34 9 On Refraction. 



tion to be about one-seventh less than in Europe*. The 

 horizontal retraction he found to be 27': but at 6' of alti- 

 tude it is 7' 4" ; and at 45° it is 44". Buuguer then gives 

 a tablet for Quito, which is more elevated above the level 

 of the sea. M. le Gen til \ found it greater at Pondieherry 

 in India, although in the torrid zone. 



The refraction diminishes when we are elevated above 

 the level of the sea. Bouguer observed § the quantity of it 

 at Chimboraco, 2388 toises above the level of the sea, and 

 found it in the horizon only 19-3-'. At the cross of Pit- 

 chinca, 2044 toises above the sea, he found it 20' 4&"; at 

 Quito, 1479 toises above the sea, 22' 50": but at the level 

 of the sea 27'. These observations, when joined with the 

 theory, produced the following rule ; That if we take the 

 excess of 515S toises above the elevation of the place, with 

 regard to the level of the sea, the refraction will be as the 

 square root of this excess. Thus the square root of 5158 

 toises is 2?', for the horizontal refraction at the level of the 

 sea, in the torrid zone : and the square root of the excess 

 of 5158 above the elevation of the place will be its hori- 

 zontal refraction. The quantity 5158 is the height above 

 which the refractive matter no longer produces any sensible 

 effect, at least in the torrid zone||. 



But although by this time considerable attention had been 

 paid to the subject, yet great differences were to be found in 

 the tables then most in use. Thus at the altitude of 30°, 

 according to Flamsteed, the refraction was l' 23"; New- 

 ton V 30"; Cassini V 42"; and de la Hire l' 55^ ; leaving 

 an uncertainty of more than half a minute: and it must 

 have been very mortifying to an observer, after having taken 

 the utmost pains to avoid errors of two or three seconds, to 

 find his reduced observations liable to so great an error, ac- 

 cording to the choice of his table of refraction. 



It is indeed rather extraordinary, that in a memoir pub- 

 lished by Cassini de Thury, among those of the Academy 

 for 1745, he attempted to reconcile a number of observa- 

 tions with each other, by considering the state of the ther- 

 mometer only, without at all noticing that of the barome- 

 ter; although at that time Hauksbee's experiments had 

 been published about 37 years. 



He concludes his paper, as is very natural to suppose, 

 without being able to make the observations agree : nor 

 does it clearly appear that the French noticed the above- 



* This Table is in the memoir above cited. 



t Mem. 1749. Conn. ties Mouv. Celest.p. 1765. J Mem. 1774. Voyage, 

 torn. i. § Mem. p. 1749. |) Encycl. Method, art. Refr. 



mentioned 



