192 WILLIAMSON ON COMETS. 



4. Are the comets also habitable ? This brings us to consider the 

 circumstances of that luminous appearance called the tail of the comet. 

 Is it a fiery vapour discharged from the comet when ignited by the heat 

 of the sun? Or is it part of the atmosphere of the comet thrown behind 

 the nucleus, as he approaches the sun, lest his inhabitants should suffer 

 by the heat of too great an incumbent atmosphere ? The latter appears 

 to be the case, as we shall endeavour to prove. 



Perhaps it may be advisable, before we attempt to establish this opi- 

 nion, to show that the opinion that formerly prevailed cannot possibly 

 be correct. When I speak of comets I shall chiefly refer to that of 

 1680, because it was very remarkable, and its period supposed to be 

 about five hundred years. That comet came so near the sun that, 

 according to the received opinion of heat being inversely as the square 

 of the distance from the sun, the comet, in perihelio, had acquired a 

 degree of heat that was two thousand times hotter than redhot iron. 

 But it has been computed that a globe of redhot iron, large as this 

 earth, would require fifty thousand years to cool ; therefore, if the comet, 

 which was as large as the earth, had cooled slowly as iron, it would have 

 required one hundred millions of years to cool ; but though it had cooled 

 two hundred times faster than iron, it would have been five hundred 

 thousand years in cooling. Now it is certain that a comet, heated as 

 above, must have retained a glowing heat, and, like a star, would have 

 been visible to the naked eye, during the whole of its circuit: but we 

 are assured, that the comet referred to disappeared within three months, 

 although it was in a situation where it should have been seen, if lumi- 

 nous. Another fact respecting this very comet demands our attention. 

 On its approach towards the sun, it exhibited a tail some millions of 

 miles long, while it was yet above one hundred millions of miles from 

 the sun. That tail could not have been a flame of fire caused by the 

 heat of the nucleus. 



