahan here gives his correction of the chronometer, whidi is erron euiis.
t Calculation shows the minutes oonect*

REPORT ON THE TRANSIT OF VENUS.

** of light being uns5mimetrically situated with regard to the line joining
'* the apparent centres of the Sun and Venus ; but this is established
'* beyond all doubt — indeed, the most unpractised eye niust have noted
** the circumstance. It will be observed that the brightest part of it
** is almost exactly on the preceding portion of the disc reckoning along
*' the line of the planet's motion ; but whether this is a mere coincidence
'* or a significant fact, is not readily apparent. The ring was visible up
'* to the time of external contact, which enables one to make a rough
*• estimate of the refractive power of the planet's atmosphere ; inasmuch
" as the minimum (?) duration of a solar ray reaching the observer's eye
** after refraction when Venus is at exterior contact must evidently be the
'* apparent diameter of Venus as seen from the Earth + the apparent
** diameter as seen from the Sun. This deviation, in the present case,
** amounts to about i' 27"."

As regards this last remark, the refraction deduced is but a small
part of what has been got before by other means.

From the corrections to, and rates of, the chronometer given before,
I deduce the following sidereal times of observation : —

h, 1H, s.

Internal Contact — By Chronometer ... ... 16 13 22

Correction ... ... +1 14*93

Lahore Sidereal Time

H 3693

A.
16

1508

2408

External Contact — By Chronometer

Correction

Lahore Sidereal Time

The reductions have been made in precisely the same way as those
at Roorkee ; save that I have converted these sidereal times into Green-
wich mean time and used that for calculating places in the series.

In revising these calculations I found that I had inadvertently used
a longitude 4 seconds (or one minute of space) too small. The equations,
as quoted in the following results, have been corrected by the use of the
coefficient of AL and are now accurate*

RESULTS. ^g

RESULTS.

For the external contact at Ingress there are no observations. For
the internal contact I have the following : —

ij/. — ^The results of hour measures of the cusps near internal contact at
Roorkee.

2nd, — ^The observed internal contact at Roorkee.

yd, — The measures of distance of limbs near contact.

Of these, it will be convenient to leave the cusp measures to be con-
sidered last —

1st. — Observed Contact.

The observed time was 12A. 42W. 30*05. ± 2'05., whence I deduce
the equation —

7^58 ± 0^07 = -0^-0335 At + o-''o33SAL - AS + Ao- + 0-6418 AiR g

-07182ANPD8 +0"3338An

2nd.^^0bserved Distances of Limbs.
From these I deduce the following six equations of equal weight : —

6''-23 = -o'o3i8At +o-'o3i9AL -AS +o-6233AiR 8 -07371ANPD?

+ 02184 AIT

7''i8 = -0''0306At +0^0307 AL - AS + Aer 4-0-6091 AiR ? -07SO9ANPD 8

4-Oi3iSAn

7''i3 = -0^0295 At 4-o''o296AL -AS + A<r 40-5969AiR 8 -07623ANPD8

+00588 An

6'ii = -o'o278At +o'o279AL -AS 4 Aer +o-577iAiR 8 -07801ANPD8

- 00570 An

6'Si = -o^0266At +o'o267AL -AS +A<r +0-5632AiR8 -o 79i9^NPD 8

-oi358An

5^69= -0''O2S4At +o''0255AL -as 4A<r+OS49i^^8 -o-8o34ANPD8

—0-2139 An

REPORT ON THE TRANSIT OF VENUS.

On looking over these results it is evident that the discordances are
mainly due to errors of observation. I have therefore after some con-
sideration preferred to deduce a probable error of result from these dis-
cordances. Taking the mean equation thus, I find —

6-'47 ± 0^23 = -0^0286 At +0^0287 AL +0-833 Ao- -AS 4-o-586sAiR ?

-07710ANPD9 +oooo3An.

^rd. — The cusp measures.

For the time of contact, calculated as I have explained from each of
the cusp measures, we get the following values : —

A. m. s.

12 42 446 +4r48oAc - 22-461 Ao- +001 1 AS

42 42-0 +31-270AC -i4-2i5A(r +o*009AS

43 lo-o +25084 Ac - 9738 Ao- +0-006 AS (rejected).
42 39*3 +14-67 1 Ac - 3-643 Ao- +0003 AS.

It seems probable that the time in the third measure has been noted
wrongly half a minute : I prefer to reject this rather than apply any con-
jectural emendation. If there were any error of zero, then a sensible
value of Ac would exist, and the sign would differ in the central obser-
vations from that in the first and last. It is manifest that there is no
such correction needed. In order to weight these equations, I assume that
one measure is subject to a probable error of half a second, which then is
the probable value of 2 Ac. Hence the probable errors of the retained
values are 10-85., 7'8s., and 375., and their weights may be taken as i, 2
and 9 respectively. Hence the mean of the values will be—

12A. 42m, 40'2s, ± 3-2J. - 6*973 Ao- +0005 AS.

As the observed time of contact was 12A. 42m. 30*05. we shall get an
approximate equation for this time by substituting in the contact equation
-|-io'25. ± 3*25. — 6'973A(r +o'oo5AS for At and restoring the term in At
which gives —

7^-92 ±o'-ii = -o*'-0335At +0^-0335 AL +r234A<r -roooAS +o-64i8AiR8

-07182ANPD? +o-3338An.

RESULTS. ^j

I am veiy doubtful if th6 equations from micrometer measures
and those from actual observation of contact should fairly be combined
into one equation, but if we do so combine them, we have for their
weights-—

Measures of limb distances .,. ••• i

>, cusp ff .*• ,,, 4.

„ observed contact ... ... 11

and the mean equation will be —

7^56 ± o''-o6 = - 0^0332 At. +o'o33aAL -AS +ro68A<r +0-6401^^9

- 07197ANPD i +0-3263^0.

This contact was also observed by Mr. Hennessey at Mussoorie, and
the data are fully given in Proceedings of the Royal Society, VoU XXIII,
P- 379* I have computed a similar equation from these data ; only assum-
ing as the height of the station 6,850 feet, (for the same reason that I have
increased the levelled height of Roorkee) to get the increase to the Radius
Vector of the Normal Ellipsoid. I deduce the following equation : —

i^^o ± o'i2 = -o''o342At +0^0343 AL -as +A(r +0-6496AiR 8

-oyoqgANPDs +03564 An.

It will be seen that the observation by Mr. Hennessey differs a great
deal from mine, and can only be nearly reconciled with it by supposing an
error of three minutes of time in one of the Records. Now, I think, that the
accordance of the residuals from the various classes of observations may
be considered to be sufficient reason for trusting my own work ; but I have
conclusive evidence, I think, that the actual contact did take place at the
time I have noted very nearly. From Captain Campbell's Alt- Azimuth
Observations I have deduced errors of the tables in JR and NPD« These
give the following corrections of the tables :—

AiR = 6^502.
ANPD = - 2^-247.

Also Mr. Dunkin states that the value of AS when the mean semi-
diameter of the Sun is assumed 96i*''82 is — o*'53 from the Green-
wich Observations (Royal Astronomical Society^s Monthly Notices, Vol.
XXXIII, p. 294), and I measured the mean diameter of Venus as 63'''g489
whereas in computing I have deduced from Mr. Stonb's value 64''*io;
thus A«r would be — o^'oyG.

^2 REPORT ON THE TRANSIT OF VENUS.

Substituting these values in the above equations of condition for
contact at Roorkee and Mussoorie, we get—

AtRoorkee ... i'-34 = -o''0335A»- +o''o335^L +0-3338 An.
At Mussoorie .. . -4''97 = -0^0342 At + 0^-0343 AL +0*3 564 AH.

The discordance at Mussoorie seems unaccountable by any hypothesis
but that I have suggested. I may add that the published time is only
23 seconds later than that which is deduced from the Nautical Alma-
nac formulae of prediction, in which agreement it is, I believe, singular.
Mr. Hennessey assures me that there can be no error in th^ record
of observation, and I am quite at a loss to explain the phenomenon.*

At the egress of Venus from the Smt^s surfau.-^l have, as at the
ingress, three classes of observation near the internal contact*

From the measures of limb distances I get the following equations
of condition which as before have equal weight : —

3''-2o =Bo''o27iAT - 0^0275 AL -AS + Ao- - 0*1454 AiR J -0-9875ANPD9

- 2*2264 AIT.

3^04 =o''o287AT -o'o29oAL -AS + Ao- -oi679AiR? -o*9833ANPD8

-2-2291 AIT.

2'-92 =o''o298AT -o''o303AL -AS + Ao- - 01858 AiRg - 09795 ANPD?

-2-2275 AIT.
2^73 =:o'o307AT -0^-03 1 1 AL -AS + Ao- -o-i984AiR$ - 0*9766 ANPD?

-2*2245 AIT.

The mean value is —

2'*97 ±o'''io « 0^0291 AT -o'x)295AL -AS+Ao- -0'i744AiR9

-0-98I7ANPD8 -22269 AIT.
of which I have derived the probable error from the probable error of
one measure as deduced from my measures of the diameter of Venus.
The observed internal contact gives the equation —

-2*i889An.

