198 PROF. O. REYNOLDS ON COMETARY PHENOMENA. 



comets; and it is an explanation of the facts: — (i) that 

 whilst there are apparently a countless number of comets 

 which do not return, and, according to the laws of gra- 

 vity, a countless number of these must have been con- 

 verted by the disturbances of the planets into periodic 

 comets, there are only a very few which are known to be 

 periodic ; (2) that the size of the periodic comets has been 

 observed in many instances, if not in all, to decrease; 

 (3) that there are many meteoric stones whose orbits are 

 similar to those of many of the periodic comets, and 

 which do not show cometic appearances, — the assump- 

 tion being that numberless comets have been disturbed 

 in their paths through space, and, instead of having been 

 sent back in a parabolic orbit, have, owing to a second 

 disturbance by one of our planets (generally Jupiter or 

 Uranus) been attached to our system, and, for a time, have 

 appeared as periodic comets such as those of Halley, but 

 that they gradually lost their softer materials, becoming 

 less and less, until they finally ceased to be comets, and 

 became meteoric stones. 



The rate of evaporation on such a body as a comet 

 would obviously increase as the comet approached the sun, 

 and diminish as it receded ; but it would not depend solely 

 on the distance of the bodies from each other; for the 

 materials of the comet would take time to heat, and con- 

 sequently, as it was approaching, part of the heat would go 

 to warming the body of the comet, and for any position the 

 evaporation would be less than the sun would cause if the 

 comet were stationary. As it left the sun it would be the 

 other way ; that is, the evaporation would be more than 

 the position warranted. Thus the greatest rate of evapo- 

 ration would not be exactly at the time when the comet 

 was nearest the sun, but some time after it had passed its 

 perihelion. Now this lagging (as it may be called) of the 

 sun's action on the comet is similar to, and consequently 



