RADIANT HEAT. 381 



I. The supposition of the sun being simply a body intensely heated 

 and losing its heat by radiation or simple cooling. 



This he considers quite untenable, as well on theoretical grounds 

 advanced in some other papers, as on the simple consideration that 

 if this were true the sun would be extinguished in a very short time. 



II. The hypothesis of chemical action or combustion of any kind. 

 Supposing one of the combining bodies to be supplied from any 



■atmosphere, the products of combustion would be so enormous as to 

 choke the fire, if gaseous, by preventing the access of the air in 

 question, or, if solid or liquid, by preventing the supply of fuel; and 

 according to the mechanical theory before mentioned, a numerical 

 calculation shows that the whole mass of the sun could scarcely last 

 8,000 years without being all consumed, if generating, by its own 

 burning, the heat which is actually emitted. Hence if the sun is a 

 fire, the fuel must be supplied from external space. But a mass of 

 coal or iron or potassium could not reach the sun from external space 

 without generating thousands of times as much heat from its motion 

 as it could possibly do by its combustion. Combustion is probably, 

 therefore, insignificant, if it exists at all, as a source of solar heat. 



III. The hypothesis of meteors falling into the sun and expending 

 force mechanically has been started by Mr. Waterston*, who supposes 

 such bodies to be attracted and fall directly into the sun from remote 

 extra-planetary regions. 



The supply of meteoric matter necessary according to this theory 

 is estimated to amount to such a mass as would cover the sun's surface 

 to a depth of thirty feet in one year. 



The author, however, considers it probable that meteors actually 

 fall into the sun, not directly from distant spaces, but by the action 

 of a resisting medium surrounding the sun, which contracts the orbits 

 in which they are revolving round him. He conceives that these 

 meteors must be moving within the limits of the earth's orbit, or we 

 should be continually struck by them, and that they are probably the 

 matter of the zodiacal light. 



It is, however, quite conceivable that that cloud of small planetary 

 masses may once have extended beyond the limits of the earth' s orbit, 

 and thus in' remote periods the earth may have been exposed to such 

 falls of them as to have materially raised its temperature; and hence 

 a possible source of those high temperatures which once existed. 



But to return to the effects of the resisting medium. Owing to its 

 retardation, the approach of these bodies to the sun is gradual, and 

 on this hypothesis a calculation similar to the former would give the 

 result that the sun must be covered to a depth of sixty feet in a year, 

 or a mile in about eighty-eight years, which would occasion an in- 

 crease of 1" apparent diameter in 40,000 years — a change, of course, 

 utterly inappreciable to observation. 



The amount of matter thus abstracted by the sun would be equal 

 to the mass of the earth in about forty-seven years; but it is quite 

 conceivable that a quantity of one hundred times this amount would 



'British Association, Hull Meeting, 1S53. 



