58 REVIEWS—DONATI'S COMET. 
ellipse, and confidently predicted its return at a stated period, nothing 
was wanting to assign to the wanderers their character as material 
bodies subject to the power of gravitation like common things. Yet 
though they were thus seen to be under the influence of Father Sol, 
and to obey the same laws as his recognised children the planets, and 
though some of them even conform so far as to move about him in 
re-entering orbits, there are peculiarities which prove them to be for- 
eign to this family, alike in their origin and persistence. For while 
all the members of the planetary system move around the sun nearly 
in the same plane, all in ellipses of small eccentricity or nearly circles, 
and all in the same direction, comets on the contrary violate all these 
laws ; their orbits are of all sorts of eccentricity, most of them para- 
bolas, some even hyperbolas; their planes are inclined at all angles to 
the ecliptic, and their motion is frequently retrograde instead of direct. 
Laplace has calculated the mean position of the orbital planes of a 
great number, and has found it to lie with reference to the ecliptic at the 
angle at which it ought to lie, if there be no determining cause to one 
position rather than another. An acute remark of Herschel’s may 
here be noted in connection with the fact that the periodic comets 
have mostly their motion direct, and their planes not widely different 
from the ecliptic, a result which ought to be expected, for a comet in a 
parabolic orbit near the ecliptic, if its motion were direct, would be 
likely to be thrown by the disturbing action of the planets into an 
elliptic one, while if it were retrograde, the orbit would be converted 
into a hyperbola, and the comet would pass away from our system 
never to return to it. We may then consider a comet to be a body 
moving in the extra-planetary spaces in a straight line with uniform 
velocity, till its path approaches near enough to the sun to be sensibly 
affected by his attraction, and the comet then obeying the principle of 
universal gravitation is drawn into a conic, the nature of which de- 
pends on its initial velocity, in some cases merely passing once round 
the sun and again going off into space, in others moving round him in 
a periodic orbit which may or may not be permanent according as its 
motion takes it into the neighbourhood of other bodies which disturb 
it from this orbit, and perhaps (as we shall see presently) there may 
be a something in the commonly-called “free”? space itself which 
has an effect. The number of comets whose observation has been re- 
corded amounts to nearly a thousand, and in recent times the average 
is said to be about five a year, but this is only a small portion of the 
