[ i6g ] 
be a very great improvement in point of power : 
while others attending to the extraordinary and even 
unexpected effects of oblique vanes, have been led to 
imagine, that oblique vanes applied to water-mills, 
would as much exceed the common water wheels, 
as the vertical windmills are found to have exceeded 
all attempts towards an horizontal one. Both thefe 
notions, but efpecially the firft, have fo plaufible an 
appearance, that of late years there has feldom been 
wanting thofe, who have affiduoufly employed them- 
felves to bring to bear defigns of this kind : it may 
not therefore be unacceptable to endeavour to fet this 
matter in a clear light. 
Plate VI. fig 2d. Let AB be the fe&ion of a 
plain, upon which let the wind blow in the direction 
C D, with fuch a velocity as to defcribe a given fpace 
B E, in a given time (fuppofe 1 fecond) ; and let 
A B be moved parallel to itfelf, in the direc- 
tion C D. Now, if the plane AB moves with* the 
fame velocity as the wind ; that is, if the point B 
moves thro’ the fpace B E in the fame time that a 
particle of air would move thro’ the fame fpace ; it 
is plain that, in this cafe, there can be no preffure or 
impulfe of the wind upon the plane : but if the plane 
moves flower than the wind, in the fame direction, 
fo that the point B may move to F, while a particle 
of air, fetting out from B at the fame inflant, would 
move to E, then B F will exprefs the velocity of the 
; and the relative velocity of the wind and plane 
3e expreffed by the line F E. Let the ratio o 
F E to BE be given (fuppofe 2 : 3.) ; let the line 
A B reprefent the impulfe of the wind upon the plane 
A B, when aCting with its whole velocity BE ; but, 
Vol. LI. Z * when 
plane 
will l 
