A NEW THEORY OF THE SUN. 225 



the mean distance of the sun from the earth (95,000,000 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 2,250-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|f 000000 f tne s l ar energy is radiated into space, and 

 apparently lost to the solar system, and only 22go S ooo utilized or in- 

 tercepted. 



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 

 life-time. 



Among the ingenious hypotheses intended to account for a con- 

 tinuance of solar heat is that of shrinkage or gradual reduction of the 

 sun's volume, suggested by Helmholtz. It may, however, be argued 

 against this theory that the heat so produced would be liberated 

 throughout its mass, and would have to be brought to the surface by 

 conduction, aided perhaps by convection; but we know of no material 

 of sufficient conductivity to transmit anything approaching the amount 

 of heat lost by radiation. 



Chemical action between the constituent parts of the sun has also 

 been suggested ; but here again we are met by the difficulty that the 

 products of such combination would, ere this, have accumulated on the 

 surface, and would have formed a barrier against further action. 



These difficulties led Sir William Thomson to the suggestion that 

 the cause of maintenance of solar temperature might be found in the 

 circumstance of meteorites, not falling upon the sun from great dis- 

 tances in space, as had been suggested by Mayer and Waterton, but 

 circulating with an acquired velocity within the planetary distances of 

 the sun, and he shows that each pound of matter so imported would 

 represent a large number of heat-units, without disturbing the plan- 

 etary equilibrium. But in considering more fully the enormous amount 

 of planetary matter that would be required for the maintenance of the 

 solar temperature, Sir William Thomson soon abandoned this hypoth- 

 esis for that of simple transfer of heat from the interior of a fluid sun 

 to the surface by means of convection-currents, which latter hypothesis 

 is at the present time supported by Professor Stokes and other leading 

 physicists. 



This theory has certainly the advantage of accounting for the 

 greatest possible store of heat within the solar mass, because it sup- 

 poses the latter to consist in the main of a fluid heated to such a tem- 

 perature that, if it wfere relieved at any point of the confining pressure, 

 vol. xxi. 15 



