Emily. In the same way, the flyers of a windmill, when put in motion by the wind, would be driven straight forwards in a right line, were they not confined to a fixed point round which they are compelled

to move.

Mrs. B. Very well. And observe, that the point to which the motion of a small body, such as the ball with the string, which may be considered as revolving in one plane, is confined, becomes the centre of its motion. But when the bodies are not of a size or shape to allow of our considering every part of them as moving in the same plane, they in reality revolve round a line, which line is call the axis of motion. In a top, for instance, when spinning on its point, the axis is the line which passes through the middle of it, perpendicularly to the floor.

Caroline. The axle of the flyers of the windmill, is then the axis of its motion; but is the centre of motion always in the middle of a body?

Mrs. B. No, not always. The middle point of a body, is called its centre of magnitude, or position, that is the centre of its mass or bulk. Bodies have also another centre, called the centre of gravity, which I shall explain to you; but at present we must confine ourselves to the axis of motion. This line you must observe remains at rest, whilst all the other parts of the body move around it; when you spin a top the axis is stationary whilst every other part is in motion round

it.

Caroline. But a top generally has a motion forwards, besides its spinning motion; and then no point within it can be at rest?

Mrs. B. What I say of the axis of motion, relates only to circular motion; that is to say, to motion round a line, and not to that which a body may have at the same time in any other direction. There is one circumstance in circular motion, which you must carefully attend to; which is, that the further any part of a body is from the axis of motion, the greater is its velocity; as you approach that line, the velocity of the parts gradually diminish till you reach the axis of motion, which is perfectly at rest.

Caroline. But, if every part of the same body did not move with the same velocity, that part which moved quickest, must be separated from the rest of the body, and leave it behind?

Mrs. B. You perplex yourself by confounding the idea of circular motion, with that of motion in a right line; you must think only of the motion of a body round a fixed line, and you will find, that if the parts farthest from the centre had not the greatest velocity, those parts would not be able to keep up with the rest of the body, and would be left behind. Do not the extremities of the vanes of a windmill move over a much greater space, than the parts nearest the axis of motion? (pl. III. fig. 1.) the three dotted circles describe the paths in which three different parts of the vanes move, and though the circles are of different dimensions the vanes describe each of them in the same space of time.

Caroline. Certainly they do; and I now only wonder, that we neither of us ever made the observation before and the same effect must take place in a solid

body, like the top in spinning; the most bulging part of the surface must move with the greatest rapidity.

Mrs. B. The force which confines a body to a centre, round which it moves is called the centripetal force; and that force, which impels a body to fly from the centre, is called the centrifugal force; in circular motion these two forces constantly balance each other; otherwise the revolving body would either approach the centre, or recede from it, according as the one or the other prevailed.

Caroline. When I see any body moving in a circle, I shall remember, that it is acted on by two forces.

Mrs. B. Motion, either in a circle, an ellipsis, or any other curve-line, must be the result of the action of two forces; for you know, that the impulse of one single force, always produces motion in a right line.

Emily. And if any cause should destroy the centripetal force, the centrifugal force would alone impel the body, and it would I suppose fly off in a straight line from the centre to which it had been confined.

Mrs. B. It would not fly off in a right line from the centre; but in a right line in the direction in which it was moving, at the instant of its release; if a stone, whirled round in a sling, gets loose at the point A (plate III. fig. 2.) it flies off in the direction A B; this line is called a tangent, it touches the circumference of the circle, and forms a right angle with a line drawn from that point of the circumference to the centre of the circle, C.

Emily. You say, that motion in a curve-line, is owing to two forces acting upon a body; but when I throw this ball in a horizontal direction, it describes a curve