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 Harth, 
according to data which Professor Prazz1 Smyru 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. Povrrzet’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 Jouxr’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 335 or 3}, of its own value. 
TABLE J. Forces and Motrons in the SOLAR SYSTEM. 
oie Forces of attraction ra 
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"! (surface) 441,000 | 28-61 per lb, of matter| (equator) 1:27 
Imaginary solid 
planet close to| 1 x 207! 441,000 286,100 x 10% 277 
the Sun, . 
Mercury, . . | 870 x 107 36,800,000 35,710 x 10% 30°36 
Venase chain 10,530 x 107! 68,700,000 124,200 x 10” 22:22 
Pharthsawins a 11,920 x 107! 95,000,000 73,490 x 10" 18°89 
EATS ES a's anaes 1,579 x 10?! | 144,800,000 4,211 x 10” 15:28 
Jupiter, . . | 4,037,000 x 10"! | 494,300,000 919,400 x 10” 8-28 
Saturn, . .| 1,208,000 x 10°! | 906,200,000 | 81,855 x 10” 611 
Uranus; 35); 201,490 x 107! 1,822,000,000 3.877 x 10! 4:31 
Neptune, . . 236,380 x 10"! | 2,854,000,000 1,615 x 10" 3°44 
Distances from Harth’s | Attraction towards Earth | Velocities relatively to 
centre. in terrestrial pounds. | Harth’s centre, in miles. 
Moon, . . 136 x 107! 237,000 378 x 10% 0-615 
Earth’s equator, 3,956 1 per lb, of matter. 0-291 
