description raised from the groudd 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.

EMILY.

The hinges represent the fulcrum, our hands the power applied to the other end of the lever; but where is the weight to be moved?

MRS. B.

The door is the weight, and it consequently occupies the whole of the space between the power and the fulcrum. The whole weight 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 weight of the door, and passing through its centre of gravity. Another very common instance is found in an oar; the blade is kept in the same place by the resistance of the water and becomes the fulcrum; the resistance is applied where the oar passes over the side of the boat, and the hands at the handle are the power. Nutcrackers are double levers of this kind: the hinge is the fulcrum, the nut the resistance, and the hands the power.

In levers of the third kind (fig. 8.) the fulcrum is also at one of the extremities, the weight or

resistance at the other, and it is now the power which is applied between the fulcrum and the resistance.

EMILY.

The fulcrum, the weight, and the power, then, each in their turn, occupy some part of the middle of the lever between its extremities. But in this third kind of lever, the weight being farther from the centre of motion than the power, the difficulty of raising it is increased instead of diminished.

MRS. B.

Levers of this kind are used only when the object is to produce great velocity. In general the object in mechanics is to gain force by exchanging it for time; but it is, on the contrary, sometimes desirable to produce great velocity by. an expenditure of force. The common turning lathe affords an example of a lever of the third kind, employed in gaining time or velocity at the expense of force. A man in raising a ladder perpendicularly against a wall cannot place his hands on the upper part of the ladder; the power, therefore, is necessarily much nearer the fulcrum than the weight.

CAROLINE.

True; the hands are the power, the ground the fulcrum, and the upper part of the ladder the weight.

G

MRS. B.

Yes; as in the case of the door, the weight may be considered as collected in the centre of gravity of the ladder, about half way up it, and consequently beyond the point where the hands are applied. Nature employs this kind of lever in the structure of the human frame. In lifting a weight with the hand, the lower part of the arm becomes a lever of the third kind; the elbow is the fulcrum; the muscles which move the arm, the power; and as these are nearer to the elbow than the hand, it is necessary that their power should exceed the weight to be raised.

EMILY.

Is it not surprising that nature should have furnished us with such disadvantageous levers ?

MRS. B.

The disadvantage in respect to power is more than compensated by the convenience resulting from this structure of the arm. It is of more consequence that we should be able to move our limbs nimbly, than that we should be able to overcome great resistance with them, as it is comparatively seldom that we meet with great obstacles, and when we do, they can be overcome by art. Besides, the Creator has endowed the muscular fibres with prodigious strength, so that, upon the whole,

this kind of lever is best adapted to enable the arm to perform its various functions.

We have dwelt so long on the lever, that we must reserve the examination of the other mechanical powers to our next interview.

124

CONVERSATION VI.

ON THE MECHANICAL POWERS.

OF THE PULLEY.-OF THE WHEEL AND AXLE.

THE INCLINED PLANE. OF THE WEDGE. OF THE SCREW.

MRS. B.

THE pulley is the second mechanical power we are to examine. You both, I suppose, have seen a pulley?

CAROLINE.

Yes, frequently: it is a circular and flat piece of wood or metal, with a string running in a groove round it; by means of which, a weight may be pulled up. Thus pulleys are used for drawing up curtains, and sails of ships.

MRS. B.

Yes; but in these instances the pulleys are fixed, and do not increase the power to raise the weights, as you will perceive by this figure. (Plate V. fig. 1.) If P represents the power employed to raise the