Moving Force. 159 
grande que les filets ont de se dévier aux 
approches de cette surface; mais si la veine 
rencontre une surface plus grande qu'elle, qui 
Yoblige & changer en entier la direction de 
tous ses filets, la vitesse perdue, étant par 1a 
augmentée, la resistance devient beaucoup 
plus grande.’’* 
But in these experiments, a part only of the 
vein strikes the surface opposed to it, and the 
force of that part appears to be equal to the 
force assigned by the theory to the whole vein. 
Of all theoretical propositions, that which 
was first demonstrated by Daniel Bernoulli 
in his Hydrodynamics, page 290, and after- 
wards more fully by the same author, in the 
Comment. Petropol. vol. 8. page 120, appears 
to be the most applicable to Mr. Smeaton’s 
cases, and comes the nearest to his results. 
It is, that, when the force of an insulated vein 
of water is directed perpendicularly against 
a plane indefinitely large, its pressure against 
the plane is equal to the weight of a column of 
water, of which the base is equal to the area 
of the section of the vein, and the height 
equal to twice the height due to the velocity 
of the vein. But the circumstances of this 
case are not quite the same as those of Mr. 
Smeaton, and he found the pressure against 
* Principes d@’hydrau!, rol. 2. p. 150. 
