WILLIAM SIEMENS, F.R.S. 295 



<1 in solar physics, and the question whether and how solar 

 eiKTgy, comprising the rays of heat, of light, and the actinic rays, 

 is likely to be maintained, is one in which we have at least as great 

 a ivviTMiinarv interest as we have in landed estate or other property. 



If the amount of heat, or, more correctly speaking, of energy, 

 supplied annually to our earth is great as compared with terrestrial 

 quantities, that scattered abroad in all directions by the sun strikes 

 us as something almost beyond conception. 



The amount of heat radiated from the sun has been approxi- 

 mately computed by the aid of the pyrheliometer of Pouillet, and 

 by the actinometers of Herschel, at 18,000,000 heat units from 

 every square foot of its surface per hour ; or, expressed popularly, 

 if coal were consumed on the surface of the sun in the most 

 perfect manner, our total annual production of 280,000,000 tons, 

 being the estimated produce of all the coal-mines of the earth, 

 would suffice to keep up solar radiation for only one forty-millionth 

 part of a second ; or, if the earth was a mass of coal, and could be 

 supplied by contract to the solar furnace-men, this supply would 

 last them just thirty-six hours. 



If the sun were surrounded by a solid sphere of a radius equal 

 to the mean distance of the sun from the earth (95,000,000 of 

 miles), the whole of this prodigious amount of heat would be 

 intercepted ; but considering that the earth's apparent diameter 

 as seen from the sun is only seventeen seconds, the earth can 

 intercept only the 2250-millionth part. Assuming that the other 

 planetary bodies swell the amount of intercepted heat to ten times 

 this amount, there remains the important fact that f|-|ooooo f 

 the solar energy is radiated into space, and apparently lost to the 

 solar system, and only a ?it6o6o6o utilised or intercepted. 



Notwithstanding this enormous loss of heat, solar temperature 

 has not diminished sensibly for centuries, if we neglect the periodic 

 changes, apparently connected with the appearance of sun-spots, 

 that have been observed by Lockyer and others, and the question 

 forces itself upon us how this great loss can be sustained without 

 producing an observable diminution of solar temperature even 

 within a human lifetime. 



Amongst the ingenious hypotheses intended to account for a 

 continuance of solar heat is that of shrinkage or gradual reduction 

 of the sun's volume suggested by Helmholtz. It may, however, 



