[370] 



6°. The heat be/ow and the rate of progreffion being known, 



the height of the line of congelation may be found by the fixth 



r 1 rri. ■ *T- 1 "54 — ■^a .22,00000 

 formula. Thus .in the example 2Z — J^ o- i — — ! h I = 



40840 40846 



53,86 + I = 54,86 X 100 = 5486. 



]S/ote. — That the heat be/ow and the heat ^bove being given, if 

 the difference includes no fradion, and if the height alfo be 

 known, the rate or common difference may be found by di- 

 viding the difference of heats above and below, by the height or 



TT I. 



d — zz. _: n being expreffed in the terms of the progreffion. 



n 



Thus the height of Tinto being 1642 feet it fliould be expreffed 

 16,42. 



Some have thought tliat heat is conveyed upwards by the gra- 

 dual afcent of the lower ftrata of air which firft receive the in- 

 creafe of heat, but that this cannot happen will be fhewn in the 

 fequel. 



In ferene weather, whatever variations may occur in the tempe- 

 ratures below, ths differences between them and the heats above 

 are as nearly the fame as can be expedled, when due allowances 

 are made, for the imperfedion of thermometers on which frac- 

 tions 



