COMETS OF SHORT PERIOD. 189 



half hours in the first revolution, five in the next, seven 

 and a half in the next, and so on it is the sum of all 

 these gains (and not the gain made in the last revolution) 

 which expresses the total gain of the comet in point of 

 time. Hence the last perihelion passage occurred 

 twenty-nine days before the time at which it would 

 have taken place, but for some unknown cause which 

 has interfered with the comet's motion. What that 

 cause may be, has not yet been certainly determined ; 

 but it is at least highly probable that Encke has as- 

 signed the true cause in suggesting that so light a 

 substance as the comet may be retarded in its passage 

 through the interplanetary spaces by the existence of 

 ' a thin ethereal medium,' incapable of perceptibly 

 retarding the motion of the planets. 



At first sight, it may seem strange that we should 

 speak of the acceleration of the comet as being caused 

 by the retarding influence of such a medium as has 

 been conceived to occupy the interplanetary spaces. 

 Yet it is strictly the case that, if a planet or comet be 

 continually checked in its onward course, its velocity 

 will continually grow greater and greater. For instance, 

 if our earth were so checked, it would move in a spiral 

 which would gradually bring its orbit to that of Venus, 

 by which time its motion would be as rapid as that of 

 Venus (which moves one-third faster than the earth) ; 

 then it would continue revolving in a spiral fill it 

 reached the orbit of Mercury, when it would be moving 

 as fast as this the swiftest of all the planets. And so 



