[ I 7° ] 
when aCting with its relative velocity F E, let its im- 
pulfe be denoted by fome aliquot part of A B, as for 
inilance -JAB: then will of the parallelogram A F 
reprefent the mechanical power of the plane 5 that is, 
| AB x-j B E. 
2dly, Let I N be the feCtion of a plane, inclined 
in fuch a manner, that the bafe I K of the reCtangle 
triangle 1 KN may be equal to A B ; and the per- 
pendicular N K=B E ; let the plane IN be ftruck 
by the wind, in the direction L M, perpendicular to 
I K : then, according to the known rules of oblique 
forces, the impulfe of the wind upon the plain I N, 
tending to move it according to the direction L M, or 
N K, will be denoted by the bafe I K ; and that 
part of the impulfe, tending to move it according to 
the direction I K, will be exprefled by the perpendi- 
cular N K. Let the plane I N be moveable in the 
direction of I K only; that is, the point I in the di- 
rection of I K, and the point N in the direction N Q. 
parallel thereto. Now it is evident, that if the point 
I moves thro’ the line I K, while a particle of air, 
fetting forwards at the fame time from the point N, 
moves thro’ the line N K, they will both arrive at the 
point K at the fame time ; and confequently, in this 
cafe alfo, there can be no preflure or impulfe of the 
particle of the air upon the plane I N. Now let I O 
be to I K as B F to B E 5 and let the plane I N move 
at fuch a rate, that the point I may arrive at O, and 
acquire the polition I Q^in the fame time that a par- 
ticle of wind would move thro’ the fpace N K : as 
OQJs parallel to IN; (by the properties of limilar 
triangles) it will cut N K in the point P, in fuch a 
manner, that N P=B F, and P K=F E ; hence it 
appears. 
