PNEUMATICS. 297 



centre B, with the distance BC or 27, describe the 

 semicircle CGH ; take the arch CG equal to the 

 double latitude of any parallel ; and from G drasv 

 GO perpendicular to AC ; then is AO the mean tem- 

 perature of that parallel, according to FAHRENHEIT'S 

 scale. 



The mean temperatures thus found, agree very well 

 with observation. Springs, in which the water 

 does not considerably change its heat from one sea- 

 son of the year to another, afford an expeditious 

 and accurate way of ascertaining the mean tempe- 

 rature. 



404. If the place is at any height H above the 



TT 



level of the sea, t = M - + E cos 2! L. 

 H is understood to be expressed in English feet. 



405. On ascending into the atmosphere, at a cer- 

 tain height in every latitude, a point is found where 

 it always freezes, or where it freezes more than it 

 thaws, so that the mean temperature is below 321. 

 The curve joining all these points, from the equa- 

 tor to the pole, is called the line of perpetual con- 

 gelation. The equation to it will be found, by 



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making 32! = M -- - -h E cos 2 L. 



This 



