[ 105 ] 



XXII. The Balcerian Lecture. — On the Theory of the Astro- 

 nomical Refractions. Bi) James IvouY,/v./f., M.A.^ F.Il.S. 

 L. 4- E.j Instit. Reg. Sc. Paris, Corresp. et Reg. Sc. Gotiin. 



Corresp. 



[Continued from p. 12.] 



Atmosphere of dry air. 



IN applying the formula (A.) to the experimental ascents 

 that have been made in the atmosphere, a- may be ac- 



counted equal to 2, the height ascended : for — , which is a 



minute fraction at the top of the atmosphere, is insensible in 

 small elevations. Further, in such experiments, the depres- 

 sion of the thermometer, or the difference of the tempei'ature 

 at the upper and lower extremities of the ascent, is only a 

 moderate number of degrees ; and as j3 is a very small frac- 

 tion, the value of 5' in the formula 



will be so inconsiderable, that its powers may be neglected. 

 Attending to what is said, the formula (A.), even in those 

 cases where the ascents are most considerable, may take this 

 very simple form without much error, or rather with all the 

 accuracy warranted by the nature of such experiments, viz. 



^- p' ' f ' 1+^t" 

 or, by making D = p' (1+|3t'), 



. = i±/.(/-x).^.i. 



Now it is obvious that D is the density of the air at the earth's 

 surface, reduced to zero of the thermometer ; and hence we 



learn that -yr is independent on the magnitude oip', and has 



the same value in all atmospheres of dry air; for, D being 

 the density of the air produced by the pressure p' at the 

 fixed temperature zero of the thermometer, it will vary pro- 

 portionally top'. 



The value of the constant quantity -^ is next to be found. 



It has been ascertained, by very careful experiments, that the 

 density of mercury is to the density of dry air as 10462 to 1, 

 the temperature beuig 0° centigrade, or 32° of Fahrenheit's 

 scale, and the barometric pressure O^'^Q, or 29*9218 English 

 inches. The temperature remaining at 32° Fahrenheit, if 



