ON COMETS. 137 



ances. Of course I do not mean to deny that that very 

 minute brilliant point which some are said to have ex- 

 hibited, may not be a solid body but it must be a very- 

 small one perhaps not a tenth or a hundredth part the 

 size of the moon ; and, indeed, if there be not some little 

 solid mass, it seems impossible to conceive how the ob- 

 servations of a loose bundle of smoke, rolling and career- 

 ing about, could ever be represented by any calculation. 

 Certain it is, that what appears to be the central point of 

 a comet, is that point (and no other is) which conforms 

 rigorously to the laws of solar gravitation, and moves 

 strictly in a parabolic or elliptic orbit. 



(51.) There is a very curious feature common to all the 

 comets which have little or no tail, and which circulate . 

 about the sun in short periods ; such as that of Encke, 

 in which it has been especially observed. As they ap- 

 proach the sun, so far from dilating in size, they con- 

 tract, I mean in their real bulk, orat least their visible 

 bulk, and on receding from the sun they grow again to 

 their former size. The only possible explanation of 

 this is, that a portion of their substance is evaporated by 

 the heat that is to say > converted from the state of fog 

 or cloud into that of invisible transparent vapour. Per- 

 haps I ought to explain what is the difference. Take 

 the case of a light cloud in a clear sky when the sun 

 shines on it. If you watch it attentively, you will very 

 often see it grow thinner and thinner, and at last dis- 

 appear altogether. It has been converted from mist to 

 invisible vapour. The material substance, the watery 

 particles are there, but they have passed into another 



