-1859] WRITINGS OF JOSEPH HENRY. 43 



and though in a more northern latitude the obliquity of the 

 ray is greater, and on this account the intensity should be 

 less, yet the longer duration of the day is more than suffi- 

 cient to compensate this effect, and to produce the result ex- 

 hibited. This is an important fact, in comparing the agri- 

 cultural capacity of different latitudes ; for though there is 

 absolutely more heat at the latitude of New Orleans during 

 the year than at Madison, in Wisconsin, yet there is more heat 

 received at the latter place during the three months of mid- 

 summer than in the same time at the former place. An 

 analogous but contrary result is exhibited in regard to the 

 cold of winters, as will be seen by the table. It is from 

 this principle that as we advance toward the equator, the 

 extreme variations of the season become less and less. It 

 is important to remark in this place that the foregoing 

 tables exhibit the amount of heat actually falling upon the 

 earth during the day as unmodified by any extraneous 

 causes. They do not however exhibit the hottest portion of 

 the season. This will depend upon another condition, which 

 may be properly explained in this connection, though it is 

 not classed under the astronomical causes. It is a well es- 

 tablished principle that all bodies are radiating heat even 

 while they are receiving it. If the amount received in a 

 definite time is greater than that given off, the temperature 

 will increase ; on the contrary, if the amount given off is 

 greater than that received, the temperature will diminish. 

 The earth is constantly radiating heat into space, but only 

 receiving it from the sun during the day. As the sun is de- 

 clining towards the south, the daily amount received at 

 length becomes less than that given off in the night, and 

 hence the temperature begins to fall ; and this diminution 

 will continue until the two quantities again become equal, 

 which will not be at the point where the greatest amount of 

 heat is given off. On the twenty-first of June, in northern 

 latitudes, the earth is receiving the greatest amount of heat, 

 and hence it is becoming heated up most rapidly at this 

 time. On the tweyty-second it receives a less amount of heat, 

 but the heating continues, since the gain is still greater than 

 the loss ; and this goes on until about the 25th of July, or 



