206 SPARKS FROM A GEOLOGIST'S HAMMER. 



day is much longer. On the 15th of June, for instance, 

 the diurnal intensity at the equator is 72, while in the 

 latitude of forty degrees it is 90. 1. At the north pole, 

 where the day may be regarded as twenty- four hours long, 

 the daily intensity on the 15th of June is 97.6. The 

 amount of heat received at the pole is in excess of that 

 received at a point on the equator from the 10th of May 

 to the 3d of August, a period of eighty-five days. On 

 the parallel of forty degrees the excess of diurnal heat 

 extends from the 24th of April to the 20th of August, 

 an interval of one hundred and eighteen days. 



These contrasts, however, it must be remarked, apply 

 only to the upper stratum of the atmosphere. 



The sun's intensity at the earth's surface is materially 

 diminished by atmospheric absorption, and this effect is 

 peculiarly experienced by the slanting rays of the polar 

 regions. 



So far we have considered the temperature of a locality 

 only in its relation to astronomical conditions. The nor- 

 mal astronomical temperature is almost always disguised 

 by numerous perturbating influences of a local character. 

 The influence of winds and moisture upon the sensible, 

 and also upon the actual, temperature has already been 

 mentioned. There are other local conditions, however, 

 which exert a permanent and more important influence. 

 The most efficient of these are altitude above the sea- 

 level and proximity to great bodies of water. It is well 

 understood that the temperature falls as we ascend above 

 the level of the ocean. The rate of diminution of tem- 

 perature varies with the hour of the day, the season and 

 the latitude. In temperate latitudes it may be taken at 

 one degree for every 333 feet of ascent. Lake Superior, 



