THE MECHANICAL POWERS. 9 
The Mechanical Powers. 
A machine is an instrument by means of which force is applied to 
the performance of work, generally by changing the direction of the foree— 
as the capstan, by which sailors raise the anchor; the crane, by which 
stones or other heavy weights are raised ; or the lever, by which a heavy 
object is moved upwards by pressing down the other end. Such con- 
trivances do not increase the force applied, but merely afford the means 
by which it may be directed more advantageously to the end in view. The 
simple machines used for this purpose, or the MECHANICAL PowERs, as 
they have been called, are usually considered to be six in number—the 
Lever, the Wheel and Axle, the Pulley, the Inclined Plane, the Screw, 
and the Wedge. 
’ I.—The Lever. 
A lever, from Latin levo, to raise, means that which raises or lifts. It 
is a body of any form fixed at a point about which it can move, called 
the centre of motion, or the fulcrum [Latin, ‘a prop’]; thus, when the 
fire is stirred with a poker, the poker is, for the time being, a lever, 
and the bar on which it rests is the centre 
of motion, or fulcrum. Levers are distin- 
guished into three classes. 
(1) The first class of lever (fig. 2) has the 
fulerum between the force applied and the 
weight to be raised, or the resistance to be 
overcome, as in the case of a poker when 
stirring the fire. The common balance and Fig. 2. 
a pair of scissors—the latter being double— AS ihe ler 9% the celeron ; 
are both examples of this class of lever. power sh hs Regt oe 
(2) The second class of lever has the weight 
between the power and the fulcrum (fig. 3). The common wheelbarrow 
is a lever of this class; an oar is another example—the man pulling 
being the power, the water taken hold of by the blade being the 
fulcrum, and the boat the resistance. 

i) 
Ww | 
Fig. 3. Fig. 4, 
(3) The third class of lever has the power between the fulcrum and 

