C 148 ] 
mill-builders, tho’ they make the angle to diminifh, 
in receding from the center towards the extremity, 
yet condantly do it in fuch manner, as that the fur- 
face of the fail may be concave towards the wind. 
In this manner the fails made ufe of in N°. 8, 9, 10, 
11, 12, and 13. were condru&ed ; the middle of the 
fail making an angle with the extreme bar of 1 2°; 
and the greated angle (which was about of the ra- 
dius from the centre) of 15 0 therewith. Thofe fails 
being tried in various politions, the bed: appears to 
be that of N°. 11. where the extremities flood at an 
angle of y°k with the plane of motion, the product 
being 635? : greater than that of thofe made by the 
theorem in the ratio of 9:11, and double to that of 
N°.i.; and this was the greateft product that could 
be procured without an augmentation of furface. 
Hence it appears, that when the wind falls upon a 
concave fur / ace , it is an advantage to the power of 
the whole , tho * every part, taken feparately, fould not 
be difpofed to the befl advantage *. 
Having thus obtained the bed pofition of the fails, 
or manner of weathering, as it is called by workmen, 
the next point was to try what advantage could be 
* By feveral trials in large I have found the following angles to 
anfwer as well as any. The radius is fuppofed to be divided into 
6 parts and i-6th, reckoning from the center, is called 1, the ex- 
tremity being denoted 6. 
N°. 
1 
2 
3 
4 
5 
6 
Angle with 
the axis. 
' 72 ° - 
71 
72 ' 
74 
77 i 
83 ' 
Angle with the plane 
. of motion. 
— 18 0 
— 19 
18 middle. 
* - 1 6 
1 2 1 
extremity. 
mads 
