_Caroline._ What an idea! Were we not indebted to sir Isaac Newton for the theory of attraction, I should be tempted to laugh at him for such a supposition. What insignificant little creatures we should be! _Mrs. B._ If our consequence arose from the size of our bodies, we should indeed be but pigmies, but remember that the mind of Newton was not circumscribed by the dimensions of its envelope. _Emily._ It is, however, fortunate that heat keeps the pores of matter open and distended, and prevents the attraction of cohesion from squeezing us into a nut-shell. _Mrs. B._ Let us now return to the subject of reaction, on which we have some further observations to make. It is because reaction is in its direction opposite to action, that _reflected motion_ is produced. If you throw a ball against the wall, it rebounds; this return of the ball is owing to the reaction of the wall against which it struck, and is called _reflected motion_. _Emily._ And I now understand why balls filled with air rebound better than those stuffed with bran or wool; air being most susceptible of compression and most elastic, the reaction is more complete. _Caroline._ I have observed that when I throw a ball straight against the wall, it returns straight to my hand; but if I throw it obliquely upwards, it rebounds still higher, and I catch it when it falls. _Mrs. B._ You should not say straight, but perpendicularly against the wall; for straight is a general term for lines in all directions which are neither curved nor bent, and is therefore equally applicable to oblique or perpendicular lines. _Caroline._ I thought that perpendicularly meant either directly upwards or downwards? _Mrs. B._ In those directions lines are perpendicular to the earth. A perpendicular line has always a reference to something towards which it is perpendicular; that is to say, that it inclines neither to the one side or the other, but makes an equal angle on every side. Do you understand what an angle is? _Caroline._ Yes, I believe so: it is the space contained between two lines meeting in a point. _Mrs. B._ Well then, let the line A B (plate 2. 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, or the space between them. _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 spaces, making the angular point the centre. _Emily._ To what extent must I open the compasses?