す

ASTRONOMICAL OBSERVATIONS

MADE IN THE

JUPPER PARTS OF HINDOSTAN,

AND

ON A JOURNEY THENCE TO OUJEIN.

BY WILLIAM HUNTER, ESQ.

BEFORE delivering the following observations, it will be proper to give some account of the instruments with which they are made. The altitudes for determining latitudes and time, were taken with a sextant of ten inches radius, made by Troughton: the limb is divided into degrees and thirds of a degree, and the divisions on the vernier go to half minutes; so that, by the help of the magnifying lens, a difference of ten seconds is sufficiently perceptible. The two specula, being screwed down in their places, do not (as far as I can discover) admit of the principal or vertical adjustment: but the error was almost daily ascertained by the double mensuration of the sun's diameter, and constantly allowed for. It is subtractive; and my determination of its quantity varied from 2′ 30′′ to 3 30". These differences may have in part arisen from a real variation in the quantity of this correction; but I ascribe them chiefly to some inaccuracy in my mensuration of the sun's diameter. To form some judgment of the influence this cause might have, I have examined twenty-three of those measurements, made between the 7th of March and the 7th of June (being all of which I have any record)

by taking the medium of the sun's diameters, as mea sured on the limb, to the right and left of zero, and comparing it with the diameter for that day, as laid down in the Ephemeris. It will appear, from a list of those observations, that my measurements com monly exceeded those given in the Ephemeris; but the greatest excess was 25′′.

These mensurations may have a farther use, besides ascertaining the adjustment of the quadrant. If the eye could determine, with perfect accuracy, the contact of the limbs, the mean between the two measurements of the sun's diameter would be exactly equal to his apparent diameter, as determined by calculation, and given in the Ephemeris; but, from the imperfection of our organs, it happens that the limbs will sometimes appear to be in contact, when a little space remains between them; at others, when they overlap one another: in the former case, the diameter will appear greater; in the latter, less than the truth. But it is probable that, at nearly the same period of time, the state of the eye, or of the sensorium, by which we judge of this contact, is, in the same person, nearly the same. Of this I have made some trials, and found, that, when the sun's diameter, by my mensuration, differed from that in the Ephemeris, on repeating the mensurations, at short intervals, the difference remained nearly the same. Therefore, if we observe the sun's altitude a little time before or after measuring his diameter, the contact of the limbs will, probably, appear to take place in the same real situation of those limbs as when we measured the sun's diameter. But here, the effect of too open or too close observation will be reversed; the former making the altitude appear less; the latter, greater than the truth. These measurements then may be applied as corrections of the observed altitude. Thus, if the diameter of the sun has appeared too great, add the quantity of its excess to the angle observed, between the sun and his image in Mercury; if it appeared too small, subtract the defect, to give the true angle. Thus, March the 13th, the error of the sextant was 2′ 52" to be subtracted; but the measurement of the sun's diameter exceeds the truth by 24". Therefore, this quantity