302 Notices respecting New Books. 



gravitation, as at present recognised, are altogether insufficient to ac- 

 count for it. " Such a form, as one of equilibrium, is inconceivable 

 without the admission of repulsive as well as of attractive forces. 

 But, if we admit the matter of the tail to be at once repelled by the 

 sun, and attracted by the nucleus, it no longer presents any diffi- 

 culty." 



After some further remarks in corroboration of this view, and 

 citing the opinions of Newton and Kepler, he adds — " Supposing 

 the approach of a comet to the sun to be such as to enable the re- 

 pulsive force to overcome the attractive in those portions of its tail 

 remote from the nucleus, they would, of course, be driven off irreco- 

 verably. But what would now be the state of the remaining mass ? 

 Inertia and repulsion have been subtracted ; of course, what is left 

 has become pro quantitate materia, on both accounts more attractively 

 disposed as a whole ; the dimensions of the orbit must therefore 

 contract. The periodic time will diminish. We have here presented 

 to us a series of consequences, identical in some of their leading 

 features with those which, observed in the case of Encke's comet, 

 have been attributed to resistance of the sether. Pursued into all 

 their consequences no doubt the two explanations diverge. . . . But 

 it may be doubted whether observation of a body so ill- defined is yet 

 precise enough to decide between them." — P. 410. 



We can only make room for two further extracts : — 



" The preservation of the geometrical form of the envelope of our 

 comet is undoubtedly indicative of a high degree of tranquillity, the 

 definite action of perfectly regular forces, and at least a near approach 

 to a state of equilibrium in the 3trata of which the mass consists. It 

 proves also, if taken in conjunction with what has been said respect- 

 ing its apparent dilatation, similarity of external and internal strata, 

 precisely such as the laws of equilibrium would lead us to expect in 

 an elastic fluid mass subjected to such forces. 



•' The rapid disappearance of the coma would seem to be referable 

 neither to dissipation nor absorption into the head, but rather to its 

 being swept off into the tail by the sun's action, and there deposited 



as part thereof The ray or tail of the nucleus is also a highly 



instructive phenomenon. As the envelope dilated and grew fainter, 

 this, on the contrary, while also dilating at the same rate, grew more 

 intense up to the 1st of February, after which time it faded, while 

 the nucleus with its coma rapidly increased in comparative bright- 

 ness. It seems hardly possible not to recognise in these changes the 

 effect of the gradual deposition of the matter of the envelope ; 

 choosing the line of its axis as the course of the deposited particles 

 in their progress to rejoin the nucleus." — P. 411. 



From the above extracts the reader will be enabled to form an 

 opinion of the manner in which this very interesting portion of the 

 work is treated. At the next return of the comet, it will be regarded 

 as a fortunate event for astronomy that the time of Sir John Her- 

 schel's sojourn at the Cape embraced that of its last appearance, and 

 that the most powerful optical means then in existence, in the hands 

 of the first astronomer and philosopher of the age, were applied to the 