The cusp measures give as the values of contact time—

h» in% s, s»

16 28 287 - 24-567 A^ + 9-561 Ao- -o*006AS ± 3*6 tt//. 2*6

34*9 - 29*064 A^ +i2*772Ao- -0'008AS ± 4*4 «//. 1*7

268 -36*704A^ +i8*753Aor -0'Oo8AS ± 5*5 «//. i*l

* Captain Campbell obMrved the contact within two seconds of my observation, but the small aperture he
used, and the high probable error I should have assigned, has made me leave this observation alone.

RESULTS. ^^

of which the probable errors are derived from the assumption that a
single measure of distance has a probable error of o'''3.
The mean of these values is —

n, fft, S, Sm S, S,

i6 28 303 ± 25 +l2-444A«r -0007AS

and I derive as before the equation of condition —

3''-39 ±o'09 =o''o344AT - 0*0349 AL +r428A(r -AS - 0*255 rAiR?

-o-96ioANPD? -2'i77^An.

The weights of these three equations of condition are, i, 11 and i'2;
and the mean equation resulting is —

3''o8 ±o''03 =oro34oAT - 0^0345 AL +i039A<r -AS -o-2So6AiR8

- 09621 ANPD 8 - 21908 AIT.

Captain Strahan's observation at Lahore gives —

3'-23 =o'o344AT -0'0349AL -AS +A<r -0-255iAiR8 -0'96ioANPDs

^2-2641 An.

I should think that the probable error of this observation of time might
well be several seconds, but the class of observation is so different from
mine that I cannot assign a probable error to it with any confidence.

From Mr. Hennessey's observation of this phase at Mussoorie I
deduce the equation —

3^77 ±0*07 =cro344AT - 0*0348 AL - AS +A(r -a2542AA9 - 0*9612 ANPD;

- 2*2052 All.

External contact at Egress.

For this I have only the eye observations of each station. These
give—

1st. — At Raorkee,

4'-88 ±o''o8 =oo399AT - 0*0405 AL -AS -A<r -o-3404AiR8 -0'9294ANPD8

- 2X1)5 84 AIT.

2nd. — At Lahore.

3'-38 sso'atOOAT -o'O406AL -AS -A<r -0-3424^*8 -0-9285ANPD8

* 2* 1492 An.

^rd, — At Mussoorie.

4'-96 ±0*04 «:0'0399AT - 0*0405 AL - AS - A<r - 0*3405 AiR 8 - 09294 ANPD 8

-20725 All.

^ REPORT ON THE TRANSIT OF VENUS.

Captain Strah AN *s observation is discordant: probably this is due to the
same cause which showed the bright line round Venus to him and none
to me.

It will be seen that the Egress observations point to the necessity
for a sensible decrease* in the semi-diameter of Venus, though Captain
Strahan's observations at Lahore would make the semi-diameter very
near what I measured it. I believe the solution of this small value must
be sought in the irradiation, which would especially affect the observa-
tion of the last contact and make it occur too soon, while Captain
Strahan's eye might have been guided by the fact that the disc of Venus
was visible outside the Sun.

It is more difficult to explain the fact, that while the cusp measures
point to a time of contact when to the eye the planet was entirely within
the Sun's disc, the measures of limb distances point to one when the
internal contact was visibly incomplete. A negative value of A<i« would
increase the discordance and appears inadmissible in this case. If the
value of the micrometric screw were sensibly affected by temperature, we
should probably find the measured distances too small, though the
accordance of results would be greater if the measurements were some*
what increased. I see, however, no way of determining the effect of the
Sun's rays on the scale, unless the temperatures of the object-glass and
micrometer can be separately estimated : it is clear that a temperature
coefficient, derived from a general change in temperature of the whole
instrument, could not be relied on.

I am disposed to think, that where the spectroscope isf not used,
external contacts should be observed with a pale dark glass, so as to
facilitate the seeing of Venus outside the Sun's limb, but that a smoke-
colored glass (brown yellow) as deep in tint as is convenient should be
used for internal contact to diminish, as far as may be, the irradiation
and the haze round the planet.

♦ Allowing for the errors of place before given, I find for the scmi-diamcter of Venus—

By my direct observations. . .

• •»

•••

... 3o"'87

M Mr. Hennessey's

■ • (

• •*

... 3i"i8

M Captain Strahan's

I*.

...

... 3»"70

Hence the mean semi-diameters would be—'

•

ri6o,8"-a4a,8--379,

and the mean of all, 8 "'260,

net far from Eucke's determination from the Transit of 1761.

RESULTS.

Micrometer observations are certainly valuable as checks on the
direct observation ; whether they are sufficiently accurate and otherwise
suitable for combination must be determined from a larger experience
than that of one station, but the following conditions seem essential to
give them the greatest value in future Transits : —

is/. — The plane glass reflector should not be cemented into its place,
but capable of expanding and contracting freely independ-
ently of the brass surroundings.
2nd. — Means must be supplied for determining the temperatures
of the object-glass and micrometer separately as a check
on the scale value.
^rd, — As it is possible that any reflection may alter the working focal
length of the object-glass, it would be desirable that the
scale should be determined with the reflector in situ. If the
first of these conditions be fulfilled, the plane glass might be
temporarily silvered by Liebig*s process, when it would be
possible to use stars for determining the scale, while the
removal of the silvering would make the reflector available
as usual for reducing the Sun's brilliancy and heat.

aQ report on the transit of VENUS.

THE ALT-AZIMUTH OBSERVATIONS AT ROORKEE.

These observations were so designed that if the photographic opera-
tions failed, there might still be a series of observations of the relative
position of the Sun and Venus throughout the Transit. I had many
reasons for doubting the full success of the photographic class of observ-
ations : their principle is of course sound, but the application of it
requires the solution of several problems which I think can hardly be
considered as having their difficulties cleared up even at this date : I
had no means of entering on them myself, and it was impossible to
get full information of what was the progress made elsewhere. From
my previous knowledge of the Alt-Azimuth I had very high ideas of its
probable accuracy when used as I have attempted to use it: I think now
these were too great, and my present experience has very much raised my
estimate of the accuracy to be attained by observations of contacts and
micrometer measures, so that I should now doubt whether any other class
of observations can compete with such as' I took with the equatoreal
when the circumstances are favorable, but the various discussions as to
past Transits of Venus, and the accounts of Transits of Mercury (which
I never had any opportunity of observing with good optical appliances)
formerly threw great doubt on the adequacy of such observations for
Parallax determination.

As it was in any case necessary to employ the times of observations
in the reductions, I had determined to use these alone independently of
the divisions on the instrument's limb, making the observations strictly
differential. The accuracy with which the adjustments of a good Alt-
Azimuth can be made and their errors determined seemed to me to give
this instrument great advantages for making the requisite observations ;
and the instrument itself which was available was especially steady in
all its adjustments ; it was the new Great Theodolite made by Messrs.
Troughton and Simms for the Great Trigonometrical Survey of India

THE ALT-AZIMUTH OBSERVATIONS AT ROORKEE. a^

under Colonel Strange's supervision. I had no time, as it turned out, to
study the peculiarities of the instrument ; but I was fortunate in Captain
Campbell, who was quite competent to work out all the details of my
general idea.

The instrument is described (so far as is necessary) in the Proceed-
ings of the Royal Society, Vol. XX, p. 317. It was mounted under a
revolving dome on a brick pillar capped with stone, and a platform was
constructed round the pillar to enable the Observer to reach the eye-end
of the Telescope with facility. The errors of the vertical axis were very
small and very steady for long times, so that it was considered safer to
trust to the instrument than to introduce the uncertainty of level readings.
The supports of the transit axis admit of no adjustment at all.

The neglect of reading the level along with the observations enabled
far more work to be done during a given time, and it had incidentally
another advantage : there was no necessity for a rigorous investigation of
the value of the level scales. If this had been necessary, it is doubtful if
it could have been carried out, as the levels were all found to have their
runs greatly varying with the temperature and unequal in different parts,
so that the difficulty of getting a sufficient number of observations at a
low temperature would have been very great indeed.

Of course it was of the first importance to protect the instrument,
while in use, from the direct action of the Solar rays, and for this pur-
pose the following arrangements were made by Captain Campbell. A
disc of millboard, 24 inches in diameter, was stiffened with brass, and so
fixed on to the object end of the Telescope that it could without difficulty
be removed and re-attached. The weight of this of course disturbed the
equilibrium about the transit axis, which was carefully restored by the
weights provided for balancing. A thick screen of cotton cloth was then
placed over the opening of the dome, so as entirely to prevent any rays
from the Sun passing it except through a hole 12 inches in diameter made
in it and which could be adjusted, by moving the whole screen, so as to
allow the rays to fall only on the object-glass and surrounding millboard.
A flap closed this hole when it was not wanted and completely shut out
the Sun : the result was quite satisfactory. The millboard of course
prevented the reversal of the Telescope while it was attached, and it thus

REPORT ON THE TRANSIT OF VENUS.

became necessary only to reverse occasionally instead of after each com-
plete observation as would perhaps have been desirable, but this is of
small importance in such purely differential work.

Great care was taken to take the transits of the Limbs of the Sun
and Planet over the same parts of the wires, so as to remove any errors
from their position being slightly erroneous.

All the observations were registered on the chronograph, on which
one strip of paper was devoted solely to the instrument.

There were five vertical and five horizontal wires in the telescope
whose intervals were duly determined : those of the horizontal wires by
transits of a Persei over them when near its greatest eastern elongation,
and those of the vertical wires by transits of small stars as ne^r the
pole as the instrument could be conveniently set and near the meridian.

