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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 



ON 



THE PREPARATIONS FOR, AND OBSERVATIONS OF 



THE 



TRANSIT OF VENUS, 

AS SEEN AT ROORKEE AND LAHORE, 



On December 8, 1874. 



i 1 1 -^ .'A ',' -..' ■,) 



BY 






^v- 



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




/ 



y 



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 



ON 



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, 

B 



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 



I 



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. 



13 



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 


23 


27 


5, 
50*20 


Greenwich 7 y. Cat. for 1864. 


yy 144^ *** *** 


10 


24 


19*48 


Ditto and Results for 1868, 1869, 
and 1870. 


Groombridge, 2053 


13 


42 


08*67 


Greenwich 7 y. Cat. for 1864. 


3834 


22 


30 


03-38 


Ditto. 


c Eridani ••• ••• ••• 


3 


26 


59-68 


Nautical Almanac, 1875. 


17. Tauri ... ••• 


3 


37 


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 : — 



1 

Wires. 


Mean Reading on Cross. 


Distance from D. 


1 

A 

1 


rev, 
2*0128 


rev, 
13*8788 


B 


8*9550 


6*9366 


C 


12*4x60 


3*4756 


D 

1 


15*8916 




E 


19*3468 


3'455« 


! F 


22*8000 


4-9084 


G 


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. 



H 



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. 



15 



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. 


s. 


s. 


December 3 


• • • 


2 181 


— 201 


1 


4 


• • 


2 13-8 


— 2-35 


— 0-34 


5 


* • * 


2 34-8 


— 278 1 

' 1 


— 0-43 




• •• 


2 143 


-3*53 


- 0-38 


8 


• • • 


2 13-8 


- 3-87 i 


— 0-34 ! 


9 


• •• 


2 524 


- 392 


— 0-05 


10 


■ • • 


2 524 


- 3*94 


— 0'p2 


1 


.«. 


2 42-8 


— 414 

1 


— 0-20 1 

1 



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



J 



i6 



REPORT ON THE TRANSIT OF VENUS. 



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PHOTOHELIOGRAPH. 



17 



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. 



i8 



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. 



19 



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. 



21 



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. 


M 

( \ 

1 


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 


m 

••• 
••• 
•• • 
••• 

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. 



23 



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. 


1 

' Micrometer. 


Space. 


h, m, s. 


rev. 


rev. 


rev. 


V 




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 


1 


10 54 

11 58 


7*o6o 
13-868 


SO 

to 

/ 1 

M 


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 





24 



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. 



25 



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. 


rev. 


reo. 


m 


m 


m 


16 3a 14 


13-028 


\ 


1 2-648 
3-894 


42-534 


•'• 


42-534 


33 13 


13*274 




46-485 






46-485 


34 24 


I3'576 




3" 1 96 


51-336 






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 




• • • 


••• 






••« 


1 


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 


1 


V 


••• 




»•• 


• •< 





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. 



27 



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). ^]. 

dt 

= 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.^ 

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. 

H 



30 



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. 


h. 


m. t. 


• 





# 




12 


49 3575 


47-633 


0-014 


47-65 


Centre of 9 from Limb of . 


12 


54 49*25 


24*688 


0-007 


24-70 


Distance near Limbs. 


12 


59 06*25 


32-683 


o-oo8 


32-69 


Ditto ditto. 


13 


05 4975 


45*594 


o-oii 


45'6i 


Ditto ditto. 


13 


10 20-25 


52765 


0-014 


5278 


Ditto ditto. 


'3 


14 4575 


60-712 


0-015 


60-73 


Ditto ditto. 




■ 


Nfap 


. Egress. 




15 


58 53-25 


56-416 


0-044 


56^46 


Distance near Limbs. 


i6 


04 38-50 


46734 


0-036 


46-77 


Ditto ditto. 


i6 


09 21*25 


38-366 


0-029 


38-40 


Ditto ditto. 


i6 


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. 



31 



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' 



32 



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 



34 



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. 



35 



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. 


S. 


J, 


S. 


December i 


I 21*2 


+ 16-87 


• •• 


••• 


„ 2 


I 29*2 


2392 


+ 7-05 


+ 7401 


3 


I 29*2 


31-48 


7-56 


7-56 


„ 4 


I 13-8 


39*42 


7-94 


8-03 


5 


I 44-4 


4776 


774 


7-58 


6 


I 28-8 


54-53 


7-37 


7-45 


7 


I 44*2 


62*17 


764 


7-56 


8 


4 217 


71-04 


8*87 


8-00 


9 


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 



36 



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. 



37 



•' 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* 



38 



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 



16 


H 3693 


A. 
16 

+ 


I 


s. 

09 

1508 


16 


42 


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 



40 



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. 

L 



^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 — 

3''o6 ±©''•03 =o*0344AT -o''*0349AL -AS+Ao- -o*255iAiR? -o-96ioANPD? 

-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. 



I 

I 



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. 









1 



RESULTS. 



45 



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. 



M 



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 



V 



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 



48 



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 ; 



I 



REDUCTIONS. 



49 



^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, ♦]• 

N 



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. 



51 



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. 






• 










A 


B 


c 


D 


E 








s. 


s. 


s. 


s. 


J. 






'\ / 


o-oi 


rn 


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 


iS 














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 


1 

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%% 



s. 



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. 



52 



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. 



53 



.« 



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. 



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 



t 




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- 

B 



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. 



m 



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,, 

b2 



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. 



12 



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." 



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