xxiv INTRODUCTION TO MECHANICS. 



is kept in the same place by the resistance of the water, and becomes the 

 fulcrum ; the resistance is applied where the oar passes over the side of 

 the boat, and the hands at the handle are the power. Nut-crackers are 

 double levers of this kind : the hinge is the fulcrum, the nut the resist- 

 ance, and the hands the power. 



In levers of the third kind (Jig. 28), the fulcrum is also at one of the 

 extremities, the weight or resistance at the other, and it is now the power 



which is applied between the fulcrum 

 and the resistance. Thus the fulcrum, 

 the weight, and the power, each in it's 

 turn, occupies some part of the middle 

 3 of the lever between its extremities. But 

 in this third kind of lever, the weight being 

 |H further from the centre of motion than the 

 "W" power, the difficulty of raising it, instead of 



being diminished, is increased. Levers of this description are used when 

 the object is to produce great velocity. The aim of mechanics, in general, 

 is to gain force by exchanging it for time ; but it is sometimes desirable 

 to produce great velocity by an expenditure of force. The treddle of a 

 common turning lathe affords an example of a lever of the third kind 

 employed in gaining time, or velocity, at the expense of force. A man, 

 in raising a long ladder perpendicularly against a wall, cannot place his 

 hands on the upper part of the ladder : the power, therefore, is neces- 

 sarily placed nearer the fulcrum than the weight, for the hands are the 

 power, the ground the fulcrum, and the ladder the weight, which, as in 

 the case of the door, may be considered as collected in the centre of 

 gravity of the ladder, about halfway up it, and consequently beyond the 

 point where the hands are applied. Nature employs this kind of lever 

 in the structure of the human frame. In lifting a weight with the hand, 

 the lower part of the arm becomes a lever of the third kind : the elbow 

 is the fulcrum ; the muscles which move the arm, the power ; and as 

 these are nearer to the elbow than the hand is, it is necessary that their 

 power should exceed the weight to be raised. 



You may perhaps wonder that nature should have furnished us with 

 such levers, but the disadvantage is more than compensated by the con- 

 venience resulting from the structure of the arm. It is of more conse- 

 quence that we should be able to move our limbs nimbly, than that we 

 should be able to overcome great resistance ; for it is comparatively 

 seldom that we meet with great obstacles, and when we do, they can be 

 overcome by art. Besides, the Creator has endowed the muscular fibres 

 with prodigious strength, so that, upon the whole, this kind of lever is 

 best adapted to enable the arm to perform its various functions. 



The pulley, which is the second mechanical power we are to examine, is 

 a circular flat piece of wood or metal, with a strino- 

 running in a groove round it, by means of which a 

 weight may be pulled up. Thus pulleys are used for 

 drawing up curtains, the sails of a ship, &c. When, 

 as in the examples alluded to, the pulley is fixed, it 

 does not increase the power to raise the weight. If P 

 represent the power, to raise the weight W (Jig. 29), 

 it is evident that the power must be greater than the 

 weight, in order to move it. A fixed pulley is useful, 

 therefore, only in altering the direction of the power, 

 and its most frequent practical application is to 

 make us to draw up a weight by drawing down the string con- 



