Of the Mechanical Powers.

must always be allowed for, in calculating the power necessary to do a given work. It would be wrong, however, to suppose that the loss was equivalent to the gain, and that no advantage is derived from the mechanical powers. We are unable to augment our strength, but by the aid of science, we so divide the resistance, that by a continued exertion of power we accomplish that which it would be impossible to effect by a single effort.

If in attaining this result, we sacrifice time, we cannot but see that it is most advantageously exchanged for power.

5. It is plain, that in the moveable pulley, all the parts of the cord will be equally stretched, and hence, each cord running from pulley to pulley, will bear an equal part of the weight; consequently the power will always be equal to the weight, divided by the number of cords which reach from pulley to pulley.

EXAMPLES.

1. In a single immoveable pulley, what power will support a weight of 60 lbs ?

2. In a single moveable pulley, what power will support a weight of 80 lbs?

QUEST.-5. In the moveable pulley, what proportion exists between the cord and the weight?

Of the Mechanical Powers.

In two moveable pulleys with 5 cords, (see last fig.,) what power will support a weight of 100 lbs?

Ans. 20 lbs.

WHEEL AND AXLE.

1. This machine is composed of a wheel or crank

The power to the weight, as the radius of the axle, to the length of the crank, or radius of the wheel.

EXAMPLES.

1. What must be the length of a crank or radius of a wheel, in order that a power of 40 lbs. may balance

QUEST.-1. Of what is the machine called the wheel and axle composed? How is the axle supported? Give the proportion between the power and the weight?

Of the Mechanical Powers.

a weight of 600 lbs. suspended from an axle of 6 inches radius ?

Ans. 7 feet.

2. What must be the diameter of an axle that a power of 100 lbs. applied at the circumference of a wheel of 6 feet diameter may balance 400 lbs?

INCLINED PLANE.

Ans. 1 feet.

1. The inclined plane is nothing more than a slope or declivity, which is used for the purpose of raising weights. It is not difficult to see that a weight can be forced up an inclined plane, more easily than it can be raised in a vertical line. But in this, as in the other machines, the advantage is obtained by a partial loss of

ley at F, and the cord from the pulley to the weight be parallel to the length of the plane AB, the power P, will balance the weight W, when

P: W height BC: length AB.

QUEST.-1. What is an inclined plane? What proportion exists be tween the power and weight when they are in equilibrium ?

Of the Mechanical Powers.

It is evident that the power ought to be less than the weight, since a part of the weight is supported by the plane.

EXAMPLES.

1. The length of a plane is 30 feet, and its height 6 feet what power will be necessary to balance a weight of 200 lbs?

Ans. 40 lbs.

2. The height of a plane is 10 feet, and the length 20 feet: what weight will a power of 50 lbs. support? Ans. 100 lbs.

3. The height of a plane is 15 feet, and length 45 feet what power will sustain a weight of 180 lbs?

attraction of cohesion of the body which it is employed to separate. The wedge acts principally by being struck

QUEST.-1. What is the wedge? What is it used for? What resistance is it used to overcome?

Of the Mechanical Powers.

with a hammer, or mallet on its head, and very little ef fect can be produced by it, by mere pressure.

All cutting instruments are constructed on the principle of the inclined plane or wedge. Such as have but one sloping edge, like the chisel, may be referred to the nclined plane, and such as have two, like the axe and the knife, to that of the wedge.

THE SCREW.

1. The screw is composed of two parts-the screw S, and the nut N.

The screw S, is a cylinder with a spiral projection winding around it, called the thread. The nut N is perforated to admit the screw, and within it is a groove

into which the thread of the screw fits closely.

The handle D, which projects from the nut, is a lever which works the nut upon the screw. The power of the screw depends on the distance between the threads. The closer the threads of the screw, the greater will be

QUEST.-1. Of how many parts is the screw composed? Describe the screw? What is the thread? What the nut? What is the handle used for? To what uses, is the screw applied?