Mr Milne's Prize Essay on Comets. 351 



45° X {27,000 — ^^,}, or nearly 1,215,000 degrees of Fahrenheit, 

 for the quantity of caloric abtracted. Now, Newton, judging from the 

 proximity of the Comet of 1680 to the sun at its perihelion, shews 

 that its temperature ought to be about 2000 times greater than the 

 temperature of iron red hot, or about 9000 times greater than the heat 

 of boiling water; the boiling point of water being 212° of Fahrenheit, 

 the sun communicated to this comet a supply of caloric amounting to 

 1,908,000°. But the loss, which, as we have just seen, must have been 

 sustained by the rarefaction above supposed, amounted to two-thirds of 

 this quantity ; so that the actual influence of the sun, in raising the tem- 

 perature of the comet, will undoubtedly be diminished in the same pro- 

 portion. In a corresponding manner, when the comet retires towards 

 its aphelion, where the heat of the sun becomes so much weakened on 

 account of the distance, the condensation of the nebulous matter form- 

 ing the tail and envelope serves not only to furnish the nucleus with 

 continual supplies from the heat acquired at the perihelion, but even to 

 render the warming influence of the solar i-ays much more efficacious 

 than at a less remote part of the comet's orbit. 



" It appears, then, that the variations of heat and cold, to which 

 comets are exposed in the opposite points of their course, are by no 

 means so great as to be incompatible with the supposition of their being 

 fit abodes for animated beings : and if we recollect the facility with 

 which our own bodies can adapt themselves to great and sudden ex- 

 tremes of temperature, as exemplified by various experiments, we may 

 even conjecture these beings to possess a constitution not very dissi- 

 milar to that of the human species. Individuals, we know, have often 

 allowed themselves to be confined for a considerable time in apartments 

 heated to 260° and 280" of Fahrenheit, without feeling much inconve- 

 nience ; and though we cannot as easily ascertain the extent to which 

 cold may be endured by the human frame, we know that it is frequent- 

 ly exposed, without any injurious effects, to an intensity far surpassing 

 what is necessary for the congelation of mercury *. In order, then, to 

 be capable of sustaining those variations of temperature to which a 

 comet may be subjected, it is not necessary that the constitution of its 

 supposed inhabitants should be very different from the constitution of 

 the beings belonging to the Earth. And when we recollect that these 

 variations proceed in a gradual manner, not by the rapid transitions 



* Gay Liussac inentionsit that natural colxl hiis been ubserved, and there- 

 fore sustained by the human frame, so severe as— 58° of Fahrenheit. Bretp- 

 iter's Journal^ iii. 181. 



