440 Prof. Tait on the Laws of Motion. 
which he employed to qualify the word vis. That this is the 
true state of the case is made absolutely certain by the fol- 
lowing : — 
Definitio IV. Vis impressa est actio in corpus exercita, ad 
mutandum 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 hsec vis in actione sola, neque post actionem per- 
manet in corpore. Perseverat enim corpus in statu omni novo 
per solam vim inertias 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 uniformly forward in a right line.'''' 
" 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 inertia; only. 
Impressed forces are of different origins ; as from percussion, 
from pressure, from centripetal force. " 
The difficulty which Motte here makes for himself, and 
which he escapes from only by leaving part of the passage in 
the original Latin, is introduced solely by his use of the word 
force as the equivalent of the Latin vis. 
We may quote two other passages of Newton bearing defi- 
nitely on this point. 
Definitio III. Material vis insita est potentia resistendi, qua 
co7'pus unumquodque, quantum in se est, perseverat in statu suo 
vel quiescendi vel movendi uniformiter in directum. 
It is perfectly clear that, in this passage, the phrase vis 
insita is one idea, not two, and that vis cannot here be trans- 
lated by force. Yet Motte has 
" The vis insita, or innate force of matter, is " &c. 
Definitio V. Vis centripeta est, qua corpora versus punctum 
aliquod, tanquam ad centrum, undique trahuntur, impelluntur, 
vel utcumque tendunt. 
It is obvious that the qualifying term centripeta here includes 
the idea suggested by impressa, defining in fact the direction 
of the vis, and therefore implying that its origin is outside the 
body. 
After what has just been said, no further comment need be 
added to show the absurdity of the terms accelerating force, 
innate force, impressed force, &c. All of these have arisen 
simply from mistranslation. Vis, by itself, is often used for 
force ; but vis acceleratrix, vis impressa, vis insita, and other 
