250 OUTLINES OF NATURAL PHILOSOPHY 



513, &c. LA PLACE, Mecanique Celeste, torn. in. 

 p. 289. BIOT, Astron. Phys. torn. in. Additions, 

 2d edit. Also, Memoire sur la Formule Barome- 

 trigue* par L. RAMOND. 



Though much has been done by the authors just 

 named, the barometrical method of measuring 

 heights is probably not yet so perfect as it may be 

 rendered. Its exactness depends on the compari- 

 son of trigonometrical measures with barometrical 

 observations. Now, the trigonometrical measures 

 of heights are subject to considerable inaccuracy, 

 from the variations of refraction, unless the refrac- 

 tion itself be observed at the same time with the 

 angle of elevation. If this is done, or if the height 

 be taken by levelling, the result may be obtained 

 with great precision. The number of instances, 

 however, in which either of these precautions has 

 been used, is not great enough to afford an exact 

 determination of the constant quantities that en- 

 ter into the barometric formula. It would be use- 

 ful also to ascertain by more accurate experiments, 

 the law according to which the temperature of the 

 air diminishes on ascending from the surface of the 

 earth. 



343. The temperature of the air diminishes on 

 ascending into the atmosphere, both on account of 

 the greater distance from the earth, the principal 

 source of its heat, and the greater power of absorb- 

 ing heat that air acquires, by being less com- 

 pressed. 



The 



