44? JWr. Vince's Observations on the Theory ' 
two thin pieces of lead with the edges feathered off, and of the 
same weight with the two solids. These must first be put upon 
the opposite arms, and a weight w found as before. Then the 
leads are to be taken off, and the solids put on in their place, 
with that side to go foremost whose resistance is required, and 
then find W as in the case of the planes ; and the absolute re- 
sistance will be^x W upon one of the solids. 
By this machine we may find the absolute resistance upon 
the planes in a direction perpendicular to that of their motion. 
For let the lower end of the axis, instead of resting upon the 
base of the frame, stand upon one end of an horizontal lever, 
like that in figure the seventh, and let it be balanced by a 
weight in a scale hanging at the same distance on the other 
side of the fulcrum, when the sails have acquired an uniform 
motion, with the planes horizontal, or when moving edge-ways. 
Then turn the planes to any angle, and add equal weights to 
the scales R and T, until the planes have acquired the same 
uniform velocity as before, and put a weight P into the scale at 
the other end of the lever, which shall now just balance it, and 
P will be the absolute resistance of the fluid in a direction per- 
pendicular to the motion of the planes. 
The law of resistance, when the velocity varies, may be thus 
found. Let w, as before, be the sum of the two equal weights 
which will give the planes an uniform horizontal motion when 
they move edge-ways. Then set them perpendicular to the ho- 
rizon, and let W be the sum of the two equal weights, put one 
into each scale, in order to give the sails the same uniform ve- 
locity. Take out these two equal weights, and put in two other 
equal weights, together equal to Q, such as shall give the planes 
an uniform velocity double to that before given; then the 
