July 12, 1895.] 



SCIENCE. 



37 



Earth is more than equivalent to the power 

 of all these animals exerting themselves to 

 the utmost and continuously. 



It is easy to show that " the amount of 

 heat emitted in a minute by a square metre 

 of the Sun's surface is about 46,000 times as 

 great as that received by a square metre at 

 the Earth, * * * that is, over 100,000 horse- 

 power per square metre acting continu- 

 ously." * * * (Young.) If the Sun were 

 solid coal this rate of expenditure would 

 imply its entire combustion in about 6,000 

 years. The effective temperature of the 

 Sun's surface is difficult to determine, and 

 has been variously evaluated, from the 

 enormously high estimates of Secchi, Erics- 

 son and Zollner, to the more moderate fig- 

 ures of Spoerer and Lane, who deduced 

 temperatures of 80,000° to 50,000° Fahren- 

 heit. According to Eosetti, it is no less than 

 18,000° Fahrenheit, an estimate probably 

 not far wrong. M. Le Chatelier, however, 

 in 1892, found the temperature a little short 

 of 14,000°, and Wilson and Gi-ay about 

 12,000°. Dr. Scheiner's recent observa- 

 tions upon the peculiar behavior of two 

 lines in the spectrum of magnesium confirm 

 these lower values in a remarkable waj^, 

 apparently showing that the Sun's temper- 

 ature lies between that of the electric arc 

 (about 6,000° ) and that of the electric spark 

 (probably as high as 20,000° ). A still later 

 value is 40,000° C, derived by Herr Ebert 

 of Kiel in 1895, by a method, however, in- 

 volving much theoretic uncertainty. 



The maintenance of this stupendous out- 

 lay of solar energy is explainable on the 

 theory advanced by Von Helmholtz in 

 1856, who calculated that an annual con- 

 traction of 250 feet in the Sun's diameter 

 will account for its entire radiation in a 

 year — a rate of shrinkage so slow that 

 many centuries must elapse before it will 

 become detectable with our best instru- 

 ments. Accepting this theory. Lord Kelvin 

 estimates that the Earth cannot have been 



receiving the Sun's light and heat longer 

 than 20,000,000 years in the past; and Pro- 

 fessor jSTewcomb calculates that in 5,000,- 

 000 years the Sun will have contracted to 

 one-half of its present diameter, and it is 

 unlikely that it can continue to radiate suf- 

 ficient heat to maintain life of types now 

 present on the Earth longer than 10,000,000 

 years in the future. But it is now known 

 that there are elements neglected in this 

 computation which render a revision neces- 

 sary and will probably extend this time 

 very greatly. Assuming that solar heat is 

 radiated uniformly in all directions, com- 

 putation shows that all the known planets 

 receive almost a two-hundred-millionth part 

 of the entire heat given out by the Sun, 

 the Earth's share being about fV of this. 

 The vast remainder seems to us essentially 

 wasted, and its ultimate destination is un- 

 known. 



To epitomize Professor Young's state- 

 ment of the theory of the Sun's constitu- 

 tion, generally accepted: 



(a) The Sun is made up of concentric 

 layers or shells, its main body or nucleus 

 being very probably composed of gases, but 

 under conditions very unlike any laboratory 

 state with which we are acquainted, on ac- 

 count of the intense heat and the extreme 

 compression by the enormous force of solar 

 gravity. These gases would be denser than 

 water, and viscous, in consistency possibly 

 resembling tar or pitch. 



(b) Surrounding the main body of the 

 Sun is a shell of incandescent clouds, 

 formed by condensation of the vapors which 

 are exposed to the cold of space, and called 

 the photosphere. Telescopic scrutiny shows 

 that the photosphere is composed of myriad 

 ' granules ' about 500 miles in diameter, 

 excessively brilliant, and apparently float- 

 ing in a darker medium. 



(c) The shallow, vapor-laden atmosphere 

 in which the photospheric clouds appear to 

 float is called the ' reversing layer,' because 



