PNEUMATICS. 293 



397. The first of these depends on the latitude 

 of the place, or its distance from the equator, by 

 which the intensity of the heat and light from the 

 sun, and also the length of the day, are determined 

 for different seasons of the year. 



The intensity of the sun's rays, when they strike on 

 any plane, is as the quantity that falls on a given 

 space, or as the sine of the sun's elevation above the 

 plane. The nearer the sun is to the zenith of any 

 place, at a given moment, the greater the intensity 

 of heat produced by his rays. 



The heat for an entire day, depends also on the length 

 of the day, and as the day is longer where the dis- 

 tance from the zenith is greater, the inequality in the 

 distribution of heat arising from the one of these 

 causes, compensates that proceeding from the other, 

 and brings their combined effects nearer to an equa- 

 lity than might be imagined. FONT AN A has shewn, 

 that the heat of the day of the summer solstice at 

 Pavia, is greater than the heat of the same day at 

 Petersburgh, in a ratio not greater than that of 63 

 to 62, though the latitude of the former be 45 11', 

 and of the latter 59 56'. 



The same author finds, that when the sun's declination 

 exceeds 18, or from about the 10th of May to the 

 30th of July, the heat in twenty-four hours proceed- 

 ing from the sun's rays is greater at the north pole 

 than at the equator. GREG. FONTANA, Disquisi- 

 tiones Physico-Mathematicce, l ma & 2 da . 



The 



