COMETS. 99 



that this period has regularly decreased in every suc- 

 cessive revolution from 1786 to 1838 ; the diminution 

 amounting in an interval of 52 years to 1'8 days. After 

 a careful consideration of all the planetary perturbations, 

 this remarkable phenomenon has led to the adoption, for the 

 purpose of bringing observation and calculation into harmony, 

 of the not improbable supposition of the existence of a fluid 

 of extreme rarity, or ether, dispersed through space, forming 

 a resisting medium; the resistance lessens the tangential 

 force, and with it the major axis of the comet's orbit. The 

 value of the constant of resistance appears to be somewhat 

 different before and after the perihelion; and this may 

 perhaps be ascribed to the change of form of the small 

 nucleus, or to inequality in the density of the ether in the 

 vicinity of the sun ( 53 ) . These facts, and the investigations 

 to which they have given rise, are amongst the most inte- 

 resting results of modern astronomy. Encke's comet has 

 also led to a more rigorous examination of the mass of 

 Jupiter, so important an element in all calculations of 

 perturbations ; and has still more recently obtained for us the 

 first, although it is only an approximate, determination of a 

 smaller mass for the planet Mercury. 



To this first discovered comet of short period, there was 

 soon added a second planetary comet, having its aphelion 

 beyond the orbit of Jupiter, but within that of Saturn. 

 Biela's comet, discovered in 1826, has a period of revolution 

 of 6.75 years, and its light is still fainter than that of Encke's. 

 The motion of both these comets is direct, or the same as 

 *hat of the planets, whereas Halle/s is opposite or retro- 

 grade. Biela's comet presents the first certain instance of the 

 orbit of a comet intersecting that of the earth ; its path 



