INTRODUCTION TO MECHANICS. xxiii 



pensated by its superior velocity ; as we 



^g* 25. observed in the see-saw. Therefore when 



r ~~2\ a great weight is to be raised, it must 



be fastened to the shortest arm of a lever, 



^p and the power applied to the longest 



"W arm; but if the case will admit of putting 



the end of the lever under the weight, 



no 'fastening will be required, as you may perceive by stirring the fire. 

 The poker is a lever of the first kind ; the point where it rests against the 

 bars of the grate, whilst stirring the fire, is the fulcrum ; the short arm, or 

 resisting part of the lever, is employed in lifting the weight, which is 

 the coals, and the hand is the power applied to the longest arm, or 

 acting part of the lever. A pair of scissars is an instrument composed 

 of two levers, united in one common fulcrum ; the point at which the two 

 levers are screwed together is the fulcrum; the handles, to, which the 

 power of the fingers is applied, are the extremities of the acting part of 

 the levers, and the cutting part of the scissars are the resisting parts of 

 the levers ; therefore the longer the handles, and the shorter the points of 

 the scissars, the more easily will they cut. Thus when pasteboard, or any 

 hard substance is to be cut, that part of the scissars nearest the screw or 

 rivet, is used. Snuffers, and most kinds of pincers, are levers of a similar 

 description, the great force of which consists in the resisting part of the 

 lever being very short in comparison of the acting part. 



In levers of the second kind, the weight, instead of being at one end, 

 is situated between the power and the fulcrum (Jig. 26). In moving it, 

 the velocity of the power must necessarily be greater than that of the 

 weight, as it is more distant than the centre of motion. You may, perhaps, 

 have seen a snow-ball moved by means of a lever of the second 

 order, as well as by one of the first. The end of the stick (Jig. 27) that 

 is thrust under the ball rests on the ground, which becomes the 



Fig. 26. Fig. 27. 



fulcrum ; the ball is the weight to be moved, and the power the hands 

 applied to the other end of the lever. In this instance there is an 

 immense difference in the length of the arms of the lever, the weight being 

 almost close to the fulcrum, and the advantage gained is proportional. 

 Fishermen's boats are thus raised from the ground to be launched into 

 the sea, by means of slippery pieces of board, which are thrust under the 

 keel. The most common example that we have of levers of the second 

 kind is in the doors of our apartments : in these the hinges represent the 

 fulcrum, the hand, the power applied to the other end of the lever, and the 

 door, or rather its inertia, is the weight which occupies the whole of the 

 space between the power and the fulcrum. The whole weight and inertia 

 of the door may be regarded as collected into its centre of gravity; that 

 is to say, the resistance of the door is the same that would be offered by 

 a force equal to the inertia of the door, and passing through its centre 

 of gravity. Another very common instance is found in an oar : the blade 



