94 
REVIEWS. 
somewhat farther on, in the book by Mr. Tyndall, of the Royal Institution. 
The subject treated is “ Natural Philosophy.” This gives the writer a wide 
range; and, therefore, as his space is limited, he must choose a few sub¬ 
jects to illustrate his views. We confess we were charmed with the luci¬ 
dity of his style, his clearness of thought, and vivid illustrations. It is 
well for Mr. Hughes that, if mistakes be made in one part of his series, we 
can often find their correction in another. Thus, in Mr. Tyndall’s prefa¬ 
tory remarks on Natural Phenomena, and those agencies called forces, by 
writers on Physics, we get a rational account of what the “ cleavage of 
minerals” means. When common salt is referred to, for the sake of illus¬ 
tration, its composition is correctly given. Of the subjects he selects for 
treatment, we shall not speak much, as they do not properly fall within our 
province; but we cannot refrain from saying, that he has succeeded, to a 
marvellous degree, in making plain and simple that difficult elementary 
notion in dynamics—viz., that the force which exists in a body in motion 
is measured by mass X velocity, or by its momentum, as Newton de¬ 
fined it. We have alluded to the vividness of his illustrations ; we cannot 
omit the following, which is most felicitous. lie is speaking of the impact 
of two equal and perfectly elastic balls, moving in opposite directions, with 
the same velocity:—“If, for example, each ball possessed a velocity of 
nine feet a second before impact, this velocity is not only destroyed in 
each case, but is converted into one in the opposite direction ; and in this 
case it is usual to say that the loss of velocity is doubled. I know that 
this will appear a new notion of loss to many boys, inasmuch as the abso¬ 
lute velocities after impact are the same as before. The case may be illus¬ 
trated by a parliamentary candidate, who reckons on the support of a 
certain voter; if the voter dies, the candidate may be said to lose a vote ; 
but if the voter change his opinion, as our ball has changed its direction, 
and votes for the opposition candidate, then the loss may be said to be 
doubled.” 
A Manual of Marine Zoology for the British Isles. By Philip 
Henry Gosse, A.L.S. Parti. Pp. 204. Price 7s. 6d. London: 
John Van Voorst, Paternoster-row. 1855. 
Britain is a favoured country so far as natural history is concerned. Mo¬ 
nographs on various subjects connected with the natural sciences are con¬ 
tinually pouring from its press, and, with all this, her naturalists are still 
bringing before our notice new forms and new species, plucked from the 
world of chaos, and proving to us how much is still to be done in these 
