[ *7* ] 
appears, that the plane I N, by acquiring the por- 
tion O (^withdraws itfelf from the aCtion of the 
wind, by the fame fpace N P, that the plane A B 
does by acquiring the pofition F G ; and confequently, 
from the equality of P K to F E, the relative im- 
pulfe of the wind P K, upon the plane O Q, will be 
equal to the relative impulfe of the wind F E, upon 
the plane F G : and fince the impulfe of the wind 
upon A B, with the relative velocity F E, in the di- 
rection B E, is reprefented by -i A B ; the relative 
impulfe of the wind upon the plane I N, in the di- 
rection N K, will in like manner be reprefented by 
■J I K ; and the impulfe of the wind upon the plane 
I N, with the relative velocity P K, in the direction 
I K, will be reprefented by -J N K : and confequently 
the mechanical power of the plane I N, in the direc- 
tion I K, will be -i the parallellogram I Qj that is 
I K x N K : that is, from the equality of I K=A B 
and N K=B E, we fhall have 4 I ABx|BE 
=4 ABx^B E=| of the area of the parallellogram 
A F. Hence we deduce this 
General Proposition, 
'That all planes however fituated y that intercept 
the fame fetlion of the wind, and having the fame re- 
lative velocity, in regard to the wind , when reduced 
into the fame direction, have equal powers to produce 
mechanical ejfefts. 
For what is loft by the obliquity of the impulfe, 
is gained by the velocity of the motion. 
Hence it appears, that an oblique fail is under no 
difadvantage in refpeCt of power, compared with a 
direCt one $ except what arifes from a diminution of 
Z a its. 
