Brown. — The Maintenance of the Sun's Heat. 895 



of some very large figures. It is evident that we must first find 

 out how much heat the sun loses in an hour, or a week, or 

 a year, and then compare this amount with the quantity which 

 could be evolved by a hot or burning body as big as the sun. 



We are all of us fully aware that the sun radiates heat, but 

 there are probably only a few here who have ever thought about 

 the quantity of heat thus radiated, or who have any definite idea 

 of the enormous thermal loss which the sun daily undergoes. 



To learn how much heat the sun loses in a given time, we 

 must measure the amount of radiation on a given area of the 

 earth's surface. Since the radiation is going on simultaneously 

 in all other directions, it is clear that every square mile on the 

 surface of a sphere of which the radius is 95,000,000 miles will 

 be equally warmed ; we must therefore, to find the total radiation, 

 multiply the number we have obtained by the number of square 

 miles on the surface of this sphere, that is, by 108,000 million 

 million. Many measurements of the solar radiation have been 

 made with more or less perfect apparatus. The first, which were 

 carried out at the Cape of Good Hope by Herschel, led to the 

 conclusion that the solar radiation on a square mile would raise 

 47,500,000 lbs. of water from freezing to boiling in an hour. In 

 obtaining this result, however, no account was taken of the 

 large amount of heat absorbed by the atmosphere, an amount 

 which varies with the humidity of the air, and with the obliquity 

 of the sun's rays. Allowing for this atmospheric absorption, 

 and basing our calculation on the experiments of Violle, which 

 are probably the most exact, we find that the solar radiation per 

 square mile per hour, just outside the earth's atmosphere, would 

 raise 85,500,000 lbs. of water from freezing to boiling. Multi- 

 plying this number by 108,000 million million, we obtain an 

 expression for the total hourly solar radiation which is, accord- 

 ing to Tyndall, sufficient to boil 700,000 millions of cubic miles 

 of ice-cold water per hour. If, now, we consider how this loss 

 would affect the temperature of a hot body of the mass of 

 the sun which received no heat from any source, we find that it 

 would result in a fall of the sun's temperature of at least 2° C. 

 annually, or 10,000° in 5,000 years ; yet all the evidence accu- 

 mulated by geologists goes to show that in bygone ages the sun's 

 rays were no hotter than they now are. If, on the other hand, 

 we suppose that the sun is a vast burning mass, we find that if 

 it were made of solid coal, and were burning at a rate sufficient 

 to yield this enormous supply of heat, it would be all consumed 

 in 6,000 years. As no apparent diminution of the solar radiation 

 has taken place for thousands of years, I was justified in com- 

 paring the sun to the bush in the desert, wbich burnt, yet was 

 not consumed. 



Two well-known hypotheses have been set up to account for 

 the maintenance of the sun's heat ; the one ascribes it to a great 



