14 PHYSICS. 
IV.—The Inclined Plane. 
The Inclined Plane is a contrivance by which a heavy weight can be 
raised to a height, when it would be difficult or impossible to lift it 
directly. We have seen, in the case of the Mechanical Powers already 
considered, that no additional power is actually gained by the use of the 
machine, or is generated by it, but that its use is simply to enable a small | 
force, when exerted over a:larger space, or in a more extended manner, to 
have the same effect as a greater force applied directly: the same principle 
applies to the inclined plane. A force pushing a weight from A to B 
(fig. 13) only raises it through the perpendicular height BD. If, then, AB 
. be twice as long as the height BD, 
the force necessary to raise any 
weight from D to B, would, when 
pushing the weight from A to B, be 
distributed over twice as much space: 
but in consequence of this, the force 
exerted at any moment would be 
only half as great, leaving out of 

cannot be taken notice of here. Thus, 
a power, P, would balance a weight 
on the plane twice as great; that is, if the mass W weigh two pounds, 
the weight of P would require to be only one pound. The reason of this 
is, that part of the weight of W is supported on the plane, so that the 
resistance of the plane, and the tension of the string, p, support the weight 
of W between them, in the way represented in the figure. The weight of 
W acts perpendicularly downward from its centre of gravity,! c, in the 
direction of cf; now, the part of this weight supported by the plane must 
act perpendicularly to the plane, therefore along ce; and the part of it 
supported by the string, 
directly opposite to the 
tension of the string, 
along cg; and these two 
are independent of each 
other, so that the pro- 
portion of the weight of 
Fig. 14. W to be supported by the 
string is as ce is to ef. 
Now, it is known, according to a simple geometrical problem, that cg is 

1 Centre of gravity is a point in a body such that, if the body be suspended upon that point, 
it will balance itself in every position, 
account the effect of friction, which © 
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