416 Professor W. Thomson on the 



The amount of matter, drawn by the sun in any time from 

 surrounding space, would be such as in 47^^ years to amount to 

 a mass equal to that of the earth. Now there is no reason what- 

 ever to suppose that 100 times the earth^s mass drawn in to 

 the sun, would be missed from the zodiacal light (or from meteors 

 revolving inside the orbit of Mercury, whether visible as the 

 " zodiacal light '' or not) ; and we may conclude that there is no 

 difficulty whatever in accounting for a constancy of solar heat 

 during 5000 years of time past or to come. Even physical 

 astronomy can raise no objection by showing that the sun's mass 

 has not experienced such an augmentation ; for according to the 

 form of the gravitation theory which I have proposed, the added 

 matter is drawn from a space where it acts on the planets with 

 very nearly the same forces as when incorporated in the sun. 

 This form of the gravitation theory then, which may be proved 

 to require a greater mass of meteoric matter to produce the solar 

 heat than would be required on any other assumption that could 

 be made regarding the previous positions and motions of the 

 meteors, requires not more than it is perfectly possible does fall 

 in to the sun. Hence 1 think we may. regard the adequacy of 

 the meteoric theory to be fully established. 



Let us now consider how much chemical action would be re- 

 quired to produce the same effects, with a view both to test the 

 adequacy of the theory that the sun is merely a burning mass 

 without a supply of either fuel or dynamical energy from without, 

 and to ascertain the extent to which, in the third theory, the 

 combustion of meteors may contribute, along with their dynamical 

 energies, to the supply of solar heat. Taking the former 

 estimate, 2781 thermal units Centigrade, or 3,869,000 foot-lbs. 

 as the rate per second of emission of energy from a square foot 

 of the sun's surface, equivalent to 7000 horse-power, we find 

 that more than '42 of a lb. of coal per second, or 1500 lbs. per 

 hour would be required to produce heat at the same rate. Now 

 if all the fires of the whole Baltic fleet were heaped up and kept 

 in full combustion, over one or two square yards of surface, and 

 if the surface of a globe all round had every square yard so 

 occupied, where could a sufficient supply of air come frora to 

 sustain the combustion ? yet such is the condition we must 

 suppose the sun to be in, according to the hypothesis now under 

 consideration, at least if one of the combining elements be oxygen 

 or any other gas drawn from the surrounding atmosphere. If 

 ..the products of combustion were gaseous, they would in rising 

 check the necessary supply of fresh air ; or if they be solid or 



in appearance and brightness, but was always of a " ruddy " colour, may 

 have not experienced meteoric impact enough to make its surface more 

 than red-hot. 