All the chronograph records were read by Captain Campbell and
verified by him afterwards, and before he left me he had aided me in the
computations as far as the bringing up of the wire intervals.

REDUCTIONS.

The general principle of calculation has been as follows : —
It has been assumed that the instrumental corrections were so small
that they would not affect the differences of the times of transit of the
Sun and Venus over the wires ; any small effect would be further reduced
by the occasional changes of face.

15/. — A very approximate time of transit of the centre wire has been
obtained for the centre of each body from the transits of
both its Hmbs over that wire with an allowance (in the case
of the Sun) for the change in the rate of motion which
could be inferred fVom the last computed observation ;
2nd. — For these approximate times of transit, the apparent places
have been computed in exactly the same way as for the
reductions of the observations already discussed ;

REDUCTIONS.

^rd. — The next step has been to compute the apparent zenith dis-
tance or azimuth, as the case might be, with factors for
the correction of the resulting value for corrections to the
time, longitude, mean solar parallax and (in the case of
Venus) to the right ascensions and declinations of the
tables ;

4th. — By suitable formulae each Transit of a limb over a wire has
had deduced from it the corresponding time of Transit of
the centre over the central wire and a mean value of this
time has then been deduced ;

5/A. — By using the coefficient for a correction to time, the zenith
distance or azimuth at. the assumed time has been reduced
to what it would have been if computed for the mean time
of Transit of the centre, and

6th. — ^As the zenith distances or azimuths were by observation the
same, the difference of the two computed values has been
made = o, giving an equation of condition.

If we call the Apparent Right Ascension JR + v^R'.

NP. Distance NPD' + vNPD'.
Sidereal time t + A r and 0' = t — /R\
Also <^ the Latitude,
and tan. M = cot, NPD' sec. 0'.

we shall have —

A ^ 'TiA I cos.(^ - M)cos.NPD' . /w-A ^ JD'\ COS. ^ sin. NPD' sin.0

Appt. ZD=cos. -• .-^ ^ — ^-^-=rj + (isAr-ViR; ? r-?7rj

*^*^ sm. M sin. appt. ZD.

- VNPD'. cof - » sin. NHy ^^^^ jjpjj, ^^^ ^ _ ^
^ sin. appt. ZD ^ ^'

of which the first term is the Tabular Zenith distance which is used in
the denominators of the other terms.

So also —

A *. A •-«. 4.U 4.-1 COS. A tan. 0' sin.' Az. cos. ^ ^xTT>r\

Appt. Azunuth = tan.-« -^ — ^ — -=r . 6 xtpw - — ^- VNPD

'^'^ sin. (^ - M) sm.* NPD sm.

+ (15 At - V-*) sin-' Az. sin, ^ [i +cot. Az* cot. 0cosec, ♦]•

CO REPORT ON THE TRANSIT OF VENUS.

In computing both these I have used in getting the co-efficients, the
logarithms taken out for the computation of parallax which correspond
to the true values instead of those for the apparent values.

For reducing the transits of limbs over wires, and the transits of
centre over middle wire, I have used the following formulae : —

15/. — For transits over the horizontal wires —

Let (o be the apparent Zenith distance at transit of a centre wire.
$o and 0^ the Declination and Hour Angle at the same instant.
C the distance of the wire from the centre wire.
.S the semi-diameter of the body.

Asii ^ 2 p p cos. i;' sin. i'' where / is the Equatoreal Horizontal
JS=s I + 2 / p sec. i;' sin. i'' 5 Parallax of the body.

a =yV . -j~- ; *=5 15 . _- ; and / = the interval of time between the
»t« at.

observed transit and that of the
centre at the centre wire.

Then—

j^ sin, g, C ± S

li A COS. 8, cos. ♦ ■ (I - tf) sin. 0^ - * (i - tan. 8^ cot. ^ cos. 0J

15 cos, e^ " JB cot. Co cosec. g^ cos. 8^ cos. <p sin.» 0^ j^ . ^awin t'
2 • (I -a) sin. 0^ -* (I- tan. 8^ cot. ♦cos. 0J • -^ • ^* " «^ «»"• ^ •

where the / approximately computed from the first term is
used in the second.

2wrf.— And for reducing transits over a side wire to the centre vertical
wire, I have used the following formula :—

j_ C ±S

15 ' ff [cos. p cos. 8^ - sin. p sin. 8^ cos. (0^ - ^)] - (i - a) sin. p cos. 8, sin. (0^, - ^)

J2_ J COB. (0^ - ^) sin, p sin. 8, \s / • •

2 ' ff [cos. p COS. 8^ - sin. p sin. 8^ cos. (0^ - «^)] - ( i - tf) sin. p cos. 8^ sin. ' (0^ - ^) ' ^^ *^' ^* ^^^' ^

where n = — -- . —j— ; cos. p = - sin. Az. cos. ♦ ; and cos. ^ = - tan. ± cot. p.

1$ at ^ r

The following Abstract of one of these calculations will make the
procedure more plain : I have given the figures as aiFected by the error

/ =

REDUCTIONS.

in the Solar Right Ascension which, as I have explained, have been
allowed for in the results in all other places : —

Intbryals from

Zbro.

No. of Set

Clock time of
Zero.

Objects.

•

'\ /

o-oi

16*29

25-93

34-86

Sun's I St limb.

No. 5.

«9 l-l

185-85

i93'97

202*40

212*32

221-23

:. 2nd „

Horizontal

Wires.

87-23

95-09

10373

1 13*38

122-40

Venus' I St „

1 -^-^ \

93-02

100-97

109-74

119-20

127-92

II 2nd „

Approx. Trans. over C was... 13 07 00 -3 by clock, or 13 06 56*42 S. T,
Tab. Rt. Ascens. ... 255** 46' 40^-36.
„ Declination ...-22** 48' 48''-6i.

Appt.Rt.Asc. = 255**46'47''*68 + 0-0459 At - 00456 AL + 08200 AIT.
„ NPDist. = 112*48' 54''*32 + 0-0043 At - 0-0041 AL + 06392 ATT,
„ Zen. Dist. = jf 24' o2''-93 - io-5oi5At - 00347AL + 09899An.

hf f9%%

Mean of Transits from ist Limb 13 07 00-18) , , .

2nd „ 13 07 0035 i

General Mean

13 07 00*27

At which moment —

Appt. Zen. Dist. « jf 24' 03^-25 - 0-0347 AL + 0*9899 All.

Approx. Trans. 9 over C was ... 13 06 57-7 by clock, or 13 06 53*82 S. T.
Tab. Rt. Ascens. 9 ... 255** 56' 2i''*89.
„ Declination 9 ...-22' 36' 54''-40.
Appt. Rt. Asc. was ... 255*'56'49''*i8 - o*o249At + o-o«6iAL - 3-o556An

+ AiR.»

* In strictness both ^/R and ^NPD have a factor differing slightly from i, but it has been thought un-
necessary to consider them as diffierent, since the errors of the places of the Sun and Venus cannot be separated.

REPORT ON THE TRANSIT OF VENUS.

Appt. NPDist. was ... ii2* 37' is^'-ei - o*oi2SAt + o'oi32AL + rs74gAn

+ ANPD.
„ Zen. Dist. „ •.. 77' 24' oi'''8o - io-5965At + 0-0269AL + 3-6871^11

+ 07047 AiR + 0-645 8 ANPD.

Mean of Transits from ist Limb 13 06 5774> , t ..

?> clock time.
„ 2nd „ 13 06 57-So>

General Mean ... 13 06 5762

At which moment —

Appt. Zen. Dist. 9 = 77^ 24' o2''-65 + 0-0269 AL + 3-6871 AH + 07047 AiR

+ 0-6458ANPD.

Hence* —

0*60 s= 00616 AL + 26972 An + 07047 AiR + 0-6458 ANPD.

The observations were of three classes —

Complete Observations. — Where both limbs of both bodies were
observed over all the five wires (whether Vertical or Horizontal)
of the instrument.

Imperfect Observations, — Where both limbs of the Sun were ob-
served over all the wires, while Venus was only observed in part,
observations of one or both limbs over some wires being
missing.

And a few observations which were rejected as too imperfect to be
worth reducing.

It is manifest that the error of tabular semi-diaiheter could produce
no effect on the complete observations, and that these would, except a few
when the Sun was very low, all be of the same weight. But in the case
of the imperfect observations it was necessary to consider that the diame-
ter of Vends seen with the small aperture of the telescope might be very
sensibly different from the assumed value, and that such a difference
would affect the deduced time of Transit of the Planet's centre. More-
over, the observations would all be of less weight than the complete
observations, and vary somewhat in weight.

* The corrections to time have been omitted.

REDUCTIONS.

.«

The 30 complete observations were therefore reduced first, and the
result showed that the apparent semi-diameter of Venus was in fact I'^'Sa
smaller than it had been assumed, the observations being very accordant,
considering their class. This reduction was then applied to the Tabular
value in computing the 8 imperfect observations, which may^ I think, now
be considered as free from any error from this cause.

I thus obtained 38 equations of condition involving the same correc-
ions to the elements and place of observation as before, and also cor*
rections to the mean times of transit of the bodies observed.

