150 THE WOTU.D. 



nearer the poles. The following table exhibits the length of the 

 longest day for different geographical latitudes'. 



Polar elevation. Length of the longest day. 



. . ". . 12 hours, 



16 44' 13 



30 48' 14 



49 22' 16 



63 2.3' 20 



66 32' 24 



67 23' 1 month, 



73 39' 3 months, 



90 G months. 



Upon examining this table it will be perceived that within the 

 tropics, the length of the longest day never varies much from that 

 of the night, and hence, as before observed, the temperature is 

 tolerably equal. In higher latitudes (.he rays of the sun strike 

 more obliquely than within the tropics, yet the day so much ex- 

 ceeding the night, more heat is gained during the day than is ra- 

 diated during the night, and thus, what is lost in intensity, is gained 

 in the length or duration, and it thus happens that during the 

 summer it may be very hot, even at places far removed from the 

 equator. At St. Petersburg!!, for instance, during a hot summer 

 the thermometer frequently rises to 86. On the other hand, in 

 winter, at the same latitudes, the days become as much shorter 

 than the nights, as the nights were previously shorter than the days; 

 hence, since the sun's rays fall very obliquely, and are therefore 

 very feeble in their action, the earth radiating much more heat at 

 night than it receives during the day, the winter temperature is 

 very low, the difference between winter and summer tempera- 

 ture will therefore, generally be greater, the farther we remove 

 from the equator. 



" At Bogota, which is 40 35' N. of the equator the difference 

 of temperature between the hottest and coldest month amounts to 

 only 3 ; in Mexico (19 25' N. lat.) this difference is 14 ; at 

 Paris, (48 50' N. lat.) 48, and for St. Petersburgh, (59 5G' N. 

 lat.) 57." 



It appears from what has been said, that within a distance of 10 

 or 15 degrees of the equator or equinoctial line, the difference be- 

 tween summer and wiivter temperature is trifling, but when we 



