402 COMETS. 



sun, Ihave tried; and the heat ofredhot iron, (if 

 my conjecture is right,) is 3 or 4 times greater 

 than the heat of boiling water. And, therefore, 

 the heat which dry earth, on the Comet, while in 

 its perihelion, might have concord from the rays 

 of the sun, was about 2000 times greater than the 

 heat of redhot iron. But by so fierce a heat, 

 vapors and exhalations, and every volatile mat- 

 ter, must have been consumed and dissipated. 



This Comet, therefore, must have conceived 

 an immense heat from the sun, and retained 

 that heat for an exceeding long time ; for a 

 globe of iron, of an inch in diameter, exposed 

 redhot to the open air, will scarcely lose all its 

 heat in an hour's time; but a greater globe 

 would retain its heat longer, in the proportion 

 of its diameter, because the surface (in propor- 

 tion to which it is cooled by the contact of the 

 ambient air,) is in that proportion less, in res- 

 pect of the quanity of the included hot matter ; 

 and, therefore, a globe of redhot iron, equal to 

 our earth, that is, about 40,000,000 feet in dia- 

 meter, would scarcely cool in an equal number 

 of days, or in above 50,000 years."* 



So far, however, from subscribing to the 

 opinions that were entertained by Sir Isaac 

 Newton, that Comets are compact and so- 

 lid, fixt and durable bodies, like what he 

 supposes the planets to be ; I conceive that 

 they are composed of the rarest materials 



*Davis's Translation of Sir I.Newton's Principia, vol. 2. p. 285* 



