224 TEE POPULAR SCIENCE MONTHLY. 



ion was the ultimate cause of the motion of his locomotive-engine, said 

 that he thought it went by "the bottled-up rays of the sun." 



With the exception of our coal-fields and a few elementary com- 

 bustible substances such as sulphur and what are called the precious 

 metals, which we find sparsely scattered about, our earth consists es- 

 sentially of combined matter. Thus our rivers, lakes, and oceans are 

 filled with oxidized hydrogen, the result of a most powerful combus- 

 tion ; and the crust of our earth is found to consist either of quartz (a 

 combination of the metal silicon with oxygen) or limestone (oxidized 

 calcium combined with oxidized carbon), or of other metals, such as 

 magnesium, aluminium, or iron, oxidized and combined in a similar 

 manner. Excepting, therefore, the few substances before enumerated, 

 we may look upon our earth, near its surface at any rate, as a huge 

 ball of cinder, which, if left to itself, would soon become intensely 

 cold, and devoid of life or animation of any kind. 



It is true that a goodly store of heat still exists in the interior of 

 our earth, which, according to some geologists, is in a state of fusion, 

 and must certainly be in a highly heated condition ; but this internal 

 heat would be of no avail, owing to the slow rate of conduction, by 

 which alone, excepting volcanic action, it could be brought to us liv- 

 ing upon its surface. 



An estimate of the amount of heat poured down annually upon the 

 surface of our earth may be formed from the fact that it exceeds a 

 million times the heat producible by all the coal raised, which may be 

 taken at 280,000,000 tons a year. 



If, then, we depend upon solar radiation for our very existence from 

 day to day, it can not be said that we are only remotely interested in 

 solar physics, and the question whether and how solar energy, com- 

 prising 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 reversionary 

 interest as we have in landed estate or other property. 



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

 plied annually to our earth is great as compare^ with terrestrial quan- 

 tities, 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 approximately 

 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 con- 

 sumed 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 were 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 



