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LXIV. On the Laws of Motion. By Professor Tait. 
To the Editors of the Philosophical Magazine and Journal. 
Gentlemen, 
I DO not think that any useful purpose could be served by 
my entering upon a discussion with Mr. Browne, as his 
views are so entirely different from mine. 
But I must request you kindly to afford me an opportunity 
of letting your readers know precisely what are those views of 
mine which Mr. Browne regards as dangerous. 
They will see from the enclosed abstract of my paper that 
all I propose is to dispense with the sense-suggested idea of 
Force, and to introduce in its place the objective reality Energy. 
I think your readers will probably consider Mr. Browne to 
be a much more sweeping and dangerous innovator than my- 
self; for while we both acknowledge Time and Space, I main- 
tain the objectivity alike of Matter and of Energy, and base 
my system on them ; but Mr. Browne practically throws them 
both overboard, and constructs his physical universe by means 
of Force alone. Yours truly, 
College, Edinburgh, P. G. TAIT. 
November 10th, 1883. 
On the Laws of Motion*. 
The one objection to which, in modern times, Newton's 
wonderfully complete and compact system (the Axiomata sive 
Leges Motus) is liable, is that it is expressly founded on the 
conception of what is now called ' force " as an agent which 
" compels " a change of the state of rest or motion of a body. 
This is part of the first law; and the second law is merely a 
definite statement of the amount of change produced by a 
given force. 
There can be no doubt that the proper use of the terra force 
in modern science is that which is implied in the statement — 
Force is whatever changes a body's state of rest or motion. 
This is part of the first law of motion. Thus we see that force 
is the English equivalent of Newton's term vis impressa. But 
it is also manifest that, on many occasiors, but only where his 
meaning admitted of no doubt, Newton omitted the word im- 
pressa and used vis alone, in the proper sense of force. In 
other cases he omitted the word impressa, as being implied 
in some other adjective such as centripeta, gravitans, &c, 
* Communicated, by the Author, as an Abstract of a Paper read before 
the Royal Society of Edinburgh, December 18, 1882. 