Of these there were 17 from complete, and 5 from imperfect, observa-
tions over the horizontal wires, and 13 from complete, and 3 from imper-
fect> observations on the vertical wires, the incomplete observations,
being about the middle of the Transit*

To form normal equations I have divided the complete observations^
so far as they go, into groups of 5. Two complete and five imperfect
observations on the horizontal wires have been combined to form one
normal equation^ and three imperfect observations in the middle of the
Transit over the vertical wires have been combined with three complete
ones near the end for another group.

I have thus obtained the following seven normal equations^ in writing
which I have here omitted the terms depending on correction, to the
time : —

■

o'yi6o^JR + 0-6309ANPD -f 271242^11 -f o*o6ai/iL - I'^-yg = o (+i'45)

o^77oAiR + 0-6796ANPD + a'S^ysAU + o-o6oaAL - sl'-ga = o (-1-05)

o*6239AiR + 07367ANPD + rsSoiAU + 0-0573AL - a''-ao = o (+o-ao)

0-5050 AiR + 0-8335 ANPD -f a-4336An + 0*0504 AL - d'-gz = o (+0*49)

ioa86A^ - 0-39I5ANPD + o-oo33An + o-o667AL - 7''-33 = o (+o-a4)

i-0349AiR - o*44a4ANPD + ooo37An + o-o663AL - 7'-48 = o (+0*^4)

1-0976 AiR - 0*^3252 ANPD + 0*0027 AIT + 0*0728 AL - 8**25 = o (-o'38)

of which I consider the first equation to have only half the weight of the
rest, owing to the Sun^s low altitude.

From these in the usual way I get the following two equations for
determining AiR ? and ANPD ? , which alone can be got without refer-
ence to other stations : —

4*5645 AiR? + 0-2361 ANPD 9 + 5*i334An + 0*3302 AL - 29''i48 =s o.
.o-236iAA8 + 2-2536ANPD8 + 6-1591 AH + 00558AL + 3''*5*8 = o.

CA REPORT ON THE TRANSIT OF VENUS.

whence' —

^JR9 s= 6''5oa - 0*07 lAL - O'989/MI. p. e. = ± 0^-3 12.
ANPD? =-2^*247 - 0*017 AL * 2-629^0, p.e. = ± o*-439.

These probable errors are deduced by neglecting the terms in aL and
An, and the residuals on the same condition are given at the end of each
equation in brackets.

From these I deduce the following corriections to the place of Venus
in geocentric longitude and latitude —

Corm. to Longitude = 5^757 ± o'-290 - o*o67AL - i*i65An

■

„ Latitude =s 2'''82o ± o*"438 - ooiiAL + 2*528 All.

In which the Sun's place has been assumed to be accurate.

These values differ a little from those I have given elsewhere from my
having gone over the whole solution of the equations again, finding a
email error in work which I had been unable to get checked by an inde-
pendent computer.

.«

PLANS iNu SECTIONS
LARGE TOWER FOR EQUATOREAL.

i * > " 4. > ,. .

it I

> r

SELECTIONS, ETC.

ADDRESS.

Ths increasing interest which is manifested in Selenography renders it important that Students, as well as the astronomical public
generally, should be in possession of the results of each other's labours. With this view the illustrations in the following pages have
been selected. The opening and succeeding articles have been written for the purpose of indicating special lines of research which it is
believed will, if pursued systematically, lead to valuable results. There can be no doubt that the great question of physical chajtoe, as
regards the moon, is of absorbing interest ; and it will greatly assist the earnest inquirer if he can bring to bear on this subject a competent
knowledge of terrestrial chemistry and geology, combined with a determination of the reflective powers of the various rocks which
characterize the earth's suriace. The road most likely to end in the soundest conclusions is that which affords the most extensive views
of terrestrial analogies.

[I]

ON THE STUDY OF CHANGE IN THE LUNAR SURFACE.

By thb Rbv. T. W. WEBB, M-A., F.ILA.S.

To those whose acquaintance with selenography comprises rather
its general features than its minuter or more practical details, it
may appear singular that the question of change on the surface of
our satellite should remain as ^t in so undecided and unsatisfactory
a condition. It is not from any thing unattractive in the subject
itself. Th^e can be no hesitation as to the interest of the inquiry
whether all in our neighbour- world is now dead and cold and still,
or whether the mighty energies which once tore up and devastated
its exterior may not yet be working, though with diminished acti-
vity — or of the kindred doubt whether the solid materials of that
globe are in direct presentation to what is usually considered empty
space, or are shielded by an interposed gaseous stratum, the pro*
bable aliment of some form of vegetable or animal life. It is not,
affain, from scarcity of observers or means of observation. These
have of late been multiplied in a degree unprecedented in the
history of astronomy. Nor has it been from restriction as to time.
Nearly a century has elapsed since the inquiry was first seriously
mooted by Schroter ; during that interval how great has been the
advance of other departments of astronomy ; how many inquiries of
apparently a less promising nature have arisen and been prosecuted
vrith success ; and yet this question stands much as it was left by
the diligent old Hanoverian astronomer. Schmidt has indeed effec-
tually reopened it of late by his very interesting discovery of the
altered aspect of the spot LinnS ; and Birt and otibers, including the
present writer, have pointed out localities where more perhaps than
suspicion may be entertained; but, though many observers are
sufficiently persuaded of the existence of variations of some kind
independent of mere optical effect, others remain unsatisfied, or
withhold their opinions. The inquiry, at any rate, is still very
incomplete ; and it may not be unsuitable to point out some of the
causes which may have operated in checking for such a length of
time the growth of this interesting investigation.

In the first place, the subject has been approached with an
amount of prepossession which would inevitably retard the attain-
ment of truth. That Schroter was one of the most diligent and
careful of observers admits of no doubt, or that as far as singleness
of purpose went, he^could, as he himself asserts, have made affidavit
of what he has recorded. But we are all liable to an unconscious
bias ; and in his case it may have been that his extreme interest
in the almost untrodden path of discovery which he had chosen

led him to overlook some necessary precautions, and to form some
premature oondusions. His great successor, Madler (for Beer is
believed to have had little to do with actual observation), seems
to have been at least equally biased in an opposite direction, and
to have undertaken his task with a determination to ignore rather
than to examine the previous evidence or supposed evidence of
change. There is little risk in the assertion that the real progress
of knowledge has been retarded in this, as it must be in every case,
by such opposing prepossessions.

In the next place, we must bear in mind that the objects among
which our search must lie are for the most part of very incon-
siderable dimensions, and subject to corresponding difficulty of
observation. This cannot, indeed, be affirmed without exception,
as some of the supposed variations in reflective power are on a con-
siderable scale ; but these have been comparatively less regarded ;
while as to the traces of eruptive action, even the most zealous
maintainers of its present continuance would hardly look for any
extensive result. As far as existing delineations can guide us, it is
only among the smallest class of craters or fissures or similar
objects that we may hope ft> find evidence of change. Much of the
past history of the moon is so legibly written in the obvious chro-
nological succession, and correspon^ngly diminished magnitude, of
the results of volcanic force, that there is littie ground to expect
any renewal of similar magnificentiy developed eruptions ; and
though it by no means follows that we have reached a period of
absolute extinction, we have great reason to believe that we look
upon a comparatively quiescent surface, and must be satisfied with
tracing the feeble struggles of expiring power. And so with the
atmospheric question : while it is deducible alike from theory and
observation that our satellite is destitute of an aerial envelope bear-
ing any comparison with our own, we are not justified in denying
the possible, or rather probable, existence of something of an
analogous nature ; but from its extreme tenuity we must infer cor-
responding uncertainty of detection. In either case it obviously
follows that our inquiry is thus limited in a way that greatiy
enhances its difficultjr. Coarser drawings and inferior telescopes
will do little for us here.

Again, we have to remember that the minute details of the lunar
surface are subject to various causes of illusion in regard to their
visibility. It would readily occur that in proportion to their deli-

these sources of inaccuracy, which attach alike to ail telescopic
details, those of which we speak are exposed to their own liabilities
to illusion. They lie beneath a perpetually changing light ; they
reflect that light back to us at continually varying angles; the
shadows through which their true relief is made known fall on a
very irr^ular surface, and are thus subject to frequent accidental
distortionB; and the change of lunar seasons, though small in
amount, is capable of altering the direction of the incident light
and projected shadow quite sufAciently to give a fresh aspect to
objects in suitable positions. From such causes alone many optical
variations might be expected, and will be found to occur. But
selenography would be an ec»ier task if these were all. We meet
with ano&er very influential source of apparent change in Ubration.
The additional unsteadiness, so to speak, of the landscape, the per-
petual shifting of its inclination botii to incident and reflected light,
the constantly varying amount of perspective, and the slow recur-
rence of precisely identical conditions must necessarily add to the
difficulty of accurate discrimination of details.

Such are some of the inevitable hindrances which have always
opposed the rapid prosecution of our inquiry, even in the ablest
and most careful hands ;^ and these may suffice to account for the
incomplete solution of the problem up to the present time.

