MECHANICS. 



iron of this description, called a crow-bar. Pincers, 

 scissors, and similar instruments are levers of the 



Fig. 6. 



first kind with two arms, the rivet at the centre 

 being the fulcrum of both. 



A common scale-beam for weighing is an ex- 

 ample of the first kind of lever, formed with two 

 arms of equal length. 



There is another kind of balance, called a steel- 

 yard, which consists of a lever with arms of 

 unequal length, and acts upon the principle of 

 distance from the fulcrum on the long arm com- 

 pensating for weight on the short arm, as already 

 defined. Fig. 7 is a representation of the steel- 

 yard balance, in which the weight, W, is sus- 

 pended from the short arm, CA. On the long 

 arm a number of divisions are set off, marked 

 i, 2, 3, &c. ; Ci being equal to CA, C2 equal to 



B 



o 



Fig. 7. 



twice CA, &c. P is a weight of a certain heavi- 

 ness, and being movable by a ring, it can be 

 slipped along the bar to any required point. In 

 proportion as the article to be weighed in the 

 scale W is heavy, so is the weight P slipped 

 along to a greater distance from the fulcrum, C ; 

 and when it is brought to a point where it 

 balances the article, the figure on the bar at that 

 point indicates the amount of the weight. If P 

 be one pound, and if, when suspended from the 

 division at 6, it balance the weight at W, it is 

 evident that the weight will be six times P, or six 

 pounds. And so on with all the other divisions. 



The steelyard, 

 though not so ancient 

 as the common bal- 

 ance, is of consider- 

 able antiquity. It was 

 used by the Romans, 

 and has long been 

 in use among the 

 Chinese. 



Fig. 8. 



Fig. 9. 



also common. One of the most familiar is that of 

 a man pushing or lifting forward a bale of goods, 

 as represented jn fig. 8, in which the bale or 

 weight W presses against the lever between the 

 power P and the fulcrum F. 



Two men carrying a load between them on a 

 pole is also an example of the second kind of 

 lever. In the case of 

 porters carrying a 

 barrel slung from a 

 pole, each man acts 

 as the power in lifting 

 the weight, and at the 

 same time each man 

 becomes a fulcrum in 

 respect to the other. 

 If the weight hang 

 fairly from the centre 

 of the pole, each man 

 will bear just a half of the burden ; but if the 

 weight be slipped along, so as to be nearer one 

 end of the lever than the other, then the man who 

 bears the shorter end of the pole supports a 

 greater load than the man who is at the long end. 

 In yoking horses to the extremities of cross-bars 

 in ploughs, coaches, or other vehicles, if the cross- 

 bar is not attached to the load by its middle 

 point, one horse will have to pull more than the 

 other. 



The instrument used for cracking nuts (fig. 10) 

 is an example of the second 

 kind of lever with two arms 

 or limbs. The oar of a boat 

 in rowing is also a lever of 

 this kind. The hands of 

 the sailor who pulls, con- 

 stitute the power ; the boat 



Fig. 10. 



Examples of the second kind of lever-power are 



is the weight to be moved ; and the water against 

 which the blade of the oar pushes, the fulcrum. 



In a lever of the third kind, the power must, 

 from its position, be always greater than the 

 weight ; and from this circumstance it has some- 

 times been called the losing lever. But this gives 

 a wrong impression ; for what is lost in the amount 

 of the power is gained in the velocity, or in the 

 space over which the resisting force is made to 

 move. Levers of this kind are used where the 

 object is not to gain power and overcome great 

 resistance, but to produce rapid motion where the 

 resistance is comparatively small. 



An example is found in the footboard of the 

 turning-lathe (fig. u). 

 The foot of the work- 

 man presses on the 

 board or plank near the 

 end which rests on the 

 ground, or fulcrum, and, 

 at the cost of a short 

 movement of its own, 

 causes the opposite ex- 

 tremity of the board to 

 move in a downward 

 direction over a con- Fig. II. 



siderable space. 



The movements in the limbs of animals are 

 mostly produced by the action of levers of the 

 third kind. The tendons or ropes which move 

 the bones are attached near the joints, which arc 

 the pivots or fulcra of the bone-levers. The struc- 

 ture of the human arm (fig. 12) is a very good 

 example. The fulcrum is the socket, C, of the 



