896 Transactions. — Astronomy. 



shower of meteorites, the other to the gradual contraction of 

 the sun's mass. To appreciate the meteoric theory we must 

 remember that whenever the motion of a body is destroyed, and 

 no other motion set up in its place, heat is evolved ; thus, when 

 I bring this hammer on this piece of lead its motion is stopped, 

 and the lead thereby becomes hot. (Exp. with thermopile.) 



The heat thus generated is proportional to the mass of the 

 moving body and the square of its velocity, and is so great that 

 if the earth fell into the sun, the heat generated would be equal 

 to that obtained from the combustion of 5,600 worlds of solid 

 carbon. There is, therefore, nothing improbable about this 

 meteoric theory, and its supporters go so far as to point to the 

 zodiacal light as material evidence of it, saying that this light 

 is emitted by a vast meteoric cloud. The adequacy of this 

 theory, as regards the possible supply of heat, is well brought 

 out in the following table, which is due to Sir W. Thomson : — 



Mercury 



Venus 



Earth 



Mars 



Jupiter 



Saturn 



Uranus 



Neptune 



Total . . 



It is, however, very doubtful whether there is any such 

 supply of meteoric matter as is required by this hypothesis. 

 The earth encounters but little, and there is no valid reason to 

 suppose that the zodiacal light results from meteoric matter. 



The second hypothesis, which is due to Helmholtz, refers 

 the sun's heat to the simple contraction of its mass ; and, in 

 order to show the sufficiency of this theory, it has been calcu- 

 lated that the contraction of the sun from a nebula the size of 

 the orbit of Neptune to its present bulk would yield a sufficient 

 heat to maintain the present rate of radiation for 120,000,000 

 years, while a contraction of the sun's diameter of about 300 

 feet per annum would make up the yearly loss. The chief 

 obstacle to the acceptance of this explanation of the origin of 

 the sun's heat is the fact that the heat due to contraction would 

 be set free throughout the sun's mass, and that it is almost 

 impossible to imagine it reaching the surface in time to prevent 

 the sun's surface from becoming cold. 



Either of the two hypotheses which I have briefly put before 

 you account fairly well for the fact that, in the period of a few 

 thousand years during which some sort of written record has 

 been kept, no diminution of the sun's heat has been observed ; 