We are not, however, entitled to infer from all this any thing
unfavourable to future success. The causes of illusion being inva-
riable and recurrent in their nature, their effects will disappear in
a broad and continuous average : nor is too much under ordinary
circumstances to be ascribed to them ; for otherwise, as Bchroter has
justly remarked, we should be constantly witnessing variations in a
single prolonged observation which are contrary to all experience.
Careful and enduring attention will enable the patient selenographer
to extricate his work from this apparent entanglement, to elimi-

bered with hasty and imperiect drawings, careless descnphons, and
precipitate inferences. Whatever is done, should be done thoroughly
as fSaor as it goes ; and where it is left incomplete, the deficiency
should be noted at the time ; and we must be tolerant of uncer-
tainty, and resolute against hasty conclusions. It must, after all,
be strictly matter of evidence, and of such evidence as has borne
rigid comparison and cross-examination. Photography has lately
rendered most important assistance, but should not supersede actual
observation : the artificial eye, though more comprehensive and un-
erring, is less keen and clear, as well as more limited in its oppor-
tunities, than the natural organ. Ultimately we may certainly
venture to hope that, by the careful collection of many independent
results, the effects of personal and instrumental equation, atmo-
spheric indistinctness and optical variation will be so clearly dis-
tinguishable and separable that we shall be able to decide with
certainty what the earlier observers could only infer with much
risk of error, whether there is or is not an amount of apparent
change not to be explained on any of these grounds. Should we
be thus led to an affirmative conclusion, it may be expected to be
of more than one kind ; and this, while adding to the complexity,
will certainly not diminish the interest of the inquiry. In some
cases, as probably in lAnni, there may be indication of alteration of
form ; in others, as in Werner and near Heard, we may trace change
of reflective power ; in others, as probably in some of the minute
variations on the floor of Flato, we may suspect variable density in
a superjacent gaseous stratum. These localities are alluded to as
mere instances of an enumeration which might be largely extended
on another occasion ; at present there is neither oppoitunity uor
space for such a full discussion of the observations of Schroter and
others bearing on these several poiilts as might at some future time
do good service to the student in this interesting investigation.

REMARKS ON THE NATURE OF THE LUNAR SURFACE AND ENVELOPE.

By EDMUND NEISON, F.R.A.S., F.CS.

Mr. Proctor, in his paper in the * Quarterly Journal of Science,'
January 1873, pp. 29-55, on the " Condition of the Moon's Sur-
face," appears to touch too lightly on the primary question which
presents itself when an inquiry into the history of the moon is
commenced — namely, the nature of the surface, from the appearance
of ^hich we are mainly to draw our conclusions. It would appear
more satisfactory, before endeavouring, as Mr. Proctor does, to
ascertain what brought the moon's surface to its present condition,
to determine what that surface really is.

When it is remembered that we are fustly limited to conceiving
only of the same elements in connexion with the moon as we know
to exist on the earth, it will, we apprehend, be found possible to
draw sound condusions as to what this surface must be ; otherwise
we open a boundless field for the imagination, and are in danger of
placing the whole subject beyond the pale of practical discussion.
Arguing, then, from what chemistry demands, there can perhaps be
no doubt that oxygen must enter as extennuely into the constitu-
tion of the moon's surface as we find it does on the earth's ; or the
general appearance of the disk would, we might suppose, be of an
entirely different character £rom what it is *. It is of course quite

» [" Sttpposing the moon to be oonstituted of similar materials to the earth,
it must be, to say the least, doubtful whether there is oxygen enough to oxidate
the metals of which she b composed ; and if not, the 8ur£oe which we see must
be metallic, or nearly so." — Sir William GroTe's Address to the British Asso-
ciation, Nottingham, 1866, p. Ix.

Sir William further suffgests that it is a &ir subject of in<)uiry to investigate
lunar formations with me especial view of ascertaining if tliey present the
appearance of '* congealed metaUio masses, as they may have set in cooling from
i^eous fusion.'* In connexion with this question we would solicit the reader's
attention to Mr. Neison's view of the surface consisting of mixed silicates, and
also to the cognate view of Hattieu Williams, founded on the fact that silica

impossible to enter into any details on such subjects within the
limits of this paper, even had it not required an acquaintance with
the special branch of science concerned to understand them pro-
perly. Had not physical science most unmistakably aaserted its
general truth, probability would naturally lead to the conclusi^ij
that the primitive formations of both earth and moon were of the
same character. And a careful consideration of the whole subject
will necessitate the further conclusion, that the basis of the lunar
surface consists of the mixed silicates of the same nature as ter-
restrial gneiss, granite, basalt, lava, &c. ; and it is from this basis
that we must' build up our lunar formations, with due regard to its
capabilities. Whatever, then, may have been the original condition
of the matter now constituting the lunar surface, it must at some
period have been intensely heated to have formed these silicates, to
such an extent, in fact, as to have fused every thing and to have
volatilized most.

Prom the nature of the surface thus determined from chemicai
considerations, it is possible to deduce with some degree of accuracy
some important points with regard to the progress of the lunar sur-
face to its present condition ; and this will enable us to apply a test
to several hypotheses that have been advanced. And here we find a
little difficulty in the way of the hypothesis discussed by Mr. Proctor
in his paper as to the immense influence brought to bear by

enters larsely into the composition of the earth's crust. (Monthly Notices B. A. S.
vol. xxxiii. p. 360.)

Professor PhilHps remarks (Phil. Trans. 1868, p. 339), in rektion to the dif-
ference of aspect of Ariatarehua and Herodotus, on the dazzling brightness of
Arittarchus as arising from its consistinff of material similar to white trachyte,
while the dark colour of Herodotut may he due to something analogous to basalt
or to augitic compounds. — ^Ed.]

oompofiition of the moon as above suggeBted. Thig difficulty might
be removed by supposing these early meteors to have had a different
constitution ; but every such assumption must most seriously weaken
an hypothesis requiring such assistance,

LiriuE AncosPBEBS.

It is to be regretted that Mr. Proctor practically waives the
question as to a lunar atmosphere, and dismisses the subject as
of little importance, being apparently of the general opinion that
whether no lunar atmosphere, or one of a very limited extent ex-
ists, is a matter of indifference ; but upon careM consideration it
will appear that this opinion is entirely inadmissible. We have
here matter for the gravest attention, and on the decision of this
point conclusions of the most serious importance depend. K, for
example, we decide against the absolute existence of any atmosphere,
such as a thin gaseous envelope, we demand very improbable cir-
cumstances with regard to the lunar constitution ; but if, on the
contrary, we admit the existence of an atmosphere, however limited,
it imperatively requires consideration as to how far this might
effectually conduce to the same purposes as our own. It is therefore
t« be regretted that Mr. Proctor does not enter upon this considera-
tion, but waives the entire matter, with the exception of assuming in
certain points the inability of any lunar atmosphere to do this. As,
however, we examine into our decision as to what consequences the
existence of a lunar atmosphere carries with it, the subject gradually
assumes a very different aspect ; and while we see that non-exist-
ence, or a very limited existence, are not synonymous, it gradually
becomes clearer that for an immense period a lunar atmosphere must
have existed, and cannot but still exist. Although it is impossible to
enter into details here, stiU it may be observed that " no atmosphere"
implies practically the absence of those elements from the moon's

to condemn an hypothesis requiring it ; yet admit their presence, as
every thing demands, and an atmosphere in the sense of a gaseous
envelope is the unavoidable sequence. And it must be remembered
that we have no evidence against the existence of an atmosphere
except of littie importance and purely negative ; on the other hand
we have certain phenomena explainable by the existence of such
an atmosphere, and perhaps alone by that ; while we, finally, see
portions of the lunar surface apparentiy in a condition which could
not occur without one.

We may, then, legitimately assume the existence of an atmo-
sphere as being at tibe very least the most probable ; and it then
becomes important to examine as to what condusions we may arrive
at concerning its nature and limits. Once grant its existence, and
there is absolutely no reason against giving it a similar constitution
to our own, excepting the vapour of water, and probably with a
very much greater proportion of carbonic anhydride. With regard
to its extent, it is perhaps impossible to draw at present any positive
conclusions ; but a remark on one point may be made. Were we t«
assume that the weight of the lunar atmosphere bore the same pro-
portion to the lunar mass as that of the earth's atmosphere does to the
earth's mass, it would lie very dose to the surface, no portion capable
of exerting a refractive power equal to one sixtieth of that of the
earth extending further than two miles from the surface ; but in all
probability it would bear a much smaller proportion ; and in either
case, considering the rough nature of the limb, it could hardly^ even
if it extended at all, reach one second of arc beyond the limb.
And as it is impossible to say what condensation and diminution
would occur at the dark limb from the cold, but littie expectation
could be entertained of detecting it by means of the refraction of a
star at the bright limb, owing to the existence of an atmosphere of
practically very maoh less bulk, and with, at the highest, from one
fiftieth to one sixtieth of the refractive power of our own.

[III.]

ON THE DETERMINATION OF LUNAR TINTS-

Br HENRY PRATT, F.R.A.S.

In the Report of the British Assodation for 1869, ' Transactions
of Sections,' p. 15, is a paper on '* Secular Variations of Lunar
Tints," by W. R. Birt, F.R.A.8., it which this sentence occurs : —
** One of the most promising lines of research having reference to
the physical aspect of the moon's surface, consists in an examination
from time to time of the tints which characterize every portion of
the visible disk." And interesting as this branch of lunar study
assuredly is, it is certainly not a little difficult to render it as per-
fectiy satisfactory in its results as one could desire, unless some
simple and thoroughly effective method be adopted both generally
and systematically by lunar observers. From the time of Schroter
estimations of the lunar tints have been made and referred to
scales ; and numerous observations have accordingly accumulated.
But the scales of Schroter, Lohrmann, Beer and Madler, and others
are not directiy comparable one with another, although their dif-
ferences could be easily reduced to any staiidard scale if such were
generally adopted; and although that would be useful work in
relation to the labours of the selenographers just mentioned, yet it
does seem in some measure undesirable, in connexion with friture
observations, to continue the mode of estimating tints according to
different numerical scales. Indeed all scales are open to the objec-
tion of being to some extent arbitrary in their character ; for the
reference of the tint of any lunar object to a particular value,
according to the scale adopted, depends very much upon the judg-
ment of the observer, and is not the result of any exact measure or
direct comparison. And this remark applies with equal force if
certain lunar objects are chosen as standard points in the scale ; for
then the standards themselves are subject to variation in tint, and

the determination of other objects by this means becomes subject to
additional inexactness.

