224 Mr. J. Croll on the supposed greater Loss of Heat 



in the sandstone would, if applied to water, raise the tempera- 

 ture of the water 1° C. to a depth of 39 feet 6 inches ; that 

 stored up in the sand would raise the temperature of the water 

 1° C. to a depth of 21 feet 8 inches, and that stored up in the 

 trap would raise the water 1° C. to the depth of 32 feet 6 inches. 

 We may take the mean of these three results as representing 

 pretty accurately the quantity stored up in the general surface 

 of the country. This would be equal to 31 feet 3 inches depth 

 of water raised 1° C. The quantity of heat lost by radiation 

 during winter below the mean was found to be about equal to 

 that stored up during summer. 



The total quantity of heat per square foot of surface received 

 by the equator from sunrise till sunset at the time of the equi- 

 noxes, allowing 22 per cent, for the amount cut off in passing 

 through the atmosphere, is 1,780,474 foot-pounds. In the 

 latitude of Edinburgh about 938,460 foot-pounds per square 

 foot of surface is received, assuming that not more than 22 per 

 cent, is cut off by the atmosphere. At this rate a quantity of 

 heat would be received from the sun in two days ten hours (say, 

 three days) sufficient to raise the temperature of the water 1° C. 

 to the required depth of 31 feet 3 inches. Consequently the 

 total quantity of heat stored up during summer in the latitude 

 of Edinburgh is only equal to what we receive from the sun 

 during three days at the time of the equinoxes. Three days' 

 sunshine during the middle of March or September, if applied 

 to raise the temperature of the ground, would restore all the 

 heat lost during the entire winter; and another three days' 

 sunshine would confer on the ground as much heat as is stored 

 up during the entire summer. But it must be observed that 

 the total duration of sunshine in winter to that of summer in 

 the latitude of Edinburgh is only about as 4 to 7. Here is a 

 difference of two months. But this is not all ; the quantity of 

 heat received during winter is scarcely one-third of that received 

 during summer; yet notwithstanding this enormous difference 

 between summer and winter, the ground during winter loses 

 only about six days' sun-heat below the maximum amount pos- 

 sessed by it in summer. 



But if what has already been stated is correct, this loss of heat 

 sustained by the earth during winter is not chiefly owing to the 

 fact of the longer absence of the sun during winter, but to the 

 decrease in the quantity of heat received in consequence of 

 his longer absence combined with the obliquity of his rays 

 during that season. But in the case of the two hemispheres, 

 although the southern winter is longer than the northern, the 

 quantity of heat received by each is the same. But suppo- 

 sing it held true, which it does not, that the loss of heat sus- 



