REVIEWS—DONATIS COMET. 7\ 
sun exerts on the comet a force distinct from that of attraction, capa- 
ble of producing this rotation: that is, a polar force which tends to 
direct one radius of the comet towards the sun, and the opposite ra- 
dius in a contrary direction. Such a force as magnetism for instance, 
Granting the existence of this force, the explanation of the oscillatory 
movement is obvious. Our readers may like to see Bessel’s own 
words :—“Le mouvement oscillatoire de l’aigrette antour du rayon 
vecteur ne peut pas s’expliquer par l’attraction du Soleil, il faut sup- 
poser que le Soleil exerce en outre sur la comete une force de rota- 
tion ; de plus, il est nécessaire que le noyau de la cométe participe 4 
ce mouvement. . . . . . . II est nécessaire d’admettre une 
force polaire que tende a diriger un des rayons de la cométe vers le 
Soleil, et le rayon opposé dans le sens contraire; il n’y a aucune 
raison pour rejeter @ priori une pareille force. Le magnetisme sur la 
Terre nous offre exemple d’ une force analogue, quoiqu’ il ne soit pas 
encore prouvé qu’ elle se rapporte au Soleil [it has been since]; si 
cela €tait, on en pourra voir l’effet dans la précession des équinoxes [?]. 
‘Une fois cette force admise, il est facile d’ expliquer le mouve- 
ment oscillatoire de l’aigrette ; la dureé des oscillations dépend de la 
grandeur de cette force, et leur amplitude d’une constante relative au 
Mouvement initial des molécules.” 
Undoubtedly if the sun exerts any other force than that of attrac- 
tion, it must be a force of the kind Bessel calls “polar,” for the 
action summed throughout the whole system must be zero. The 
analogy drawn by him between this supposed force and the terrestrial 
magnetism seems quite imperfect, nor can we understand how Bessel 
could conceive that the polarity induced in the particles of the earth 
-by magnetism would affect the position of the earth’s axis, so as to 
make itself apparent in the precession of the equinoxes. Certainly we 
ought to be very sure of our facts before we have resort to this ex- 
treme hypothesis, and we have already mentioned that Prof. Bond not 
only did not discover any such oscillation in the *58 comet, but makes 
it tolerably evident that a less searching scrutiny might have led to 
van assertion of its existence. The only rotation which Prof. Bond 
detected—rather by way of inference than direct observation—was 
-that of the nucleus, so as to present the same face always to the sun, 
as the moon does to the earth, which would seem to involve the con- 
-clusion that a rotation had been originally impressed upon or possessed 
by the comet exactly adapted to its orbital motion, which is so wildly 
improbable that we may be glad to escape from it at any price. 
To explain the emission of the particles in a cone towards the sun 