Another mode of conducting these investigations is by the direct
comparison of the objects wi^ a series of tinted disks, as in the
Homochromaseape invented by Mr. Birt, for a description of which
see the end of his Monogram of the * Mare Serenitatis,' 2nd ed., and
' Monthly Notices R. A. S.,' vol. xxii. p. 11. One of the difficulties
of this mode appears to be the production of duplicate copies of pre-
cisely the same tint ; but this is not insurmountable. It would abo
be necessary to employ a standard illumination at a stated distance,
together with one or two further details, all easily to be effected.
If this instrument could be purchased as an accessory to the tele-
scope, and it were to receive the imprimatur and recommendation of
such a body as the British Association or the Royal Society, it would
most likely be generally adopted by lunar observers as a standard
of reference, and the work of different individuals would thus*
become directiy comparable without the trouble of having first to
extricate them from the confusion of different scales. Thus the
Homoehromoscope would become to observers of lunar tints what
Admiral Smyth's chromatic scale is to the inquiry into the colours
of double stars, until some as yet unthought-of development of prac-
tical optics shall produce a means of photometric analysis similar in
its advance to the spectroscope. Unfortunately for lunar observers,
Mr. Birf s invention has not been brought forward or developed as
it deserves to be.

In the mean time there is another mode of investigation which
may prove to be the best for the purpose, and but littie open to
objection : it is recorded in that noble work of Sir John Herschels,,

direction in detertniniiig the varioiu tduts of the localities under his
scratiny. Although the reeulte of this mode are not absolnte, bat
relative, yet perhaps they are the best vhich at present can be
obtained. And there are distinct advantages to be olaimed for it.
No ezpenrive or specially contrived apparatos is needed, as in any
absolute determination of actual tint, sll that is requisite being a
good telescope and a sensitive eye. In regard to the telescope,
perhaps nothing is more enit«d to this purpose than one of With's
incomparable apeoula in conjunotioti with Browning's aohromatio
eyepieces. Another advantage of this method over that of affixing
any arbitrary numerical scale-value to a tint is, that at the moment
of observation but one idea has to be kept at«adily in the mind, viz.
the relative intensity of two spots; and therefore, as the mental
operation at the moment is simplified, the concentrated judgment is
more likely to be correct in determining the -simple precedence
jMtween the two. Those who are unaccustomed to similar work
may think this advantage onworthy of mention ; but in practice it
requires oonidderable care in fixing the order of two or three spots
6tareelj difiering in tint.

In order to sscertain the variations of the tints of any locality, it
is neceeeary first to sketoh out the immediate neighbourhood of the
spot, and divide it into portions aooording to difierences of tint,

vation of the whole region is completed. The sequence of the
detdgnatious thus obtained will represent the sequence of the
tints at the time of observation. Now, if the re^on under study
be divided into ten or twenty portions, as the memory is not
charged witii the precise order of the letters at any epoch of
obe^ation, obviou^y the method is free from the efieot of bias in
that way, and is capable of yielding excellent reeults. That change
of some kind has occurred in the tints of the selected locality will
be evident on the accumulation of even a few nights' carefol work
should the sequences vary amongst themselves. It then only re-
mains to discuss the observations according to solar altitude itc., in
order to bring out the peculiarities of tiie variations. We may
perhaps hope some day to have a means of performing actual
measures of tints and reading ofi^ the resulting values ss on a
miorometer-head or a spectroscope-scale ; but for the present such
instrumental means are wanting. In the mean time it is believed
that, by employing this method, which is capable of yielding the
beet results attainable with present appliances, and in the more
general application of it by the now numerous and oonstanUy
increasing band of lealons observers, we may look for new and
interestii^ lights upon some of the hitiierto unsolved quesCioca
relating to the past and presrat history of the moon.

[IV.]

MADLER (Schmidt), Theophilus A {B. and M.), IV B*« {Birl).

On the m of April, 1873, 7^ 20" to 7* 45-, Mr. "Webb, witii hia
9-38-inch silvered glass reflector, power 212, examined thb crater
and its surroundings. The objects mentioned in his record are a
bright point a at the extremity of a light streak, and a smaller
bright point a north or N.N.W. of it. The bright streak, which
was about as broad as the ring of Uadler, separated two regions, a

lighter one south of it from a darker one north of it. This streak,
c, met another^ s, which ran soutlierly through USdler. On the

dark ground between Hiidler and Theophilus "iir. Webb noticed a
small bright speck, b, from which a abort bright streak ran aoutherlj.
The accompanying sketch exhibits the positions of these objects.

The previous history of Madler is recorded in ' Scientific 0|ttnion,*
No. 81, May 18, 1870^ p. 449. Lohnnann, who marks the crater o
in Section U., gives a ridge adjoining the N.£. border, but no central
mountain. Beer and Uadler, at a later epoch, give a ridge termi-
nating at the north border of the crat«r,* and an interior mountain
ezcentrically utuat«d towards the south border. In fistherfurd's
photogram, March 6, 1865, a ridge is seen within the crater con-
tinued from the exterior ridge, as given by Beer and Madler; the
termination of this ridge appears to be concealed by the bright
illnmination of the interior of the aouth border. On the evening
of May 6, 1870, 8* 0- to 8" 20- G.M.T., the editor made tiie fol-
lowing record in his Obeerving-book : — "IVB*" west of Theophilus
is a remarkable crater ; a ridge, IV B* '", in continuation of a ' crater
row' north-csst of Isidorus, has penetrated into its interior nearly
to its south-east border. Xikia ri^ a^^ars to have been produced
eubseqnentiy to the formation of tlie crater, having pushed up its
north border."

We trust fiirther attention will be given to thia r^on, eapeciaUy
to the bright streaks figured by Mr. Webb.

Ur. Webb's figure is exceedingly well shown on the second edi-
tion of B. &, M.'s map, the bright point A {IV B"i) being very
consjiicuous. The streak C (IV W^ is marked (. B. & M., while
showing the streak D (IV B***) as attached to the crat«r Mddkr,
separate it from C. In this respect Mr. Webb's sketch indicates an
addition to our knowledge in the connexion between C and D at the
angular point depicted. Webb's bright points B and E are not on
B. & M. ; the bright streak from B is alao wanting on the map.
Mr. Webb appears to have missed the " crater row " from the north
end of Ifidorut. The numerals on fig. 1 represent the following
objects ; they are all preceded by the designation IV B* : —
1'. The N.W. central peak of the mountua in Tkeophihu.

2. The S.E. central peak of the same.

5. The cleft or valley between the peaks.

4. A crater or depression within the N.E. ring of Theop^itvi
(BofB. &M.).

7. A ravine within the E. border of TheophUua.
The streak C, with the bright spot A, which m Lohrmann's map
appears as a craterlet, and the sharp angle at the conjunction of C
and D, appear quite distinctly on Be La Huo's photogram of
February 22, 1858, and GiU'a photogram of Pebruary 19, 186S,
the streak D passing close by the N.E. border, as shown by Ixthr-
mann. Fig. 1, by Webb, is In dose agreement with Bntherford's
photogram of March 6, 1865.

]

ore Hour.')

<, the latest edition, with Additions.)

:E8.

rty Plains.

I The Sinus If edii.

I The Sinus Aestuum.

!f The Ooeanus Prooellarum.
)• The Mare Imbrium.
B The Mare Frigoris.

Ranges.

a The Carpathians.

A The RipWan Mountains.

t# The Percj Mountains.

is, OraterSf eie,

6 Maniliua.
7#Alexander.
;8 Eudoxus.

or.

9 Aristoteles.
Autolyous.

6 1 Aristilltts.

62 Cassini.
63»6ond.
64»Terra Photogra-

65 GlaTius. [phica.

66 Maffinus.

67 IVoho.

68 Wilhelm I.

69 LongomontanuA.

70 Hainiel.

71 HelL
7% Tfaebit

73 Arsachel.

74 Alpbonsus.

75 PtolemaeuB.

76 Triesnecker.

77 Archimedes.

78 Plato.
79«Birmingham.

80 Schiller.

81 Sohickard.

82 Capuanus.

83 BulUaldus.

84 Vitello.

85 Boppeluuiyer.

86 Gassendi.

87 Mersenius.

88 Vieta.

89 Letronne.

S The Sinus Iridum.
T The Sinus Boris.
V The Mare Smythii.
W The Mare Australe.
X The Lacus Somnioruxn.

Ar»The Tsneriffe Mountains.
/»The Harbin^rs.
m The Heroynians.

90 Ghierik^.

91 Parry.

9a Bonpland.

