[ H6 ] 
(relative to the quantity of cloth) the greatefl: of all. 
But if the fails are intended, with given dimenfions, 
to produce the greatefl effect poflible in a given time, 
we muff intirely rejetfl thofe of N°. i. and, if we 
are confined, to the ufe of' planes , conform ourfelves to 
fiome angle between N°. 3. and 4. that is y not lejs than 
y 2 °, or greater than 75°, with the axis. 
The late celebrated Mr. Maclaurin has judicioufly 
diflinguifhed between the adtion of the wind upon a 
fail at reft, and a fail in motion ; and, in confequence, 
as the motion is more rapid near the extremities than 
towards the center, that the angle of the different 
parts of the fail, as they recede from the center, 
fhould be varied. For this purpofe he has furnifhed us 
with the following theorem *. ‘ c Suppofe the velocity 
<c of the wind to be reprefented by a , and the velo- 
“ city of any given part of the fail to be denoted by 
“ c ; then the effort of the wind upon that part of 
“ the fail will be greatefl; when the tangent of the 
“ angle, in which the wind flrikes it, is to radius as 
“ / 2 -j- 11 to 1.” This theorem then af- 
flgns the law, by which the angle is to be varied ac- 
cording to the velocity of each part of the fail to the 
wind : but as it is left undetermined what velocity 
any one given part of the fail ought to have in refpedt 
to the wind, the angle that any one part of the fail 
ought to have, is left undetermined alfo ; fo that we 
are flill at a lofs for the proper data to apply the theo- 
rem. However, being willing to avail myfelf thereof, 
and confidering that any angle from 15 0 to 18 0 was 
belt fuited to a plane, and of confequence the beft 
* Maclaurin’s account of Sir Ifaac Newton’s philofophical dif- 
coveries, p. 176, art. 29. 
mean 
