II.

ASTRONOMICAL OBSERVATIONS

ΙΝ

FORT WILLIAM

AND BETWEEN

MADRAS AND CALCUTTA.

BY COLONEL THOMAS D. PEARSE, Commandant of the Artillery, and Second in Command of the Bengal Army.

I

BEG leave to communicate to the Society fome Aftronomical Obfervations which I made at different times in Fort William.

The clock I ufed from December, 1775, was made by EOLICOT: It beats dead feconds: there is one hand for minutes, and the hours revolve with the plate fixed to the hour wheel.

The pendulum can be lengthened, without ftopping the clock, by means of a fcrew, which fupports the fpring by which the pendulum hangs. And the pendulum is defcribed in the 47th volume of the Philofophical Transactions, p. 479. The clock-cafe is firmly fcrewed to the wall. The tranfit inftrument was made by SISSON; it is four feet long, and has a double object glafs. This is fupported by two iron bars, which are joined to a square frame, that lies two feet under the floor, buried in brick work.

The upright bars are protected by a cafe of wood, which is fixed to the houfe, without touching them in any part.

At first I ufed the cornice of the Commandant's house to adjust by; but afterwards a flider, with a flit

in

place, behind which I could place a light to adjust with by night. There was another object alfo to the fouth, about 1500 yards off, which I could use by day; and both these were fixed when the tranfits by telescope and equal altitudes agreed, and were examined from time to time.

I had only a tolerably good HADLEY'S quadrant and quickfilver, till December, 1776, when I was lucky enough to get an 18 inch land quadrant, made by RAMSDEN, with a micrometer to fubdivide the nonius. This inverts, and is capable of the niceft adjustments. My first telescope was an 18 inch reflector, made by GREGORY.

In Auguft, 1777, I obtained Mr. SMITH's refractor, made by DOLLOND, with a triple object glafs, and a double object glafs micrometer. And I made a polar axis for it of brafs with rack work, and a declination circle not divided, which is alfo racked; to which, when the micrometer was used, the telescope was fixed.

I likewife communicate obfervations made by myself chiefly, and by Lieutenant COLEBROOKE for me, to afcertain the longitudes and latitudes of places between Madras and Calcutta.

Going to Madras in 1782, I used a HADLEY'S octant and quickfilver, which I fhall here describe.

The octant had a wooden index. I feparated the part which carries the fpeculum from the arm; then fixed it into a lath, and turned it on its own center: it was three tenths of an inch thick; the thickness was divided into three parts, and then the edge was turned away on each fide; fo that the whole piece of wood became like three wheels of different diameters joined together on

their flat furfaces, and the middle one was the biggeft; that below was the next in fize; and the upper one was the least, and only equal to the brass plate on which the fpeculum was fcrewed.

A plate of brass, nearly one tenth thick, broad enough to admit of a hole as big as the under circular part of the turned wood, and to afford a rim of half an inch broad, was then fixed into the lath, and had a hole turned in it of that fize: on one fide it had an arm as broad as the wooden index was.

A fecond plate of the fame kind was alfo prepared; but the hole was larger, though lefs than the middle part of the turned wood.

As

The turned piece was then fixed to the octant by its pin, and the plate with the smaller hole beneath it. they fitted very nicely, the brafs plate turned upon the wood round the center of the octant, if that were held faft; and both turned on the center pin if they were preffed together.

The plate with the large hole was then laid above the turned wood, its center coinciding with the common center: the wooden arm of the index had the end nearest the center cut away, above and below, equal to the thickness of the plates of brass: it was there fixed to the octant in the fame manner as before it was cut off from the centre, and the brafs plates were drilled and rivetted to it.

When these plates were preffed together, they held the turned piece as it were in a vice; when they were forced afunder, the turned piece might be moved independently; and there were in the direction of the

radius

radius two fcrews, one beyond the fpeculum, and one between it and the nonius, for the purpofe: they had button heads, and their shanks were as high as the top of the index fpeculum.

On the back of the octant there was a fcrew with a button head; the thread entered the centre pin, and the fhoulder preffed upon the plate which keeps that pin in its place.

The back ferew and vice fcrews being flacked, the index fpeculum was brought parallel to the horizon glass; then the vice fcrews were turned to join the fpeculum to the index, as before the alteration was made.

To extend the power of the octant occafionally, it was nicely adjusted: then the index was carried to 90°, and there fcrewed to the limb. Next the back fcrew of the centre pin was forced, till by its preffure the speculum was held faft: after that, the vice fcrews being flacked, the index was carried back to o°, and there fcrewed to the limb. Whilft it was in this pofition, the vice fcrews were again turned, which fixed the fpeculum piece to the index, and then the back fcrew being flacked, the fpeculum followed its motions. When it was used, the index fhewed the angle which was to be added to 90° for the angular distance.

By this contrivance, with an octant, I could take angles of 150°; and confequently meridian altitudes as far as 75° and if the horizon glafs and telescope could have been made to flide nearer towards the centre, it would have increafed ftill further.

:

In RAMSDEN'S new quadrant there is a screw to adjust the horizon glafs, and bring it parrallel to the other: provided the index fpeculum is perpendicular to the limb, this is all well; but if that be inclined, as soon as the index quits 0°, there will be an error in the angles obferved.

obferved. I found it fo experimentally, and corrected my quadrants accordingly, by turning the horizon glass round its own axis; then having adjufted as ufual, the error fideways was corrected by moving both glaffes, by means of their adjusting fcrews, and dividing the error between them. If, when the horizon glais was reftóred to its proper pofition, there fill was a lateral error, the operation was repeated. I do not find any mention of this in any of the inftructions for ufing HADLEY'S inftruments that I have feen..

The horizon was artificial, invented for the occafion, and confifted of a wooden trough about half inch deep, (or rather more,) filled nearly with quickfilver, which ferved to float a plate of thick glafs, the under furface of which had been unpolished and blacked, that only one image, might appear. This needs not any adjustment the only requifite is, that the glafs be equally thick all over, and fmooth, that which was used was a part of a very large looking glass, that had been broken by accident.

The watch was a time-keeper, by BROOKBANK, which goes whilft it is wound up, and is tolerably good, confidered as a fale watch fent to India.

The telescope had a double object glass, with a brass ftand, and was made by GREGORY: it magnifies 80 times, but, like all of this conftruction, that I have feen, it had a dark fpeck in the middle, and was not equally good in the whole field.

In the way back, we had a land quadrant, of 15 inches radius, made by B. MARTIN, and fent out by the India Company. It was used by Mr. HURST in the tranfit of Venus. This could not be inverted. But, to destroy the effects of collimation and error of level, the latitudes are all determined by ftars taken north and fouth of each place, as the obfervations will fhew.

T. D. PEARSE.