TERRESTRIAL REFRACTION. 3 



effect of the sort, and must evince the expediency of 

 obtaining' corresponding experiments in different lati- 

 tudes; for, it is obvious, that even to ascertain any 

 deviation in a system, perhaps too generahsed, 

 might be attended with incalculable advantages to 

 science. 



It must be owned, that to render experiments on. 

 terrestrial refraction pointedly useful, it would be ne- 

 cessary to shew how discoveries in this province 

 might apply and be extended to refraction in general. 

 Hitherto, on this recondite subject, nothing which 

 would immediately apply has reached my knowledge ; 

 but as so much is still to be done whenever refrac- 

 tion is concerned, we may argue, that, in the present 

 stage of our information, observations confined to 

 terrestrial objects may be deemed sufficient. 



It has been stated on experiments*, that the refrac- 

 tive power of the air is proportional to its density; 

 and this is as its weight directly, and heat in- 

 versely. It would then appear, were our barometers 

 and thermometers, sufficiently accurate, that by com- 

 paring them at any given time, the ratio of its density 

 might be had. But it has been found, on trial, that 

 in the present unimproved state of these instruments, 

 changes, not very m.inute, in the density of the at- 

 mosphere, escape our notice, when a reference is made 

 to them alone. 



Now, since we have every reason to suppose, that 

 whatever share, heat, cold, or electricity, may have 

 separately on the refractive powers of the air f , their 



♦ By Halley. 



t EuLEU, after a number of experiments on the immediate effects 

 of heat and cold, on the refractive powers of media, concludes, " that, 

 " in all translucent substances, the focal distances diminish with the 

 " heat, which diminution, he conceives, is owing to a change in the 



b2 



/ 



