ON COMETS. 119 



of their final rencontre, which took place in 1779, it is 

 clear they could not have been far asunder in 1767, 3 

 years before it became visible ; and in fact, on executing 

 the calculations necessary, it was clearly proved that 

 before 1767 this unhappy comet had been revolving in 

 a totally different orbit of much greater dimensions, and 

 was actually siezed upon then and there by Jupiter, 

 flung as it were inwards and then after making two 

 visits to the sun, again seized on, and thrown off into 

 space, into an orbit of 20 years' period, where perhaps 

 it may be quietly circulating to this day. Jupiter, in 

 fact, is a regular stumbling-block in the way of comets. 



(32.) This is a strange history but it proved a very 

 instructive one. The comet passed, as I have said, 

 through the system of Jupiter's satellites. Now the 

 motions of these bodies have been studied with a degree 

 of care and precision quite remarkable by reason of their 

 furnishing one of the means for ascertaining the longi- 

 tudes of places. And if the comet had been a heavy 

 massive body, its attraction must have produced some 

 sensible disturbance in their motions. But no, not a 

 trace of anything of the kind was detected. One and 

 all of them pursued their courses with the very same 

 precision and regularity as if nothing had happened. 

 The conclusion is irresistible. That comet at least had 

 no sensible weight or mass it was a mere bunch of 

 vapours. 



(33.) Another very remarkable periodical comet is that 

 of Encke, which makes its circuit about the sun in 1200 

 days, or about 3 years and 4 months, in the same 



