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of one gramme, produces a velocity of one centimetre per second. Again, 
the term Erg is used to denote the unit of work, or the work done by a force 
of one dyne acting through a distance of one centimetre. Before leaving 
the volume, it should be remarked that each section is accompanied by a set 
of well-chosen problems. Altogether the work is admirably adapted to the 
wants of teachers who really desire to give their students a solid ground- 
work in the principles of physical science. 
PHYSICS OF THE ETHER.* 
I N this work an attack is made with considerable energy on two giants, 
which unfortunately are but straw-stuffed — the allied theories, as the 
author calls them, of “action at a distance ” and “potential energy.” As 
a mere matter of fact, scientific men do not even use the words “ action at 
a distance ” as the title of any theory ; and what they understand as “ poten- 
tial energy ” is not by any means what our author supposes. Newton’s 
gravity, which our author supposes to be an example of force assumed to 
act at a distance, “without the intervention of material or physical agency,” 
was not so understood by Newton himself, who may be presumed to have 
known what he meant by it. He distinctly pronounced the idea in- 
conceivable, that a body can act on another, or through a vacuum, with- 
out the intervention of anything else by or through which the force may be 
conveyed from one to another. Towards the close of his “ Principia,” he 
says : “ We have explained the phenomena of the heavens, and of our sea, 
by the power of gravity, but have not yet assigned the cause of the power.” 
And then, after showing what may certainly be inferred respecting gravity, 
he proceeds, “ I have not hitherto been able to discover the cause of these 
properties of gravity from phenomena, and I frame no hypotheses.” One 
hypothesis only did he actually consider, the hypothesis namely of action 
at a distance, but only to reject it, as one which no one could for a moment 
entertain “ who has in philosophical matters a competent faculty of think- 
ing.” So, too, of electricity. Our author is quite mistaken in supposing 
that it is regarded by men of science as acting at a distance in his sense 
of the words. Nor again are either gravity or electricity supposed to act 
with infinite rapidity. Mr. Preston says, “ if we take the case of an electro- 
magnet and a piece of iron, then, when the electro-magnet is suddenly put 
in the magnetic condition by the electric current, it is assumed in accord- 
ance with the theory of action at a distance that the agency by which the 
distant piece of iron is put in motion requires no time to pass from the 
magnet to the iron.” Nothing of the sort is assumed. No reference is 
made to the time required in this case, any more than in speaking of a 
luminous object at a short distance we mention the time which the light 
requires to travel from it to the eye, for the time is practically evanescent ; 
but it is no more regarded as absolutely evanescent in the case of magnetic 
* “ Physics of the Ether ” By S. Tolver Preston. 8vo. London : E. & 
F. N. Spon. 1875. 
