180 
MOVING FORCE. 
Cases of diffi- applied to the nieasuie of force composed of the pressure and 
doc\^rincs*o^ according to that measure the quan- 
nio\ing force, tity of force communicated will be always the same, whether 
it be applied at G, D, or at any other point in A B. The pro- 
gressiva velocity generated in G, will, no doubt, be the same, 
at whichever of these points the force is comnaunicated ; that 
is, the product of the mass into its velocity in the same direc- 
tion will, in this case, as in all others, be as the product of the 
pressure into the time of its acting } and according to that mea- 
sure, the whole effect of the force communicated is found in 
the progressive motion of the mass, the rotatory motion ap- 
pearing to be produced without force. The explanation most 
. commonly given of this inconsistency is, that the rotatory 
motion, consisting of equal quantities of motion in opposite 
directions, balances itself j but can it be shown, that equal 
quantities of motion in opposite directions may be produced 
without force : Such is not the doctrine of Sir Isaac Newton ; 
he certainly understood rotatory motion, as well as rectilinear 
motion, to be a measurable effect of force. M. de Prony at- 
tempts to explain this difficulty, in the application of the pre- 
vailing measure of moving force, as follows: Puisque nous 
savons que lorsque la resultante des quantites de m’ouvement 
imprimees passe par le centre de gravite d’un corps, ce corps, 
abandonne a Taction des moteurs, n’a aucun mouvement de 
rotation, il faut en conclure, que le mouvement de rotation n’a 
lieu que lorsque la resultante des quantites de mouvement im- 
primees passe hors du centre de gravite. Ensuite, comme le 
mouvement de ce centre est le meme, soit que la resultante y 
passe on n’y passe pas, e’est done autour du point ou il est place 
pue se fait la lotation, quand il y en a, puisque ce point est le 
seul qui ne parlicipe pas i ceite rotation. Il suit de la que 
le mouvement de translation est absoluement independant du 
mouvement de rotation, puisqu’il est independant de la cause 
qui le produit, savoir, la direction de la resultante par un autre 
point que le centre de gravite*.” 
• Arch. Hydr. p. I7(i. 
But 
