398 Astronomical and Nautical Collections. 



9377 ; or if we augment the 8740 in the proportion of the ex- 

 pansion of air from 50° to 80°, it will become 9264 ; but it is 

 well known that in computations of this kind it is necessary to 

 introduce a variety of subsidiary corrections. The principal of 

 these, however, is the reduction of the height of the barometer 

 for temperature, and when this is applied, the result of the for- 

 mula thus employed appears to be extremely accurate. Taking, 

 for example, General Roy's observations on Moel Eilio, at the 

 height of 2371 feet : the temperature, below, 68°, the corrected 

 heights of the barometer 29.918 and 27.468, we haveD =.082, 

 and H = 2306, to which adding -ji^, or 84, for the excess of 

 the lower barometer above 50°, we have 2390, while General 

 Roy's computations make the height 2393. In one of Mr. 

 Greatorex's examples, the error of this method is 6 yards, while 

 that of Dr. Maskelyne's is 12. 



It may not be improper to observe that the correction of the 

 refraction for temperature, as applied in the Nautical Almanac, 

 may possibly be found to agree better with the mean tempera- 

 tures of different climates than with the occasional variations at 

 the same place, which may often be less regular in their causes 

 and extent : and there is reason to think that, in some of these 

 cases, the correction of ^^^ for each degree of Fahrenheit, as 

 employed by Bradley, is sufficient in the immediate neighbour- 

 hood of the horizon. Mr. Groombridge's observations, as pub- 

 lished in the Connaissance des Terns for 1821, require the stan- 

 dard temperature of the table to be supposed 50°, when com- 

 pared with the exterior thermometer, and not 48°, as the 

 Greenwich observations seem to indicate. But it is at present 

 impossible to expect any thing like perfect accuracy in a deter- 

 mination so liable to uncertainties of various kinds. 



v. Accmmt of some Optical Inventions of Professor Amici. 

 Fro7n the Memoirs of the Italian Society, Vol. XIX. 



1. The first, in importance, of Mr. Amici's papers is an ac- 

 count of an iconantidiptic telescope. Jeaurat's invention of a 

 telescope that should exhibit at once two images, one erect, the 

 other inverted, coinciding in the axis only, was improved by 



