420 Professor W. Thomson on the 



masses are shown in their true proportions to that of the earth, 

 according to data which Professor Piazzi Smyth has kindly com- 

 municated to the author. 



In Table II., the mechanical values of the rotations of the sun 

 and earth are computed on the hypothesis, that the moment of 

 inertia of each sphere is equal to the square of its radius multiplied 

 by only one-third of its mass, instead of two-fifths of its mass, as 

 would be the case if its matter were of uniform density. These 

 two estimates are only introduced for the sake of comparison with 

 other mechanical values shown in the table, not having been 

 used in the reasoning. 



The numbers in the last column of Table II., showing the 

 times during which the sun emits quantities of heat mechanically 

 equivalent to the earth's motion in its orbit, and to its motion of 

 rotation, were first communicated to the Royal Society on the 

 9th of January 1852, in a paper " On the Sources Available to 

 Man for the production of Mechanical Efi^ect.^' These, and the 

 other numbers in the same column, are the only part of the 

 numerical data either shown in the tables, or used directly or in- 

 directly in the reasoning on which the present theory is founded, 

 that can possibly require any considerable correction ; depending 

 as they do on M. Pouillet's estimate of solar heat in thermal 

 units. The extreme difficulties in the way of arriving at this 

 estimate, notwithstanding the remarkably able manner in which 

 they have been met, necessarily leave much uncertainty as to 

 the degree of accuracy of the result. But even if it were two or 

 three times too great or too small (and there appears no possi- 

 bility that it can be so far from the truth), the general reasoning 

 by which the theory of solar heat at present xiommunicated is 

 supported, would hold with scarcely altered force. 



The mechanical equivalent of the thermic unit, by which the 

 solar radiation has been reduced to mechanical units, is Mr. Joule's 

 result — 1390 foot-pounds for the thermal unit Centigrade-^ which 

 he determined by direct experiment with so much accuracy, that 

 any correction it may be found to require can scarcely amount 

 to j^^ or y^o^*^ ^^ ^*® ^^^ value. 



