xxii INTRODUCTION TO MECHANICS. 



order to put a machine in motion. For this purpose each boy at his 

 descent touches the ground with his feet, and the support he receives 

 from it diminishes his weight and enables his companion to raise him ; 

 thus each boy alternately represents the power and the weight, and the 

 two arms alternately perform the function of the acting and the resisting 

 part of the lever. 



A lever, in moving, describes the arc of a circle, for it can move only 

 around the fulcrum or centre of motion. It would be impossible for one 

 child to rise perpendicularly to the point A (fig. 23), or for the other to 



Fig. 23. 



descend in a straight line to B, they each describe arcs of their re- 

 spective circles ; and you may judge from the different dimensions of the 

 circle how much greater the velocity of the little child must be than that of 

 the bigger one. Enormous weights may be raised by levers of this 

 description, for the longer the acting part of the lever in comparison to 

 the resisting part, the greater is the effect produced by it ; because the 

 greater is the velocity of the power compared to that of the weight. 

 You have seen a heavy snow-ball rolled over (fig. 24) by thrusting the 

 end of a strong stick beneath the ball, and resting it against a log of 

 Fig. 24. wood, or any object which can give it support, 



near the end in contact with the snow-ball ? The 

 stick, in this case, is a lever ; the support, the prop 

 or fulcrum ; and the nearer the latter is to the re- 

 sistance, the more easily will the power be able to 

 move it. 



There are three different kinds of levers : in the first, which compre- 

 hends the several levers we have described, the fulcrum is between the 

 power and .the weight. When the fulcrum is situated equally between the 

 power and the weight, as in the balance, the power must be greater than 

 the weight, in order to move it ; for nothing can in this case be gained 

 by velocity. The two arms of the lever being equal, the velocity of 

 their extremities must be so likewise. The balance is therefore of no 

 assistance as a mechanical power, but it is extremely useful to estimate 

 the respective weights of bodies. But when the fulcrum F of a lever 

 (fig. 25) is not equally distant from the power and the weight, and that 

 in amount, though greater in effect,the power P acts at the extremity of the 

 longest arm, it may be less than the weight W, its deficiency being com- 



