C 54-5 ] 
perpendicular fpindle ; each arm is furnifhed with a 
vane, which opens one way, and {huts the other, as 
fome have attempted the making of horizontal wind- 
mills. This, by being carried thro’ the water progref- 
fively, will turn round, and the fafter, as the fhip 
moves fafter: But to judge, whether it will doitpro- 
portionabiy in all velocities of the {hip, let us con- 
sider, 
1. That a good failing {hip will frequently fail at 
the rate of io fea miles (60 to a degree) an hour, 
which is at the rate of 17 feet per fecond. 
2. Suppoftng the fide of the fly, where the vanes 
* are clofed, to be retained by the water at reft ; the 
oppoflte fide of the fly, where the vane is open, muft 
meet the water with a velocity double to that of the 
{hip, or at the rate of 34 feet in a fecond 5 as would 
be the cafe with the upper part of a coach-wheel, 
whofe velocity thro’ the air is double to that, where- 
with the coach moves forward. 
3. That a plane furface of 3 inches fquare, moving 
thro’ the water with a velocity of 34 feet per fecond, 
will meet with areftftance, atleaft, equal to 70 pounds 
avoirdupoize. 
4. That the reflftance, which the open vanes will 
meet with in the water, will, in fwift motions, be very 
conftderable, and, of confequence, the fly will move 
much flower than it ought to do, if thefe refiftances 
were lefs. 
5. That from hence there is much reafon to doubt, 
whether the reflftance of the medium, and fridiion of 
the machine, taken together, will always produce fuch 
diminution, in the number of turns, as that the num- 
ber of revolutions, adlually fhewn by the indexes, 
Z z z may 
