Asiatick Researches, Or, Transactions of the Society Instituted in Bengal, for Inquiring Into the History and Antiquities, the Arts, Sciences, and Literature of Asia Volume 16 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1828 Excerpt: ...adverted. Having premised these observations, in illustration of the superiority of lunar transits, I shall now proceed to explain, in as concise a manner as possible, the method of deducing the longitude from the transit of the moon's limb. It is presumed, that the reader has made himself acquainted with the several adjustments necessary to the portable transit instrument previous to its being placed in the meridian. These adjustments consist, first, in setting the vertical wires truly perpendicular to the horizon; secondly, in rectifying the parallax of the telescope and the line of collimation; and lastly, in levelling the axis of the instrument. When these adjustments are made, there is no difficulty within the northern tropics in bringing the instrument into the plane of the meridian. The small altitude of Polaris, renders almost the first adjustment by that star unsusceptible of any further correction; and it seldom happens that some distant object may not be found intersected by one of the wires, which may always be referred to as a meridian mark. To adjust the instrument by Polaris, the observer should have a chronometer, whose rate is uniform and welldetermined, and he may, if he knows the equatorial interval between the wires, calculate the exact instant it should intersect the first and second wires, which will prepare him for the final adjustment to the central wire. The transit instrument is the most perfect that has been invented for the improvement of astronomy. Instruments for measuring angles, no effort of art can render perfectly free from errors of centering, division, &c. But the transit instrument is not susceptible of any instrumental error that may not be obviated. When the instrument is accurately adjusted to the meridian, it wil...