of Edinburgh, Session 1882-83. 9 
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. 
(Next comes a digression as to what was Newton's expression for 
what we now mean by the word force, when it is used in the correct 
signification above.) 
There can be no doubt that the proper use of the term 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 occasions, hut only where Ms meaning admitted of no 
doubt j Newton omitted the word impressa 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 centri- 
peta, gravitans, &c., which he employed to qualify the word vis. 
Thus (Lemma X.) he says i—Spatia, quoe corpus urgente qudcunque 
vi finitd describit, &c. It is needless to multiply examples. But 
that this is the true state of the case is made absolutely certain by 
the following: — 
Definitio IV. Vis impressa est actio in corpus exercita., ad mu- 
tandum ejus statum vel quiescendi vel movendi uniformiter in 
directum. 
Contrast this with the various senses in which the word vis is 
used in the comment which immediately follows, viz, : — 
Constitit haec vis in actione sola, neque post actionem permanet 
in corpore. Perseverat enim corpus in statu onini novo per solam 
vim inertise. Est autem vis impressa diversarum originum, ut ex 
ictu, ex pressione, ex vi centripeta. 
These passages are translated by Motte as below : — 
“ Definition IV. An impressed force is an action exerted upon a 
body, in order to change its state, either of rest, or of moving uni- 
formly forward in a right linef^ 
“ This force consists in the action only, and remains no longer in 
the body when the action is over. For a body maintains every new 
state it acquires, by its vis inertioe only. Impressed forces are of 
different origins ; as from percussion, from pressure, from centripetal 
force. ” 
