Page:Proceedings of the Royal Society of London Vol 1.djvu/393

Rh He next compares the refraction aboverdeduced, with the results of other astronomers. Piazzi, having an instrument which turns in azimuth, has deduced the actual refractions at all distances from the zenith, by means of numerous observations on Proeyon, n Lyrae, and Aldebarau, at various altitudes, from 38° to 89° zenith distance, in addition to several circumpolar stars. Piazzi's result is, that the mean refraction at 45° is 57"‘3, which is less by eight tenths of a second than that of the author; but by the present French tables it is stated to be 58"“2, which, on the contrary, is rather greater. But beside the diﬁ'erence in the quantity of mean refraction at 45°, Piazzi observes that the law assigned by Bradley does not obtain ; for though the actual refractions, so far as 80° from the zenith, are, in fact, greater than was supposed by Bradley, the refractions within the remaining 10° of the horizon are less than he supposed them to be.

In the series of observations given by the author, a similar want of conformity to Bradley's law is observable; and he observes, that the change of diﬁ'erence, from greater to less, takes place at 80° zenith distance, which is the same point of the heavens assigned by Piazzi.

Mathematicians, who have endeavoured to reconcile the known laws of refraction through diﬁ'erent media, with the actual quantity deduced from observation, have proved that the refractions vary nearly as the tangents of zenith distance; but in order to reconcile this rule with the fact at low altitudes. they have found it necessary to intro- duce a correction of the zenith distance, and have invented a formula, consisting of a tangent of the zenith distance, diminished by some multiple of the refraction. The magnitude of this multiple has been estimated diﬁ'erently by different authors. By Simpson it is rated at 2‘75; by Dr. Bradley 3: by Bong-oer 3'23 ; by Cassini 3226. Mr. Groombridge computes that this multiple should be as much as 3'3625.

In addition to the above endeavours to determine the mean re- fraction, and its variations at diﬁ'erent altitudes, the author also con- siders the corrections which should be made for the states of the ba- rometer and thermometer, and explains the means by which he de- duced those that he has adopted, in order that any error therein may he more easily detected.

The principal object of Dr. Maskelyne in making this communication, is to inform the Society of a discovery, made by Dr. Brinkley, of the parallax of the annual orbit, which he has ascertained by oh. serrations on a Lyrae.