THE CLIMATE OF THE LAKE REGION. 203 



The temperature of the earth's surface, and all those 

 incidents of climate conditioned by temperature, are de- 

 termined by the solar energy. It is indeed true that the 

 earth's interior exists in a highly-heated condition, and we 

 must probably admit that parts of the central portion still 

 remain in a molten state. In any event, the interior can 

 only be in a solid state as the consequence of pressure 

 sufficient to counteract the liquefying tendency of intense 

 heat. But notwithstanding the intensity of the internal 

 heat, very exact experiments seem to have proved that the 

 central heat is escaping to the surface with such extreme 

 slowness that the superficial temperature is affected to a 

 barely appreciable extent from this cause. 



The total amount of heat received by the earth from 

 the sun varies with the distance between the two bodies. 

 As the form of the earth's orbit is an ellipse instead of a 

 circle, while the sun occupies one of the centers or foci, 

 the earth approaches considerably nearer the sun in one 

 extremity of its orbit than in the other. The difference 

 in the distances is about three millions of miles, while. the 

 mean distance is about ninety- two and a third millions of 

 miles. In consequence of the diminished distance of the 

 earth from the sun at perihelion, the intensity of the sun's 

 rays is three and one-third per cent greater than the mean 

 intensity. At aphelion his intensity is three and one-third 

 per cent less than the mean. 



It is an interesting fact, and one of momentous conse- 

 quence to our race, that the annual period of greatest 

 intensity occurs during the winter of the northern hemis- 



ifjan, in Annual Report of the State Horticultural Society for 1880. See, also, 

 s. B. McCracken ; The State of Michigan, embracing sketches of its History, Posi- 

 tion, Resources and Industries, 1876, 8vo, 136 pp.; and Dr. H. F. LySter, Sixth 

 Annual Report of the Secretary of the State Board of Health of the State of Mich- 

 igan, pp. 167-250. 



