PERIODIC COMETS. 393 



PERIODIC COMETS. 





O.v another occasion, I gave at some length the history of Halley's comet, 

 by far the most interesting of all the periodic comets yet discovered. I shall 

 now bring under your notice the remaining bodies of this class. 



A periodic comet, as the name implies, is one which is known to return at 

 regular intervals to our system, and whose reappearance in the heavens can 

 therefore be predicted. The paths of these bodies round the sun are eccentric 

 ellipses, having the centre of the sun in one of their foci. / 



ENCKE'S COMET. 



In the year 1818, a comet was observed at Marseilles, on the 26th of No- 

 vember, by M. Pons. In the following January, its path being calculated, M. 

 Arago immediately recognised it as identical with one which had appeared in 

 1805. Subsequently, M. Encke of Berlin succeeded in calculating its entire 

 orbit inferring the invisible from the visible part and found that its period 

 round the sun was about twelve hundred days. This calculation was verified 

 by the fact of its return in 1822, since which time the comet has gone by the 

 name of Encke's comet, and returned regularly. 



This comet exhibited the appearance of a mass of nebulous vapor, so trans- 

 parent, even at its centre, that stars can be seen through it. It is round, or 

 rather oval, in its form, and is too attenuated and feeble in its light to be dis- 

 covered without the aid of a telescope. The annexed figure, 1, is that which is 

 usually given as a representation of its telescopic appearance. 



The orbit of Encke's comet is an oval, whose length is about double its 

 breadth. At its nearest approach to the sun, the distance of the comet is about 

 thirty-four millions of miles, which is about the distance of the planet Mercury 

 When most remote from the sun, its distance is about four hundred and forty- 

 three millions of miles, which is nearly four and a half times the earth's dis- 

 tance, and is little less than the distance of Jupiter. The orbit is inclined to 