93 Lanosberg.

94 Grimaldi.

95 Riocioli.

96 Lalande.

97 Euclides.

98 Flamsteed.

99 Sohroter.
100 Gkmbart
10 X Beinhold.

102 Sncke.

103 Kepler,

104 Marius.

105 Reiner.
X06 Herel.

107 Olbers.

108 Stadius.

109 Copernicus,
no Eratosthenes.

"'UBeerandMadler.
1x21

113 Timochari.s.

114 Lambert.

115 Pytheas.

116 Euler.

X17 Aristarcbus.
1x8 HerodotuR.
ix9«0tto Strure.
120 Helicon.
12 X Le Verrier.
122 Messala.
X23 PosidoniuB.

n account of the excessive foreshortening, the scale being too small for the purpose.
ied (#) have been added since 1862.

»^

„.uu to nmdei' its eHecta vi«ibl»— to such obaerrers as
... uue the trouble (not occuionaUv, but) with nntinng undui^ to
notice the sppeantncei nf the fiooi and faithfully record them — has oeen
muiifeeted between April 1869 and April 1871. The observers to whom
this result is dus were, dorinfr the period of the obeerv&tioDS, entiielj
independent of each other; thej were not even in correHpondence, but
forwBided theii obterrations t^i the selenogtapher charged with the dis-
cussion. Their testimonj is therefore of the strongest cWvcter ; it would
hare been wenli had but ft few occanonai obterjiatiina been compiued,
bad but one obsurver have given his attention to the inquiry. ' ''
the astronomicalpublic have in the two Reports published in i
volumes of the British Association for 1871 and 1872 the whole matter
before them, ^m which it will be seen that the evidence has been most
carefiillj sifted and examined ; and we have no doubt that an unbiased
perusal will contribute in no smalt degree to the formation and advance-
ment of a trul; scientific method in the stud; of selenography.

Two full years have elapsed since these obaervntions were made ; a few
have been received since ; but two circumstances conspire to retard the
further prosecution of the subject — viz. the labour attendant on the dis-
eusnon itself, and the absence of a recommendation bj the late Com-'
mittee appointed by the British Assodatdon for a renewal of the grant.
While speaking of the interesting and important change which had been
fairly shown — the darkening of the floor with an increase of the sun's
altitude — the Report is silent on any further steps that may be token
relativ to " this important branch of astronomical inquiry," except that
the Committee " tru^ that in future years the Association will not over-
IjokiL"

The resnlt to which allunon has been made is, that a series of changes
has occurred on the north-west part of the floor which requires for its
explanation something more than the mere variations of illuminating and
reflecting angles. Near the north-west border of Fialo are three spots
known by the numbers 16, 10, and 13 (see tinted plat« in ' Stndent,'
April 1870. p. 161, and plan of I^ato, British Association Beport, 1872,
p. 347). These spots are ordinarily aituated on a somewhat broad light
streak, as in fig. 3, which is a repreeentation of the fioor as seoi by

Mr. Pntt on August 29, 1869. On September 26, 1869, the streak as-
sumed the form shown by Mr. Qledhill in fig. 4, except that the streaks
( and t axe not eoimtettd as in the engraving. On November 15, 1868,

the light streak was seen by Mr. Pntt in contact with the border. On
December 16, 1809, a light portion of the north-west fioor was ob-
served in contact with the boroer by Mr. Elger (see fig. 6). The area of
this light portion was increased the following lunation ; for on Januair
12, 1870, Mr, Elger observed it as shown in fig. 0. Again, on April 14,
1870, 8 change had supervened on this part of the floor ; for the streaks
had cMumed the form shown in fig. 7. From the month of November
1868 to the month of May 1870 the whole of this part of the floor waa

It may be well to mention

1 that on October 17, 18%, Mr. Gledbill B%ied
Fig- 5.

the two (seen as one streak), and found that the alignment, if produced
to KN.E., would cut the north border of the crater 436 (Mrs. Jackson
Qwilt). WM, fAtrd edU. The history of the western streak " o " is

curious. It appears to have been lost after the subsidence of the bright-
ness on the north-west part of the floor. Its latest appearance in the
courae of the luni-aolar day was in interval 72 to 00 hours before sunset

on September S5, 18li8. It was not seen after the sun's meridian passage
at Plato later than November 19, 1869. The last distinct mention of
"u" appears to hare been on April 11, 1870, when it was recorded as
very haiy and ill-defined. In May and June it appears to have merged
into the general hrightnees of the floor. In July a condensed brightness

of eject* towarda the west bolder. In Dncember 1869, Jtinuaxj, March,

Apnl, and May 1870, the eruption might be Buppoeed to have proceeded

■mth greater energy, a flowing of ejecta occuiring about the 14th of Apnl

in xbe old channel, e; the ejecta in Mrj and June overspreading the

whole of the north-west part of the floor and producing the brightness

witnessed. As this brightneea subsided, the

newcondidon of the flowing of eJectA towards Fig, 8.

die arm of the trident e along the channel e

became permanent in August and September

1870 (see figa. 6 and 7),

Mr. J. W. Durrad has furnished a drawing
of PWo, 1872, Dec. 8, which is reproduced
(fig. 8) for the purpose of directing attention
to a crater (a) in the N.W. gap of the ^order.
This crater has not, ao far as oui knowled^
extends, been previously observed; and it u
very deeiiable that it should be conftrmed or
otherwise.

Wa invite critjciBro on this communication, wilh the esnecial object
of showing that these changes are explicable on the principles of varia'
tion of illumination and libistion ; and we shall be glad to receive records
of observations of tbe appeoroueeB of the north-west part of the floor for
comparison with the above and other drawings in our possession,

Mr, Neison has furnished the following remarks on Mr. Proctor's sug-
gestious relative to the darkening of the floor of Ftalo : —

" The hnmlheaii so utroirvbly elaborated bv Mr. Proctor as an enlanation
of tbe dwkening of tbe floor of Plaio at the aolar altitude incceaaei (' Quartcrlj
Journal of Soienoe,' Januan 1873, pp. 50-63) is op«i apparently to very grave
objeetioa. Mr. Proctor asks, are we not to regard thii change as due to phy-
siological causes 7 and ' whether, in bet, tbe neighbourhood of the dark portion

Sicaaionsily) it becomes nearly as light as the Mare laibrium, and then rajridly
rkeni to an iron grey ; ArcMinudM, hoirever, vhile likewise rapidly lightening
Bt Grst, remains unchanged of a pure greriih yellow. How is thia difl£«nae to
be explained if we find the obinge in Plato dependent upon a physiolMical
effect 7 Architaedei as well as PI/& is in tbe vicinity of the terminator. Even
if «e take a apot as dark aa Haio, aa; Boicovich. we find this spot retaining
its dark colour practically unchauged throughout tbe lunation ; and a similar
coniparatiTe Siily of tint is observed in the itan Imbrimn between Plato
and Arehivuda. This difTerenoe is surely aufflcienC to show the difficult; of
ascribing the ohange of tint in Plato to phyiiologioal effect or oonCrast ; for, be
it remembered, the wall of ArcXimedet is even brishler than that of iWo, while
the dark colour of tbe latter is fully aa well marked in pbotogrspbs as to tbe ejt,

1 showing it haa not ila origin eutiralv in that organ, seeing that I^o it ooca-
uon^y nearly aa light in tint as tbe Vara Imbriitm. Dr. De La Rue's reaults

I may be made to beer out apparently, to some extent, Mr. Proctor's bnwtbesis ;

I bat it is perhaps open to question how much reliance may Im placed on tbem
for this especial purpose.

" It is to be regretted that a more rigorauH and accurate test than mere eye
estimationa could not be used, as errors may esaity be introduced in combining
observations made by different observer! ; nevertheless with a sufficient number
of observations, eadi observer's being treated separately as well as combined,
fairly accurate results are easily obtainable. Any such method aa that EUBgest«d
by Mr. Proctor would undeniahly be of assistance ; but it is not miited for the
genwali^ (^ obsefters. Ifot only does it reqmre a perfectly mounted instru-
ment, irith special adjustments, but there are no objects on (be moon's surface
that are, including P^i^, entirely uniform, ila'o, in fact, ia covered by numerous
very taint, light grey atraaka, very constant in tint it is true, but which would
render any contrivance to remove the effects of contrast inoperative."

[In re^rence to Mr. Proct«r's remark on the attainment of the greatest
diurnal heat about two hours after noon (p. 62), tbe Bev. F. W. Stow shows
(■Quarterly Journal of the Meteorological SociBty,' April 1673. p. 138) that tbe

[VI.]

