248 THE LEVEE AND WHEELWORK. 



Scissors, shears, nippers, pincers, and other similar instruments, are com- 

 posed of two levers of the first kind ; the fulcrum being the joint or pivot, and 

 the weight the resistance of the substance to be cut or seized ; the power being 

 the fingers applied at the other end of the levers. 



The brake of a pump is a lever of the first kind ; the pump-rods and piston 

 being the weight to be raised. 



Examples of levers of the second kind, though not so frequent as those just 

 mentioned, are not uncommon. 



An oar is a lever of the second kind : the reaction of the water against the 

 blade is the fulcrum ; the boat is the weight, and the hand of the boatman the 

 power. 



The rudder of a ship or boat is an example of this kind of lever, and explain- 

 ed in a similar way. 



The chipping-knife is a lever of the second kind. The end attached to the 

 bench is the fulcrum, and the weight the resistance of the substance to be cut, 

 placed beneath it. 



A door moved upon its hinges is another example. 



Nut-crackers are two levers of the second kind ; the hinge which unites 

 them being the fulcrum, the resistance of the shell placed between them being 

 the weight, and the hand applied to the extremity being the power. 



A wheelbarrow is a lever of the second kind ; the fulcrum being the point 

 at which the wheel presses on the ground, and the weight being that of the 

 barrow and its load, collected at jheir centre of gravity. 



The same observation may be applied to all two-wheeled carriages, which 

 are partly sustained by the animal which draws them. 



In a lever of the third kind, the weight, being more distant from the fulcrum 

 than the power, must be proportion ably less than it. In this instrument, there- 

 fore, the power acts upon the weight to a mechanical disadvantage, inasmuch 

 as a greater power is necessary to support or move the weight than would be 

 required if the power were immediately applied to the weight, without the in- 

 tervention of a machine. We shall, however, hereafter show that the advan- 

 tage which is lost in force is gained in despatch, and that, in proportion as the 

 weight is less than the power which moves it, so will the speed of its motion 

 be greater than that of the power. 



Hence a lever of the third kind is only used in cases where the exertion 

 of great power is a consideration subordinate to those of rapidity and despatch. 



The most striking example of levers of the third kind is found in the animal 

 economy. The limbs of animals are generally levers of this description. The 

 socket of the bone is the fulcrum ; a strong muscle attached to the bone near 

 the socket is the power ; and the weight of the limb, together with whatever 

 resistance is opposed to its motion, is the weight. A slight contraction of the 

 muscle in this case gives a considerable motion to the limb : this effect is par- 

 ticularly conspicuous in the motion of the arms and legs in the human body ; 

 a very inconsiderable contraction of the muscles at the shoulders and hips giv- 

 ing the sweep to the limbs from which the body derives so much activity. 



The treddle of the turning-lathe is a lever of the third kind. The hinge 

 which attaches it to the floor is the fulcrum, the foot applied to it near the 

 hinge is the power, and the crank upon the axis of the fly-wheel, with which 

 its extremity is connected, is the weight. 



Tongs are levers of this kind, as also the shears used in shearing sheep. In 

 these cases, the power is the hand placed immediately below the fulcrum, or 

 point where the two levers are connected. 



When the power is. said to support the weight by means of a lever, or any 

 other machine, it is only meant tha.t the power keeps the machine in equilib- 



