MECHANICAL POWERS. 



The compound axle, (fig. 14), is s contrivance by which the power is increased 

 without increasing the diameter of the wheel. This axle has one half of it twice 

 the diameter of the other half. A moveable pulley being attached to the weight 

 to be raised, the rope is passed round the wheel, and coiled in the same direction 

 upon both parts of the axle: upon every revolution of the axle, a portion of the 

 rope equal to the circumference of the thicker part will be drawn up, but at the 

 same time, a portion equal to the circumference of the thinner part will be let 

 down : hence, every revolution of the cylinder raises the weight through a space 

 equal to half the difference between the circumferences of the thicker and thinner 

 parts of the axle. 



The Pulley is a sma11 wheel turning on an axis, and having a groove upon 

 its circumference for the reception of a rope. It is either fixed or moveable. The 

 fixed pulley (fig. 15) possesses no mechanical advantage, but is used to change 

 the direction of the power, or to give convenience in pulling. 



The moveable pulley, however, affords mechanical assistance by dividing the 

 weight. This is illustrated at fig. 16, where the weight of the barrel is equally 

 divided between the part of the rope affixed to the beam, and that held in the 

 hand. Fig. 17 shows this more clearly; if the large weight be twenty pounds, 

 ten pounds is borne by A, and ten pounds by p. The fixed pulley, B, is of no 

 other use than to change the direction of the power. 



The power of pulleys is increased by their combination. Fig. 18 illustrates 

 this : here the weight is equally distributed between four ropes, consequently it 

 may be supported by a power only a fourth of its own weight. Fig. 19 is a sys- 

 tem of pulleys, or a tackle, as it is usually called, by which a power of one 

 hundred will balance a weight of six hundred. Fig. 20 exhibits a system of pulleys 

 in combination, in which a power of one will balance a weight of sixteen. The 

 power of this system may be greatly augmented by substituting fixed pulleys for 

 the hooks to which the ends of the ropes are attached, in the manner shown at 

 fig. 21. In order to obviate the loss of power occasioned by the friction of the 

 separate axles, where several pulleys are employed, an ingenious arrangement has 

 been resorted to in White's patent pulley (fig. 22), by which all the pulleys in 

 each block turn on the same axis. 



The Inclined Plane* It is not difficult to understand that a body may 

 with much greater ease be drawn up a slope, than it can be raised the same height 

 perpendicularly. Hence the advantage of the inclined plane, which acts as a 

 mechanical power, in partly supporting the weight (fig. 23). The -.longer the 

 inclined space is in proportion to the perpendicular height, the greater is the 

 advantage afforded. Suppose the inclined plane A B (fig. 24), to bear the pro- 

 portion to the perpendicular height B c, as three to one, then a power of one will 

 balance a weight of three. 



Persons have often been struck with astonishment when viewing Stonehenge, 

 how those enormous cross-pieces of stone were raised to the elevation at which 

 we see them, but the mechanical feat is by no means very wonderful, for it 

 would only be necessary to raise an inclined plane of earth in the direction 

 of the line A B (fig. 25), and by means of rollers placed under the stone, pass 

 it to its situation on the top ; the earth being removed, the stone would remain 

 secure. 



The Wedge is a combination of two inclined planes united at their base 

 (fig. 26). The wedge derives its great power chiefly from the way in which the 

 force is applied by being struck. 



The Screw is a machine of great mechanical power, and is a modification of 

 the inclined plane, as will be seen by reference to fig. 27. The screw has no 

 power by itself, it can operate only by working in spiral grooves corresponding 

 to its threads : these grooves are formed on the inside of a box or nut, 

 through which the screw winds itself. Fig. 28 shows the screw, and its nut, 

 fig. 29, exhibits the screw with a section of the nut, showing the spiral groores, 

 The power is applied by a lever, the screw, therefore, acts with the combined 

 power of the lever and the inclined plane, thus becoming in reality a compound 

 machine. 



Screws are much used in presses of all kinds. The bookbinders' standing 

 press (fig. 30), affords one of the best examples of this application of its 

 powei. 



