Mrs. B. Well then, let the line A B (plate II. fig. 1.) represent the floor of the room, and the line C D that in which you throw a ball against it; the line C D, you will observe, forms two angles with the line A B, and those two angles are equal.

Emily. How can the angles be equal, while the lines which compose them are of unequal length?

Mrs. B. An angle is not measured by the length of the lines, but by their opening.

Emily. Yet the longer the lines are, the greater is the opening between them.

Mrs. B. Take a pair of compasses and draw a circle over these angles, making the angular point the

centre.

Emily. To what extent must I open the compas

ses?

Mrs. B. You may draw the circle what size you please, provided that it cuts the lines of the angles we are to measure. All circles, of whatever dimensions, are supposed to be divided into 360 equal parts, called degrees; the opening of an angle, being therefore a portion of a circle, must contain a certain number of degrees the larger the angle, the greater the number of degrees, and the two angles are said to be equal when they contain an equal number of degrees.

Emily. Now I understand it. As the dimensions of an angle depend upon the number of degrees contained between its lines, it is the opening, and not the length of its lines, which determines the size of the angle.

Mrs. B. Very well: now that you have a clear idea of the dimensions of angles, can you tell me how many degrees are contained in the two angles formed by one line falling perpendicular on another, as in the figure I have just drawn?

Emily. You must allow me to put one foot of the compasses at the point of the angles, and draw a circle round them, and then I think I shall be able to answer your question: the two angles are together just equal

to half a circle, they contain therefore 90 degrees each; 90 degrees being a quarter of 360.

Mrs. B. An angle of 90 degrees is called a right angle, and when one line is perpendicular to another, it forms, you see, (fig. 1.) a right angle on either side. Angles containing more than 90 degrees are called obtuse angles (fig. 2.); and those containing less than 90 degrees are called acute angles, (fig. 3.)

Caroline. The angles of this square table are right angles, but those of the octagon table are obtuse angles; and the angles of sharp-pointed instruments are acute angles.

Mrs. B. Very well. To return now to your observation, that if a ball is thrown obliquely against the wall it will not rebound in the same direction; tell me, have you ever played at billiards?

Caroline. Yes, frequently; and I have observed that when I push the ball perpendicularly against the cushion, it returns in the same direction; but when I send it obliquely to the cushion, it rebounds obliquely, but on the opposite side; the ball in this latter case describes an angle, the point of which is at the cushion. I have observed too, that the more obliquely the ball is struck against the cushion, the more obliquely it rebounds on the opposite side, so that a billiard player can calculate with great accuracy in what direction it will return.

Mrs. B. Very well. This figure (fig. 4. plate II.) represents a billiard table; now if you draw a line A B from the point where the ball A strikes perpendicular to the cushion; you will find that it will divide the angle which the ball describes into two parts, or two angles; the one will show the obliquity of the direction of the ball in its passage towards the cushion, the other its obliquity in its passage back from the cushion. The first is called the angle of incidence, the other the angle of reflection, and these angles are always equal.

Caroline. This then is the reason why, when I throw a ball obliquely against the wall, it rebounds in an opposite oblique direction, forming equal angles of incidence and of reflection.

Mrs. B. Certainly; and you will find that the more obliquely you throw the ball, the more obliquely it will rebound.

We must now conclude; but I shall have some further observations to make upon the laws of motion, at our next meeting.

CONVERSATION IV.

ON COMPOUND MOTION.

COMPOUND MOTION, THE RESULT OF TWO OPPOSITE FORCES. OF CIRCULAR MOTION, THE RESULT OF TWO FORCES, ONE OF WHICH CONFINES THE BODY TO A FIXED POINT.-CENTRE OF MOTION, THE POINT AT REST WHILE THE OTHER PARTS OF THE BODY MOVE ROUND IT.-CENTRE OF MAGNITUDE, THE MIDDLE OF A BODY.-CENTRIPETAL FORCE, THAT WHICH CONFINES A BODY TO A FIXED CENTRAL POINT.-CENTRIFUGAL FORCE, THAT WHICH IMPELS A BODY TO FLY FROM THE CENTRE. FALL OF BODIES IN A PARABOLA.—CENTRE OF GRAVITY, THE CENTRE OF WEIGHT, OR POINT ABOUT WHICH THE PARTS BALANCE EACH OTHER.

MRS. B.

I MUST now explain to you the nature of compound motion. Let us suppose a body to be struck by two equal forces in opposite directions, how will it move?

Emily. If the directions of the forces are in exact opposition to each other, I suppose the body would not move at all.

Mrs. B. You are perfectly right; but if the forces, instead of acting on the body in opposition, strike it in two directions inclined to each other, at an angle of ninety degrees, if the ball A (fig. 5, plate II.) be struck by equal forces at X and at Y, will it not move?

Emily. The force X would send it towards B, and the force Y towards C; and since these forces are equal, I do not know how the body can obey one impulse rather than the other, and yet I think the ball