REVIEWS. 
295 
attraction than in the case of light, where it is known certainly not to be 
evanescent. All we assert of gravity is that its action certainly travels with 
a velocity enormously exceeding that of light. This is known from phe- 
nomena. What its velocity is we have no means of determining, simply 
because the motions of the heavenly bodies show as yet no signs of the 
finiteness of the velocity with which the action of gravity is transmitted. 
This velocity is therefore practically infinite, that is, it is infinite in the 
same sense in which the velocity of light is infinite in ordinary terrestrial 
observations. 
“ Potential energy ” is an expression really belonging to modern science ; 
but it is not what Mr. Preston supposes. It is in one sense a form of energy 
depending on position, precisely as the tension of an elastic string connecting 
two bodies depends on the position of the bodies ; but it can no more be 
said to depend on position only than can the tension of such a string. We do 
not know what it actually depends on besides position, for we do not 
know how gravity is caused ; and we do know what the tension of the 
string depends on, for we can see and examine the string. That is the 
chief distinction between the two cases. No man of science, no one in fact 
11 with a competent faculty of thinking,” has ever supposed that in one case 
more than in the other mere difference of position was a form of energy. 
It will be manifest from what we have said about Mr. Preston’s work, 
that whatever value his own ideas respecting the ether may have, his cor- 
rections of the views he supposes to be accepted by men of science have no 
value whatever. He says “ the present work is the result of much thought 
and careful study.” He may have given much thought to it, but can scarcely 
have studied his subject very carefully. It is hardly to be expected that 
in a matter of such transcendent difficulty as he has selected for the subject 
of reflection, much can be accomplished without the most thorough investi- 
gation of at least all the verified phenomena, and no slight study of the re- 
searches and thoughts of the leading men of science who have dealt with 
the matter. We have seen how very little Mr. Preston appears to know 
about the theories of men of science. As to observed phenomena, his know- 
ledge is very unequal. In some cases the facts he depends upon are cor- 
rectly stated, in others they are doubtful, in others they are absolutely in- 
correct. As an instance of the last-named kind, we find at p. 130 the state- 
ment that the rate of transmission of electricity in materials of the greatest 
diversity is appreciably uniform — a circumstance which electricians will be a 
good deal surprised to learn. 
We have given more space to Mr. Preston’s work than its intrinsic value 
merits, because the erroneous ideas which he entertains respecting the 
accepted views of science about gravity, electricity, potential energy, and so 
forth, are very common. It is worth while to indicate, even at some expendi- 
ture of space, the views really entertained by men of science on this subject. 
