again till it has touched some other body;4. If the tube be rubbed by a moist hand, or any thing that is wet, it totally destroys the electricity;-5. Any body, except air, being interposed, stops the electricity;6. The tube attracts stronger, when rubbed over with bees-wax, and then with a dry woollen cloth; -7. When it is well rubbed, if a finger be brought near it, at about the distance of half an inch, the effluvia will snap against the finger, and make a little crackling noise; and if this be performed in a dark place, there will appear a little flash of light.

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PLATE II.
All bodies

to motion or rest. Fig. I.

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LECTURE II.

Of Central Forces.

WE have already mentioned it as a neequally in-cessary consequence arising from the deadness different or inactivity of matter, that all bodies resist any change of state, or endeavour to continue in the state they are in, whether of rest or motion. If the body A were placed in any part of free space, and nothing either drew or impelled it any way, it would for ever remain in that part of space; because it could have no tendency of itself to remove from it. If it receive a single impulse any way, as suppose from A toward B, it will go on in that direction; for,. of itself, it could never swerve from a right line, nor stop its own course. When it has gone through the space A B, and met with no resistance, its velocity will be the same at B as it was at A; and this velocity, in as much more time, will carry it through as much more space, from B to C; and so on for ever. Therefore, when we see a body in motion, we conclude that some other substance must have given it that motion; and when we see a body fall from motion to rest, we conclude that some other body or cause must have stopped it.

All motion

rectili

As all motion is naturally rectilineal, it apnaturally pears, that a bullet projected by the hand, or shot from a cannon, would forever continue to move in the same direction it received at first, if no other power diverted it from its course.

neal,

Therefore, when we see a body move in a curve PLATE. II. of any kind whatever, we conclude it must be acted upon by two powers at least; one putting it in motion, and another drawing it off from the rectilineal course it would otherwise have continued to move in: and whenever that power, which bent the motion of the body from a straight line into a curve, ceases to act, the body will again move on in a straight line, touching that point of the curve in which it was when the action of that power ceased.For example, a pebble moved round in a sling ever so long a time, will fly off the moment it is set at liberty, by slipping one end of the sling-cord, and will go on in a line touching the circle it described before; which line would actually be a straight one, if the earth's attraction did not affect the pebble, and bring it down to the ground. This shows, that the natural tendency of the pebble, when put into motion, is to continue moving in a straight line, although by the force that moves the sling it is made to revolve in a circle.

combined

The change of motion produced is in pro- The ef portion to the force impressed; for the effects fects of of natural causes are always proportionate to forces. the force or power of those causes.

By these laws it is easy to prove, that a body will describe the diagonal of a square or parallelogram, by two forces conjoined, in the same time that it would describe either of the sides, by one force singly. Thus, suppose the body A to represent a ship at sea; and that it Fig. 2. is driven by the wind, in the right line A B, with such a force as would carry it uniformly from A to B in a minute: then suppose a stream or current of water running in the di- . rection A D, with such a force as would carry

Fig. 3.

PLATE II. the ship through an equal space from A to D in a minute. By these two forces, acting together at right angles to each other, the ship will describe the line AC in a minute: which line (because the forces are equal and perpendicular to each other) will be the diagonal of an exact square. To confirm this law by an experiment, let there be a wooden square ABCD so contrived, as to have the part BEFC made to draw out or push into the square at pleasure. To this part let the pulley H be joined, so as to turn freely on the axis, which will be at H when the piece is pushed in, and at h when it is drawn out. To this part let the ends of a straight wire k be fixed, so as to move along with it under the pulley; and let the ball G be made to slide easily on the wire. A thread m is fixed to this ball, and goes over. the pulley to I; by this thread the ball may be drawn up on the wire parallel to the side AD, when the part BEFC is pushed as far as it will go into the square. But, if this part be drawn out, it will carry the ball along with it, parallel to the bottom of the square DC. By this means, the ball G may either be drawn perpendicularly upward by pulling the thread m, or moved horizontally along by pulling out the part BEFC, in equal times, and through equal spaces; each power acting equally and separately upon it. But if, when the ball is at G, the upper end of the thread be tied to the pin I, in the corner A of the fixed square, and the moveable part BEFC be drawn out, the ball will then be acted upon by both the powers together; for it will be drawn up by the thread toward the top of the square, and at the same time be carried with its wire k toward the right hand BC, moving all the while in

the diagonal line L; and will be found at g PLATE II. when the sliding part is drawn out as far as it was before; which will then have caused the thread to draw up the ball to the top of the inside of the square, just as high as it was before, when drawn up singly by the thread without moving the sliding part.

If the acting forces be equal, but at oblique angles to each other, the sides of the parallelogram will be so also; and the diagonal run through by the moving body will be longer or shorter, according as the obliquity is greater or smaller. Thus, if two equal forces act conjointly upon the body A, one having a tenden- Fig. 4. cy to move it through the space AB in the same time that the other has a tendency to move it through an equal space AD; the body will describe the diagonal AGC in the same time that either of the single forces would have caused it to describe either of the sides. If one of the forces be greater than the other, then one side of the parallelogram will be so much longer than the other. For, if one force singly would carry the body through the space AE, in the same time that the other would have carried it through the space AD, the joint action of both will carry it in the same time through the space AHF, which is the diagonal of the oblique parallelogram ADEF.

If both forces act upon the body in such a manner, as to move it uniformly, the diagonal described will be a straight line; but if one of the forces acts in such a manner as to make the body move faster and faster, then the line described will be a curve. And this is the case of all bodies which are projected in rectilineal directions, and at the same time acted upon by the power of gravity; which has a constant

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