bodies are fastened together, by a line, string, chain, or any power whatever, they are to be considered as forming but one body; if the two bodies be of equal weight, the centre of gravity will be in the middle of the line which unites them, (fig. 7.) but if one be heavier than the other, the centre of gravity will be proportionally nearer the heavy body than the light one. (fig. 8.) If you were to carry a rod or pole with an equal weight fastened at each end of it, you would hold it in the middle of the rod, in order that the weights should balance each other; whilst if it had unequal weights at each end you would hold it nearest the greater weight, to make them balance each other.

Emily. And in both cases we should support the centre of gravity; and if one weight be very considerably larger than the other, the centre of gravity will be thrown out of the rod into the heaviest weight. (fig. 9.) Mrs. B. Undoubtedly.

CONVERSATION V.

ON THE MECHANICAL POWERS.

OF THE POWER OF MACHINES.—OF THE LEVER IN GENERAL.—OF THE LEVER OF THE FIRST KIND, HAVING THE FULCRUM BETWEEN THE POWER AND THE WEIGHT. OF THE LEVER OF THE SECOND KIND, HAVING THE WEIGHT BETWEEN THE POWER AND THE FULCRUM, OF THE LEVER OF THE THIRD KIND, HAVING THE POWER BETWEEN THE FULCRUM AND THE WEIGHT.

MRS. B.

WE may now proceed to examine the mechanical powers; they are six in number, one or more of which enters into the composition of every machine. The lever, the pulley, the wheel and axle, the inclined plane, the wedge, and the screw.

In order to understand the power of a machine, there are four things to be considered. 1st. The power that acts: this consists in the effort of men or horses, of weights, springs, steam, &c.

2dly. The resistance which is to be overcome by the power; this is generally a weight to be moved. The power must always be superior to the resistance, otherwise the machine could not be put in motion.

Caroline. If for instance the resistance of a carriage was greater than the strength of the horses employed to draw it, they would not be able to make it move.

Mrs. B. 3dly. We are to consider the centre of motion, or as it is termed in mechanics, the fulcrum ; this you may recollect is the point about which all the

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parts of the body move; and lastly, the respective velocities of the power, and of the resistance.

Emily. That must depend upou their respective distances from the axis of motion; as we observed in the motion of the vanes of the windmill.

Mrs. B. We shall now examine the power of the lever. The lever is an inflexible rod or beam of any kind, that is to say, one which will not bend in any direction. For instance, the steel rod to which these scales are suspended is a lever, and the point in which it is supported the fulcrum, or centre of motion; now, can you tell me why the two scales are in equilibrium? Caroline. Being both empty, and of the same weight, they balance each other.

Emily. Or, more correctly speaking, because the centre of gravity common to both is supported.

Mrs. B. Very well; and which is the centre of gravity of this pair of scales? (fig. 1. plate IV.)

Emily. You have told us that when two bodies of equal weight were fastened together, the centre of gravity was in the middle of the line that connected them ; the centre of gravity of the scales must therefore be in the fulcrum F of the lever which unites the two scales; and corresponds with the centre of motion.

Caroline. But if the scales contain different weights, the centre of gravity would no longer be in the fulcrum of the lever, but remove towards that scale which contained the heaviest weight; and since that point would no longer be supported, the heavy scale would descend and out-weigh the other.

Mrs. B. True; but tell me, can you imagine any mode by which bodies of different weights can be made to balance each other, either in a pair of scales, or simply suspended to the extremities of the lever? for the scales are not an essential part of the machine, they have no mechanical power, and are used merely for the convenience of containing the substance to be weighed. Caroline. What! make a light body balance a heavy one? I cannot conceive that possible.