17 3 Velocity of Light. [ B ook II I . 



principles it is found, that the atmofphere is not 

 warmed by the mere paffage of the fun's rays through 

 it, but chiefly by the heat which is collected by the 

 earth, and which is thence imparted to the air. Thus 

 the air, at the fummits of high mountains, is always 

 cold, bccaufe they are too much elevated above the 

 general furface of the earth to derive any confiderable 

 advantage from this circumftance. 



When thefe fafts are fairly confidered, ihe fyftem 

 which fuppofes light to be a modification of the matter 

 of fire, or a combination of that element with fome 

 unknown principle, muft be allowed to be at lea-ft 

 probable. It has, however, been cuftomary to confider 

 ciiftindtly the properties of light, without a reference 

 to its analogy with the matter of fire, and in this 

 mode it will be necefiary, on the prefent occafion, to 

 proceed. . 



The firft remarkable property of light is its amaz- 

 ing VELOCITY. In the fhort fpace vionejecond a parti- 

 cle of light traverfes an extent of one hundred and f evenly 

 tkoujand miles *, which is fo much fwifcer than the 

 progrefs of a cannon ball, that the light is enabled to 

 pafs a fpace in about eight minutes, which could not 

 be palled with the ordinary velocity of a cannon ball 

 in lefs than thirty-two years f. The velocity of light 

 is allb found to be uniform, whether it is original, 

 as from the ftars, or rtiiecled only, as from the 

 planets. 



The mode of calculating the velocity of light is a 

 branch of atlronomy rather than of natural hiftory. It 

 "will fuffice therefore in this place to remark, that by 

 mathematical obfervations made upon the tranfits of 



Nicholfon's Phil. vol. i. p. 258. -J- Ibid, p. 257. 



x 



Venus 



