MECHANICS. 
plane A B (fig. 18,) to be parallel to the ho- 
rizon, the cylinder C will keep at rest on 
any part of the plane where it is laid. If 
the plane be placed perpendicularly, as 
A B, (fig. 19,) the cylinder C wall descend 
with its whole force of gravity, because the 
plane contributes nothing to its support or 
hindrance ; and therefore it would require 
a power equal to its whole weight to keep 
it from descending. Let A B (fig. 10.) be a 
plane parallel to the horizon, and A D a 
plane inclined to it ; and .suppose the whole 
length A D to be four times as great as the 
perpendicular D B. In this case, the cy- 
linder E will be supported upon the plane 
D A, and kept from rolling, by a power 
equal to a fourth part of the weight of the 
cylinder ; therefore, a weight may be rolled 
up this inclined plane, by a third part of the 
power which would be sufficient to draw it 
np by the side of an upright wall. It must 
also be evident, that the less the angle of 
elevation, or the gentler the ascent is, the 
greater will be the weight which a given 
power can draw up ; for the steeper the in- 
clined plane is, the less does it support of 
the weight ; and the greater the tendency 
which the weight has to roll, consequently 
the more difficult for the power to support 
it : the advantage gained by this mechanical 
power, therefore, is as great as its length 
exceeds its perpendicular height. To the 
inclined plane may be reduced all hatchets, 
chisels, and other edge-tools. 
The inclined plane, when combined with 
other machinery, is often of great use in the 
elevation of weights : it has been likewise 
made use of in the late Duke of Bridge- 
water’s canal. After this canal has extended 
about 40 miles on the same level, it is joined 
to a subterraneous navigation about 12 miles 
long, by means of an inclined plane, and 
this subterraneous portion is again connect- 
ed by an inclined plane with another por- 
tion 100 feet above it. This plane is a 
stratum of stone which slopes one foot in 
four, and is about 450 feet long. The 
boats are conveyed from one level to another 
by means of a windlass, so that a loaded 
boat descending along the plane turns the 
axis of the windlass, and raise’s an empty 
boat. 
The fifth mechanical power or machine is 
the wedge ; which may be considered as 
two equally inclined planes, joined together 
at their, bases ; then D G (fig. 21.) is the 
whole thickness of the wedge at its back 
A B G D, where the power is applied ; 
E F is the depth or height of the wedge : 
VOL. IV. 
B F the length of one of its sides ; and O F 
is its sharp edge, which is entered into the 
wood intended to be split, by the force ofa 
hammer or mallet striking perpendicularly 
on its back. Thus, A B (fig. 22.) is a wedge 
driven into the cleft C E D of the wood 
F G. When the wood does not cleave at 
any distance before the wedge, there will 
be an equilibrium between the power im- 
pelling the wedge downward and the re- 
sistance of the wood acting against the two 
sides of the wedge, when the power is to 
the resistance as half the thickness of the 
wedge at its back is to the length of either 
of its sides ; because the resistance then 
acts perpendicularly to the sides of the 
wedge. But when the resistance on each 
side acts parallel to the back, the power 
that balances the resistances on both sides 
will be, as the length of the whole back of 
the wedge is to double its perpendicular 
height. 
When the wood cleaves at any distance 
before the wedge (as it generally does) the 
power impelling the wedge will not be to 
the resistance of the wood as the length on 
the back of the wedge is to the length of 
both its sides, but as half the length of the 
back is to the length of either side of the 
cleft, estimated from the top or acting part 
of the wedge. For, if we suppose the 
wedge to be lengthened down from the top 
C E, to the bottom of the cleft at D, the 
same proportion will hold ; namely, that 
the power will be to the resistance as half 
the length of the back of the wedge is to 
the length of either of its sides : or, which 
amounts to the same thing, as the w'hole 
length of the back is to tlie length of both 
the sides. The wedge is a very great me- 
chanical power, since not only wood, but 
even rocks, can be split by it; which it 
would be impossible to etfect by the lever, 
wheel, and axle, or pulley ; for the force of 
the blow, or stroke, shakes the cohering 
parts, and thereby makes them separate 
more easily. 
The sixth and last mechanical power is 
the screw ; which cannot properly be called 
a simple machine, because it is never used 
without the application of a lever or winch 
to assist in turning it ; and then it becomes 
a compound engine of a very great force, 
either in pressing the parts of bodies closer 
together, or in raising great weights. It 
may be conceived to be made by cutting a 
piece of paper, ABC (fig. 23.) into the 
form of an inclined plane or half wedge ; 
and then wrapping it round a cylinder (fig. 
