-1859] WRITINGS OF JOSEPH HENRY. 89 



All the phenomena referable to heat from the sun acting 

 under varying conditions will now (so far as they affect the 

 climate of the United States,) be considered under two heads: 



1. The effects of varying astronomical conditions, irres- 

 pective of atmospheric and other influences. 



2. The effect of all conditions, other than astronomical, 

 such as the influence of the air, the ocean, the land, &c. 



/. Results of Astronomical Conditions. 



The earth, in its annual revolution in its orbit round the 

 sun, does not describe a perfect circle, but an ellipse, of which 

 the sun occupies one of the foci ; and hence we are nearer 

 at one season of the year to this central luminary than at 

 another. It is well established by mathematical investi- 

 gation from astronomical data, that at the present histori- 

 cal period, the earth as a whole receives the greatest amount 

 of heat during any one day in the year on the first of Jan- 

 uary, and the least amount on the 4th of July. The 

 variation in the distance of the sun produces no effect on 

 the different seasons ; since the rapidity of motion or the 

 less duration of proximity to the sun, just compensates for 

 the greater intensity of the rays due to the nearer approach. 

 Were it not for this, the eccentricity of the orbit would 

 materially influence the heat of the seasons, since the fluctua- 

 tion in the heating power of the sun's ra3^s on this account 

 amounts to one-fifteenth of the whole; and it does in reality 

 increase the diurnal intensity for a few days in January, 

 as is shown from the ardor of the sun's rays under a clear 

 sky at noon in the southern hemisphere. One-fifteenth, 

 says Sir John Herschel, is too considerable a fraction of the 

 whole intensity of sunshine, not to aggravate in a serious 

 degree the sufferings of those who are exposed to it without 

 shelter, in the thirsty deserts of the south. The accounts 

 of what is endured in the interior of Australia at this season, 

 for instance, are of the most frightful kind, and seem far 

 to excel what have ever been experienced by travellers in 

 any part of the northern hemisphere. 



Another astronomical deduction is that the point of the 



