Page images
PDF
EPUB

fest in the case before us, by increasing or diminishing the half sum of the azimuths reciprocally taken at Mullapunnabetta and Savendroog, by one second only, when it will appear that a difference of nearly one hundred and fifty fathoms will be thereby occasioned in the perpendicular degree."

But if the method of measuring a degree of the perpendicular by the convergency of the meridians cannot be successfully practised, what method must be had recourse to? The measurement of this arch is very necessary for determining the difference of longitude, and is therefore an important element in the survey. Other methods of ascertaining the longitude ought no doubt to be tried, such as that which Major Lambton mentions as having been strongly recommended to him by the late Astronomer-Royal, by carrying a good time-keeper between two meridians at a known distance, which I mean, says he, to put in practice in the course of my future operations. "I had also," adds he, " devised another method, by the instantaneous extinction of the large blue lights used at Savendroog, the times of which were to be noticed by observers at Mullapunnabetta and Yerracondah, the distance of whose meridians on a parallel of latitude passing through Savendroog is nearly 135 miles. The experiments were attempted; but the weather was so dull that the lights could scarcely be distinguished. There is, besides, a difficulty in fixing

the precise moment of extinction; and even in the most favourable state of the atmosphere, when the lights may be distinctly seen with the naked eye, at near seventy miles distance, to come within half a second of the truth, would be as near as the eye is capable of, which is 7" in an angle at the pole: But the mean of a great number of observations might come very near the truth."

2

The Major then proposes the comparison of celestial observations, such as occultations of the fixed stars by the Moon, eclipses of the satellites of Jupiter, &c. for the longitude, to be made at Madras and Mangalore-almost five degrees of longitude removed from one another, and of which the distance in fathoms is perfectly determined from the survey. He concludes with a passage, full of the modesty characteristic of real talent, and breathing the spirit of ardent and persevering research, which nothing but the love of truth is able to inspire.

"In short, the difficulty of obtaining this desideratum, (the knowledge of the true figure of the earth,) and the important advantages to geography and physical science which must accrue from it, are such powerful incitements to the prosecution of the inquiry, that I may venture an assurance of leaving nothing undone, which may come within the compass of my abilities, to give every possible satisfaction on the subject; and if my endeavours shall prove successful to throw some light on the path of

future discovery, I shall close my labours with the grateful reflection, that while employed in conducting a work of national utility, I shall have contributed my humble mite to the stock of general science." P. 368.

The 12th volume of the Asiatic Researches contains an account of the extension of this survey, to the southern extremity of the Peninsula, and the measurement of another considerable portion of the meridian, amounting in all to nearly six degrees; -the longest arch, excepting that in France, which has yet been measured on the surface of the earth. When the work was first undertaken, the principal object was to connect the two coasts of Coromandel and Malabar, and to determine the latitudes and longitudes of the principal places, both on the coasts and in the interior. As the work proceeded, the design was enlarged; and, in addition to the triangles carried across the Peninsula between the latitudes of 12 and 14 degrees, as already mentioned, another series was extended from Tranquebar and Negapatam on the Coromandel coast, across to Paniany and Calicut on the opposite shore; and to render the skeleton complete, a meridional series was carried down the middle of the Peninsula as far as Cape Comorin, from which were extended other series to the east and west along the sea coasts; so that a web of triangles has been completely woven over the Peninsula of India from the paral

lel of 14° to its utmost extremity. It is to the meridional arch, of nearly six degrees, thence deduced, that this last memoir relates. It was presented to the President of the Asiatic Society by the Governor General, Lord Minto, who added this judicious and merited encomium: " I have great pleasure in being the channel of communicating to the learned Society, a paper containing matter of such high importance to the interests of science, and furnishing so many new proofs of the eminent endowments and indefatigable exertions which have long distinguished the character and labours of its respectable and meritorious author."

In this measurement, the meridian of the Dodagoontah station, or of Savendroog, was continued south to Punnae, in the latitude of 8° 10′; and the series of triangles, for the purpose of ascertaining its length, was continued to the same point. In the extent of this prolongation, two new bases were measured, one at Putchapoliam, where the meridian intersects the parallel of 11°, and another at Tinnevelly, near the southern extremity of the arch. These bases were nearly of the same length, (somewhat shorter than that at Bangalore,) and measured with the same commendable attention to every circumstance which could ensure their accuracy. The triangles were carried on in the same manner, being a part of that great system which we have already mentioned as covering all this part of the

Peninsula. In many places the country is high and difficult to penetrate; the highest mountain in the whole survey occurs here, viz. the Hill (for so it is called) of Permaul in latitude 10° 18'; its height is set down at 7367 feet. A very laudable precaution was taken throughout by Major Lambton, that of describing the positions of the great stations, and giving marks, by which an astronomical instrument may be placed in the same situation with his, if any of the observations should seem to require repetition or verification.

[ocr errors]

The observations for the latitude appear to have been conducted even with increased diligence. The practice of reversing the sector is never omitted; the latitude of Putchapoliam, the northern extremity of this prolonged meridian, is determined from the mean of 173 zenith distances of stars, all passing very near the vertex. The number of similar observations at Punnae was 226, and from these was deduced the amplitude of the arch between the stations just named, viz. 2° 50′ 10′′.5, the length being 171516.75 fathoms. The differences between the zenith distances of the same star seldom exceed 3′′, and are usually much less; so that, taking into account the number of observations, it cannot be doubted that these latitudes are determined to a fraction of a second.

On this meridian, the distances of five stations, with the corresponding latitudes, were determined

« PreviousContinue »