446 On Refraction, 



feldspar, are frequently to be met with on the surface : I 

 did not find any of this rock in its native bed. 



" The hill called the Chair of Kildarc and Dunmurry 

 Hill, situated to the south-west of the Hill of Allen, are also 

 composed of greenstone* ; the Red Hills are conglomerate. 



" Besides the general ridge which (with the exception of 

 two low passes through which the bog rivers flow) sur- 

 rounds the district, and the Island of Allen which divides 

 the interior, there are frequently minor and more detached 

 ridges, usually of moderate elevation, bounding the several 

 bogs, and preventing the passage of the waters to the rivjrs- 

 or principal streams, which usually run in valleys beyond 

 the ridges, and nearly parallel to the ed^e of the bogs. 



" These interior ridges, where there is no river, usually 

 form the line of separation between different begs." 

 J am, sir, 



Your obedient servant, 

 William Farev. 



LXXX. A short Account of the Improvements gradually 

 made in determining the Astronomic Refraction. By 

 Afr.T. S. Evans, Master of the Mathematical School at 

 New Charlton, near Woolwich, Kent ; late of the Royal 

 Observatory, Greenwich, and oj the Royal Military Aca- 

 demy, Woolwich. 



[Continued from p. 349.] 



It would be endless to notice the different opinions re- 

 specting both the terrestrial and the astronomic refraction 

 which are to be met with in the writings of various authors 

 on the subject : and it would be equally useless to notice 

 all the tables of its quantity given by them, some of which 

 differ very much from others. It will be sufficient to men- 

 tion those only who made some considerable advances to- 

 wards obtaining it with greater accuracy. 



The next of these in order was La Caillef, who in de- 

 termining it certainly bestowed very great pains, by making 

 and reducing an immense number of observations, and 

 afterwards comparing them with others made at Green- 

 wich by Dr. Bradley, at Gottingen by Mayer, at Bologna 

 b\ X noiti, and by La Laurie who was then at Berlin. 

 From these it appeared that the refraction at 45 3 of altitude 

 was tif3 u" ; but ibis, as will hereafter be seen, Was too great 



* This is the first discovery of rock ef the trap formation in this part of 

 ftrafa d. 



f Mem. dc i'Ac de Sc. 1755, p. 547. 



by 



