MECI1MICAL POWERS. 



THE mechanical powers are essentially but two in 'number, but are usually 

 considered as six ; namely, the Lever, the Wheel and Axle, the Pulley, the In- 

 clined Plane, the Wedge, and the Screw. The three first are assemblages of 

 levers, and the three last inclined planes. One or more of these powers enters 

 into the composition of every machine. 



Ths LsVSI consists of an inflexible rod or bar, resting on a support called 

 a fulcrum, for the purpose of raising, by a power applied at one end, a weight at 

 the other. The advantage gained is in proportion to the greater distance of the 

 power from the fulcrum, than is the distance from the fulcrum to the weight to 

 be raised ; thus, if the distance from the power to the fulcrum be five times 

 greater than the distance from the weight to the fulcrum, a force of one pound 

 in the power will balance five in the weight. 



There are three kinds of levers ; the first kind is that where the fulcrum 

 (F) is placed between the ws ight (w) and the power, (p, fig. 1). The com- 

 mon balance, (fig. 2) is a lever of the first kind, as is also the Roman steel- 

 yard, (fig. 3). 



The boy'-s amusement of see-saw (fig. 4) is another illustration of a lever of 

 the. first kind, the bigger boy taking the shorter end of the plank, that his lighter 

 companion at the longer end may balance him. 



Fig. 5, shows the application of a lever of the first kind in moving a heavy 

 body ; the nearer the fulcrum to the body to be moved the more powerful 

 being the leverage. 



In levers of the second kind, the weight is situated between the power and 

 the fulcrum, (fig. 6). This kind of lever is seen in the common wheelbarrow, 

 where the wheel is the fulcrum, the load in the barrow the weight, and the 

 power the man who holds up the shafts. The oars of a boat present another 

 instance ; here the water is the fulcrum against which the blades of the oars 

 press. 



Levers of the third kind are those where the fulcrum is at one end, the weight 

 at the other, and the power between them, (fig. 7). Here the power acts with a 

 considerable disadvantage, and this kind of lever is only used where the object is 

 to produce great velocity, and which can only be effected by an expenditure of 

 power. The footboard of a common turning lathe affords an example of this 

 kind of lever. But one of the most striking instances of it is seen, in the human 

 irm in the act of raising a weight in the halul, when the lower part of the arm 

 becomes a lever of the third kind, the elbow joint being the fulcrum, and the 

 muscles which move the arm, the power (fig. 8). The muscle, by contracting 

 its fibres less than an inch, raises the hand twenty inches ; and if it raises also a 

 weight of twenty-five pounds in the hand, it must act with a force at least twenty 

 times as intense, or of five hundred pounds ; thus showing the extraordinary 

 strength of the living muscle. 



Levers may be combined in a great variety of ways, and the aggregate effect of 

 such combination is as the product of the effect of thcj separate levers. Fig. 9, 

 represents a combination of levers of the first kind, in which the power of i^ie 

 small weight P brings down A, which raises B, bringing down c, and conse- 

 quently raising D ; and thus, if properly supported, will balance the large weight 

 w. By this means a weight of one pound will balance one of a hundred and 

 twenty pounds. 



The Wheel and Axle ma J be considered as a kind of perpetual lever, of 

 which the fulcrum is the centre of the axis, and the long and short arms the 

 radius of the wheel and the radius of the axle, as shown at fig. 10 ; the power P 

 acting upon the weigh* w, through the intervention of the lever A B, whose 

 fulcrum is the centre of the axle. Supposing the semi-diameter of the wheel 

 to be six times greater than the semi-diameter of the axle, a power of one 

 will balance a weight of six, exactly upon the principle of a lever of the first 

 kind. 



There are many modifications of this mechanical power ; one of the most com- 

 mon is the windlass for raising water from a well by means of buckets, (fig. 11). 

 The capstan used on board ships is an upright axle, the moveable bars or 

 levers acting as the wheel (fig. 12). Like the lever, the wheel and axle may 

 be used in combination, the circumference of one wheel acting by means of 

 teeth upon the axle of another, as shown at fig. 13 ; the effect of such 

 combination being similar to that produced by the combination of levers already 

 described. 



