Ill 



December was to the distance of the earth from the same point 

 in the ratio of 6 to 1,000, hence he concluded that its temperature 



2 



according to the law of inverse squares would be » -§■- \ 60° = 



6 



1,670,000° (approx.) Again since the distance of the comet from 

 the centre of the Sun was also four solar radii, the tempera- 

 ture of the solar surface would be the temperature of the comet 

 increased in the proportion of 4^ -f. 32 or 2,987,000° Fahr. 

 (approx.) However this value cannot be accepted as at all 

 likely, because the law on which it is founded appears to break 

 down when applied beyond a very hmited range of temperature, 

 and to furnish exaggerated and impossible results. For instance, 

 Bunsen's value for the temperature of a body heated in the oxy- 

 hydrogen flame is 2,800° C. ; but if Newton's law be applied it 

 comes out 45,000° C. 



(6) By Duleng and Petit's Law. 

 Duleng and Petit, in the second decade of the present century, 

 proposed a new empirical law, based on a complete series of 

 experiments concerning the velocity of cooling of a thermometer, 

 both in air and in vacuo. By this new rule temperature increas- 

 ing in arithmetic progression, radiation increases in geometric 

 progression. For instance, according to Newton, the velocity of 

 cooling for a temperature of 200° above an inclosure should be 

 double that for a temperature of 100°, while Duleng and Petit 

 found that it is nearly three times as much. But if Newton erred 

 by exaggerating, Duleng and Petit have equally failed by mini- 

 mizing results. For the French physicists, notably Pouillet and 

 Vicaire, adopting this formula, have deduced for the solar 

 effective temperature valves between 2600° and 3200° Fahr. 

 But this is impossible, for, as we are told, at the focus of 

 Berin^re's great burning-glass, set up in the gardens of the 

 Luxemberg, at the end of the last century, such refractory 

 substances as iron and gold, and even the diamond, were melted, 

 and ran like butter. The principle of the burning-glass is very 

 simple, for it virtually brings the Sun to such a distance from 

 the object placed at the focus, that the apparent diameter of the 

 Sun would be equal to tliat of the glass when viewed from its 

 own focus. The largest burning-glass so far constructed would 

 bring the Sun to a distance from the Earth equal to that of the 

 Moon. It follows then that the temperature of the Sun must be 

 far higher than that of molten gold, and that were he to approach 

 as near to us as the Moon, the whole earth would in a few 

 moments be dissipated in vapour. 



(c) By Rosetti's Law. 



A third law has accordingly been proposed by Professor Eosetti, 

 of Padua, like that of his French predecessors, founded on a 