THE WEDGE-SHAPED VALLEY OF THE»ALPS,

This v«iy interesting olgect has commanded much attention from
observers, bnt we are not aware that it bos been very closely studied, as,
for instance, in the manner in which Hato has been. Drawn originally
by Bianchini, scrutinized and delineated by Schriiter, and also by Lohr-
mann and Beer and Madler, it has become one of those objects that is
sure to be pointed out to observatory visitors as exceedingly remarkable ;
but as for Its correct delineation, and especially for an enumeration of its
principal features, w« have been, and as regaJtds the last-mentioned still
Are. deficient It was on January S5, 1866, that Webb surveyed it with his
7-lt. telescope, power 313, the eyepiece being formed of two achromatic
microscope object-glasses j and on that occouon he was able to obtain a
diavring of it, which in preserved side by side with Schroter's, Lohrmann's,
and B. & M.'a in the ' Litellectual Observer,' vol. ix. (April 1860), p. 175,
and which he described in tlie following words : —

" The appendages branching out from tbe east end of the valley in various
directiona will be understood to be mountains, a portion of the Lunar Aipi.
The whole interior of the valley from end to end was so perfectly deffoed by
it and its peculiar and almost unbroken evenneos, as to admit of

the form of a pear or Florence flask, whose longest diameter made a very ob-
tuse angle to the south with the general direction of the valley. Ttiis portion,
whidi must in reali^ form a noble amphitheatre, is evidently much better
represented by Schrotrr than by either of bis Buoceeao^. Its length might
amount to about one sixth of the whole. The nsci, so to speak, of the flask, of
about equal lengUi, was a part where it was throttled, especially on tbe north,
by eneroaohing mountains, till its width was only one third, more or leas, of that
of the broadest part west of it. A low ridge runnine lengthways wu here
visible in tbe bottom connected with the souu aide, and having somewhat the

it fiir from straight. Tbe
ited, giving various widths
o the bottom in different places ; the greatcHt width, somewhat exceeding that
of the flask, lay a little east of the centre. One of Sohriiter'a two Utile craters
was plainly visible ; but the other, lying on the interior declivity, oould not be
Inioed; a alight bniding towards the north could be perceived at the narrow
west extremity of the valley, but not to tim eitmt indicated by Lohrraann.
Beyond the east end a very narrow irregular gorge ooatinued the line of the

south edge through a lofty part of the iJps, r"*—' * .. i -

Mart IvMutH,"

it drawings of coiuiderable excellence by ii
Figs. 9, 10, 11.

bert and Mr. Elger. M. Oaudibert's was made on May 28, 1871, and
Mr. Elger's on December 11, 1872. M. aoudibert says :—

" While looking attentively at this valley I was not a little surprised U> find
three markings resembling three craterleu. Of those near the mouth of the
valley I am pretty certain ; but the third may be the shadow of a projecting
peak."

Mr. Elger's remarks are as follows;—

" While gazing at this remarkable formatian at lunar snnrise, one can scarcely
fiul to be impressed with its thoroughly earti-liJce character. Unlike most of
the objeota which come under the notiw of tbe student of the moon's surboe,
the 'wedge-shaped valley' has ita counterpart in many mountainous re^onK
of our globe : for example, Ibe River lAagen in Norway flows for nenrly sixty
miles through a comparatively narrow gorge which preeenta features very
similar ; but perhaps its most stiiking analogue is the higher valley of the
Arkansas in Colorado : this valley is upwards of sixty-five miles in length, it is
five miles in width at ita upper end, and gradually narrows to about two miles
at ila lowsr end ; its northern extremity, where the river has its souroe, bears a
close resemblance to Webb's ' noble amphitheatre.' "

e. The nuin portion of the Tsllej.
C. E. BuitoD, Esq., writM from Loagblinstowii u follows : —
■' April 6, 1873. I/.M.T. 8» 60" to 8» 30", S-moh lilTered glaw reflector.
Termuulor between etraigU wall, and the muill enter next to it on the eut
nnd bieecting IVato. About two third* of the leogth of the tbUc; ii (nvened

■ome milM eouthmrd ia lost unong hiUogu. [There^re some

. Ihi* cleft in Mr. Klger'a drawing.] A deep hollow like & Drater

ii plsioly vilible with 400 on the S-ioch refleotor, where the nil meeU the ftoor
of the Tul^ limiUr to that on the west aide of Hi/fnvi. Thrae ihallow canals,
two of whioh are loniewhat iodiatinctlj marked, run louthirarda lufta point*
near to the wert end of the njje; tor Sflf oc tix^ milea."

[VII.]

COPERNICUS.
Jlaroli 8, 1873, from & to 10^ p.k.

This ereiung the alt wm clew uid stetdj M a whole. I observed
the crater Copernicus. On account of so manj^ craterleta situated in
its immediate neighboorbood on the west, I tned various pawera up
to about 800 in order to me whether there were env of that kind
either on its lamparta, slopes, or floor ; but none were visible. Thoiuh
this last power snowed manj more detaila than those given in the
accompa^ing sketch, yet I could not use it with best lesulte on account
of imperfectian in the state of the atmosphere. The slcetcb ww WpAo
withpowers]20and200. Three ,

mountains, marked 1, S, and 8, ^g. 12.

attracted my eye first when ^^^^

looking towards the centre of
the floor; but with closer atten-
tion I found that mountain 1 is
formed of three, cloaely pocked
together — one on the north
marked 4j and just separated by

a black line, another (b) on the I

south and extending towards the J

east, and the third (a) extending
towards the south-weat These
two (6, a) nwty be only two di-
stinct peaks of mountain 1.
Mountam 2 has a smaller one
close on the south fshown in
sketch, fig. 12). Its north side
has a kind of semiciKular ca-
vity, like the h^ of a crateilet
(also shown in sketch), casting

at this time a verv short shadow. Mountain 3 seems single and quite
isolated. Two mmute )iil1ocks are situated just south of 3, between

3 and a. If all theae objects -mtB included in the eiptession " oentcil
mount«in," I believe nine distinct peaks could be counted there ; but
I see Mr. Webb (' Celestial Objects,' third edit. p. 87) restricts them to
sii. The other objecte, both on the floor and along the interior sJope
of the mmparts, visible at this time, need no particular description,
with three exceptions ; for they are all hillocks of Tarioos forms uid
sizes, The exoeptiotui are 18* and 20* on the floor. These objects ate
tnore difficult to aee than perhape any others shown, on account of their
being very low and their contours very undefined. The third eicepti<w
la 3* on the eastern interior slope ; this hillock, though \eiy small, was
very bright. The vacant space oetween the central mountain down to
the Qorth extremity of the floor >a not perfectiy even ; but power SCO
showed a large amount of ruKgedness which I lougd impoedble to deli-
neate. Tlie objects on the floor are numbered separately iiom thow
aloqg the slope of the ramparts,

C. GAmi»«BT.

N,B.— e, n, and m are repreeeqted liy B. & H, as a iidg«; a^d so it
looks with power about 100.

[On the 6th of Hay, )673, hetween 7.90 and 9.10 OJLT., I namised H.
Oaudibert'K drawinB at Um teleaoope, aperture 276 inch, power 100, definition
very fine. To areid miaootioeption, it psv be vsU to reinark that an the inte^
rior eaelem Blope the objecta Sgurcd by 11. Oandibert an not itoUdtd, bui art
fill Ajoieir points of ike ifiUrior ridau. Noa. ^0 and 1 1 were piade out euily ;
also So. 9, whioh wai not portiourarly remarkable for brwhtnesa. The fol-
lowing ^Ignmente were taken: — Central monntun, Ko. 1, loqgitudiiially and
throuah iCa shadow cut« the middle of the lacuna lovti of the peak on the west
sumnut, which cnaU the pointed shadow ; a line oarried through tbe two prin-
(ripal moimtaina I and 2. ci^la 10 and the northern part of 11 on the eaiit, and
a bright depKNion norti of (he pe«k A, the tafne whi(l] (mats the pointed shadow
pn thf west— Kn,]

[VIII.]

MARIU3,

Jajiuary U, 1873. 8> to 10^ ; power 100.

Best power II

i of the fl(

f white (perhapa a

Weather unfavourable for the delicate markings of this crater, tjie ur bejng " thick " and windy.

liie floor of this crater is not level, but sUghtiv raised like a wide tnud in the middle from one

the other, irom the N.W. to the S.p. Upon tjiis slightly elevated region are situated the very ^^ _

account of contrast) crateilet 1, itself at the top of another slight eminence^also the mound 2. These two objects, the
only ones yet observed, as far as I kpow, are mentioned by Jlr. Webb. That portion pf the floor (3) situated N.E.
of the middle band was visibly much darker than anyother*, and seemed depresMd. The dafk tint went up the declivity
towards 1 and 2, leaving a white space between. The opposite side of the floor was of a Jight grey. 4 is a doubtful
white spot 4 0. Gadbibibt.

• This darker band ia not shown in the engraving, but ia strongly narked in the original sketch.

Mr. J. W. Durrad otHerred Hariua on Apri) 10, 1»T3, apPHure 3625 { hp sa« the craterlst 1, and tbf dowhtfnl wluto spot 4.
The mound 2, as well as the darker N.B. part pf the floor, escaped hi^n.

[IX.]

EECENT OBSSJIVATIONS.

M. Gaudibert has communicated to the ' English Mechapic,' April 28,
1873, p. 143, an outiine sketch of Pitaltu and Seaodui with a;^les of
position and fifty-six lettered and numbered olgects, as observed op
April 6, ISrS, with his 6-inch silvered glass reflector. The following
remarks ore iraportapt : — " On the south-east rampart are two craters
(II and 16) joinod by a valley (12), the aides of which are very much
broken. B. & M. give these two craters ; but ipatead of the broken vall^

they have here three other craters." M. Gaudibert suggests Uiat the three
craters pay have crumbled down since B. k M.'s time.

" 46 is a central ctaterlet on the floor of Btiiodnt. B. & M. shade
the extremities of this floor all around, and leave the c^be perfectly
white, as if the floor were raised there ; but they give no oratarlet Had
the craterlet existed in their time, could they have nu»ed it f It is a
yray efay object now."

-1