168 
MOVIN6 FORCE. 
Cases of diffi- 
culty in the 
doctrines of 
Bjoving force. 
ously untenable. For, when water is moving upon a horizontal 
plane, we cannot doubt that the plane must support the whole 4 j| 
weight of the water. It is never supposed, that a ball loses a Jdj 
part of its weight by rolling upon a horizontal plane, excepting > 
indeed the amount of its centrifugal force from the centre of d 
the earth 5 but that exception does not apply to the case il 
in question, for the centrifugal force, whatever it is, must, 
according to M. du Boat’s theory, be added to the non-pressure. i 
In confirmation of his theory of non pressures, M. du Buat !|| 
observes, “ Qu’ayant fait mouvoir, i une certaine profondear, | 
dans une eau stagnante, un tube vertical ouv^^rt par les deux ; 
bouts, dont le superieur etoit hors de I’eau, le fluide s’est main- 
tenuedans le tube, plus bas que la superficie du reservoir, d’une 
quantite ^-peu-pres egale a la hauteur due a la vitesse avec la- 
quelle il etoit mu*.” But he has omitted to take into conside- 
ration the cohesion or the lateral action of the particles of the 
water upon each other, which has since been so well observed 
by M. Venturi 5 from whose experiments, and from those of 
Dr, Matthew Youngf, made under the recover of an air-pump, 
we may safely conclude, that, were it not for the pressure of 
the atmosphere, and the cohesion of the particles, there could 
be no depression in the tube, as observed by M. du Buat : and, 
had he been aware of these circumstances, he surely would 
never have reasoned as he has done on the subject of non- 
pressures. But to return to the subject of water-wheels. 
It has been attempted to be theoretically demonstrated by 
M.de Borda, and afterwards by Mr. Waring, of America, that 
the force of the water against the wheel is not proportional to 
the square of the velocity with which it strikes the wheel, but I 
that it is in the simple ratio of that velocity ; and that the max- 
imum effect is therefore produced when the velocity of th» 
wheel is half that of the stream. 
M. de Borda, in reference to the labours of others, says. 
On ne consideroit qu'une seule palette contre laquelle on 
• Principcs d'hydraul. Vol. 2. p. t.'Se. 
+ Irish Phil. Trans. Vol. 7. p.63. 
«herchoit 
