of the Motion and Resistance of Fluids. 43 
resistance, and let the sails make one revolution in t seconds ; 
then the velocity will be y feet in a second. 
To find the resistance when the fluid strikes the planes at 
any angle, set them to that angle, and find the resistance in 
the very same manner as before. But here we must set two of 
the opposite planes inclined one way and two the other, so 
that the fluid may strike the two former on their upper sides, 
and the two latter on their under sides, but both at the same 
angle. This caution is necessary in order to prevent any al- 
teration in the pressure, and consequently in the friction upon 
the axis in the direction thereof ; for the fluid striking the 
planes obliquely, part of the force will be employed in resist- 
ing the motion, and part will act perpendicular thereto, or in 
the direction of the axis, and this latter effect will manifestly 
be destroyed by the above disposition of the planes, because 
this force will act upwards against two of the planes, and down- 
wards against the other two, and being equal, they will de- 
stroy each other's effects. The planes may be set to any angle 
thus: Take a small quadrant divided into degrees; let mn 
(Tab. IV. fig. 10) be the outward inclined edge of the plane ; 
suspend a plumb-line A B so as just to touch it at n, and at 71 
apply the centre of the quadrant, and let the radius passing 
through go° coincide with A B, and turn the plane till n m 
coincides with that degree at which you would have the plane 
strike the fluid, and the plane stands right for that angle. 
To find the resistance of a solid, we must have two such 
solids equal to each other, and put on at the opposite ends of 
two of the arms, for with one only its centrifugal force will in- 
crease the friction against the nut, whereas with two opposite 
to each other this effect will be destroyed. We must also get 
G 2 
