296 OUTLINES OF NATURAL PHILOSOPHY. 



It has already been remarked, that this decrease seems 

 to be somewhat slower as we ascend, but not very 

 considerably, as far as our observations have ex- 

 tended. 



402. The combination of these causes gives to 

 every place a mean temperature, which remains 

 always nearly the same, and which decreases from 

 the equator to either pole, according to a law that 

 has been determined by observation. 



403. Let t be the mean temperature of any pa- 

 rallel of which the latitude is L, M the mean tem- 

 perature of the parallel of 45, and M + E the 

 mean temperature of the equator ; then is 



t = M + E cos 2 L. 



In this formula, M=r58, and E=27. When 

 % L > 90, cos 2 L is negative. 



This theorem was first given by MAYER, Opera ine- 

 dita, vol. i. p. 4., &c. Also KIBWAN, Estimate of 

 the temperature of different Latitudes, p. 18. 



From this a geometrical construction, for finding the 

 mean temperature, may be readily deduced. 



In the line AC (fig. 27.), divided into equal parts, num- 

 bered from A, so as to represent the scale of a ther- 

 mometer, let AC = 85, and AB 58. From the 



centre 



