72 



PROFESSOR W. THOMSON ON THE 



nearer and nearer the centre, until some time, very suddenly, it gets so much 

 entangled in the solar atmosphere, as to begin to lose velocity. In a few seconds 

 more, it is at rest on the Sun's surface, and the energy given up is vibrated in a 

 minute or two across the district where it was gathered during so many ages, 

 ultimately to penetrate as light the remotest regions of space. 



Explanation of Tables. 



The following Tables exhibit the principal numerical data regarding the Mechanical Energies of 

 the Solar System. 



In Table I., the mass of the Earth is estimated on the assumption that its mean density is five 

 times that of water, and the other masses are shown in their true proportions to that of the Earth, 

 according to data which Professor Piazzi Smyth has kindly communicated 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 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 January 1852, in a paper " On 

 the Sources Available to Man for the production of Mechanical Effect." These, and the other num- 

 bers in the same column, are the only part of the numerical data either shown in the Tables, or used 

 directly or indirectly in the reasoning on which the present theory is founded, that can possibly re- 

 quire 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 possibility that it can be so far from the truth), the general reasoning by which the 

 Theory of Solar Heat at present communicated 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 ^ 00 - or 5 i ? of its own value. 



Table I. Forces and Motions in the Solar System. 









Forces of attraction 







Masses in pounds. 



Distances from the Sun's 



towards the Sun. in 



Velocities, in miles 







centre, in miles. 



terrestrial pounds. 



per second. 



Sun, 



4,230,000,000 x 10 2J 



(surface) 441,000 



28-61 per lb. of matter 



(equator) 127 



Imaginary solid 



I 









planet close to 



\ 1 x 20 2i 



441,000 



286,100 xlO 17 



277 



the Sun, . 



J 









Mercury, . 



870 x 10 21 



36,800,000 



35,710 x 10 17 



30-36 



Venus, . 



10,530 xlO- 1 



68,700,000 



124,200 x 10 17 



22-22 



Earth, . . . 



11,920 xlO 21 



95,000,000 



73,490 x 10 17 



18-89 



Mars, . . . 



1,579 x 10 21 



144,800,000 



4,211 x 10 17 



15-28 



Jupiter, 



4,037,000 x 10- 1 



494,300,000 



919,400 x 10 17 



8-28 



Saturn, 



1,208.000x10-' 



906,200,000 



81,855 x 10 17 



6-11 



Uranus, 



201,490 x 10 21 



1,822,000,000 



3.377 x 10 17 



4-31 



Neptune, . 



236,380 x 10- 1 



2,854,000,000 



1,615 x 10 17 



3-44 







Distances from Earth's 



Attraction towards Earth 



Velocities relatively to 







centre. 



in terrestrial pounds. 



Earth's centre, in miles. 



Moon, , . . 



136 x 10 21 



237,000 



378 xlO 17 



0-615 



j Earth's equator. 





3,956 



1 per lb. of matter. 



0-291 



