As his instrument had the advantage of being fixed upon stone
piers, which are not liable to- partial expansion, and as the size of
the instrument itself seemed to him better adapted to determining
the real quantity of atmospherical refraction than any which had
been before employed for the same purpose, he extended the range
of his observations as low down towards his north horizon as his
situation would permit. For this purpose he selected fifty stars of
different polar distances, and of these he made, upon the Whole, up-
wards of 1000 observations.
The observed zenith distances being first corrected by the usual equations, so as to reduce them all to the same period, January 1, 1807. a correction is next made for refraction, according to Dr. Maskelyne’s lust precepts, in which the refraction at 45° is estimated at 56%", with due allowance, as usual, for the states of the barometer and thermometer, as noted at the time of observation.
Since the co-latitude is equal to half the sum of the real zenith distances of any one star that has been observed, both above and be- neath the pole, it is evident that the same result should be obtained from stars near the pole, as from those which are more distant, after all the requisite corrections have been lightly made. But since, by the author’s observations, his co-latitude deduced from distant stars, which are subject to greater refraction, was found to be about 2%" greater than from stars near the pole, he presumed that the allowance of 56%" for mean refraction at 45° was too small. For if both the greater and less refraction be increased in the same ratio, the corrections thus made will be unequal, and their difference may be made to remove the inequality of the co-latitudes, as deduced from the mean of 56%”.
From the mean of 13 stars, which do not pass lower than56" from the zenith, compared with the mean of 21 stars, between 60° and 78° zenith distance, Mr. Gmoomhridge infers that the mean refraction is really as much as 58" and a small fraction; and accordingly, in his table of observations, he gives corrections computed according to this supposition, whereby his column of co-latitudes is rendered uniform, without departing from the law of refraction at different altitudes laid down by Dr. Bradley.
The deductions thus made from observations on the fixed stars. are next compared with those obtained from the meridian altitudes of the sun at the solstices, which he thinks afi'ord satisfactory proof of their correctness; as the latitude of his observatory, by the former method, was found to be 51° 28’ 2"‘l, and by the latter 51° 28' 2"‘35.
The author proceeds to ascertain the diiference of latitude between the Royal Observatory at Greenwich and his own, by comparison of his observations of the zenith distance of y Draconis, with some of the same star communicated to him by Dr. Maskelyne; and by similar comparison of zenith distances of other stars observed at the Royal Observatory by Colonel Mudge with the zenith sector.
