36 OUTLINES OF NATURAL PHILOSOPHY. 



a. Dr BRADLEV has given a formula, expressing this 

 variation of the refraction. 



Xet b = height of the mercury in the barometer in 

 inches, k = height of FAHRENHEIT'S thermome- 

 ter, z = the zenith distance, r = the mean refrac- 

 tion computed by the rule, ( 44. 6.), the correct 



br 400 

 refraction r = -X 



29.6 350+7* 



This formula would be improved, by introducing 

 the expression for the density or specific gravity of 



the air from vol. i. } 333. It will then be 

 i> - _^L x (10.00222 (A 50))- 



A7O 



49. Not only are the stars elevated by refrac- 

 tion, but all terrestrial objects are elevated from 

 the same cause, by an angle equal to that which 

 the straight line drawn from the eye to the ob- 

 ject makes with the tangent to the path of the 

 ray at the point where it enters the eye. 



The refraction of the heavenly and terrestrial bodies 

 is differently estimated. That of the celestial bo- 

 dy is the angle contained between the tangent to 

 the curvilineal path of the ray where it is first ac- 

 ted on by the atmosphere, and the tangent to the 

 same curve when it enters the eye. The refrac- 

 tion of a terrestrial body is the angle contained 

 between the tangent at the eye and the chord of 

 the arch intercepted between the object and the eye. 



Near 



