THE MECHANICAL POWERS. ll 
of motion be desired, it may be gained by means of a lever, at the expense 
of a greater force than is required ; and it is to this end that the third 
kind of lever is applied (see fig. 4). On this principle the. foot-board 
of a turning-lathe is constructed (fig. 6). The fulerum of the lever is the 
hinge at the toe of the man’s left foot ; the power is the man’s right 
foot, which presses down the treadle, thus communicating motion to the 
string S; and although the power com- 
Geiticated to the string is not so great as 
that of the pressure of the foot, the velocity 
is many times greater. yee nails with 
a hammer is amathes example of gain of 
velocity at the expense of strength or 
force. The fulcrum is the elbow-joint ; the 
power is exerted by the muscles attached . - 
near the wrist; and although the exertion 
of the muscles is greater than the power 
communicated to the hammer, yet the velocity gained is very great. 
The common balance is a familiar application of the lever. Since it 
is used to weigh quantities of articles equal to certain weights, it will 
be readily understood, from what has been said, that it is absolutely 
necessary that the arms of the balance be of equal length, because a 
smaller weight at the end of a longer arm would balance a greater weight 
at the end of a shorter arm. ‘This is one way by which persons selling 
articles requiring to be weighed could cheat their customers, although | 
their weights were quite correct; for, if one arm of the balance were 
_ shorter than the other, by putting the weights at that side, a less quantity 
than the just weight would balance them. 
A few words will now be suflicient to explain the principle of the 
steelyard, another application 
of the lever (fig. 7). In the 
figure, C is the fulcrum or 
pivot on which it is sus- 
pended, CA is the distance 
from the fulcrum to the point 
at which the article to be 
weighed is suspended. The 
longer arm of the instrument 
is divided into equal lengths, 
marked 1, 2, 3, &c.; and the 
great convenience of this is, that many different sieaibee can be 
weighed with one weight. For instance, if the weight used be one 
pound, and it is required to weigh two pounds, “the one - pound 
weight is placed at the division marked 2, and there, as was seen 



Fig. 7. 2 
