_Caroline._ But if a man strike another on the face with his fist, he surely does not receive as much pain by the reaction, as he inflicts by the blow? _Mrs. B._ No; but this is simply owing to the knuckles, having much less feeling than the face. Here are two ivory balls suspended by threads, (plate 1. fig. 3.) draw one of them, A, a little on one side,--now let it go;--it strikes, you see, against the other ball B, and drives it off, to a distance equal to that through which the first ball fell; but the motion of A is stopped; because when it struck B, it received in return a blow equal to that it gave, and its motion was consequently destroyed. _Emily._ I should have supposed, that the motion of the ball A was destroyed, because it had communicated all its motion to B. _Mrs. B._ It is perfectly true, that when one body strikes against another, the quantity of motion communicated to the second body, is lost by the first; but this loss proceeds from the reaction of the body which is struck. Here are six ivory balls hanging in a row, (fig. 4.) draw the first out of the perpendicular, and let it fall against the second. You see none of the balls except the last, appear to move, this flies off as far as the first ball fell; can you explain this? _Caroline._ I believe so. When the first ball struck the second, it received a blow in return, which destroyed its motion; the second ball, though it did not appear to move, must have struck against the third; the reaction of which set it at rest; the action of the third ball must have been destroyed by the reaction of the fourth, and so on till motion was communicated to the last ball, which, not being reacted upon, flies off. _Mrs. B._ Very well explained. Observe, that it is only when bodies are elastic, as these ivory balls are, and when their masses are equal, that the stroke returned is equal to the stroke given, and that the striking body loses all its motion. I will show you the difference with these two balls of clay, (fig. 5.) which are not elastic; when you raise one of these, D, out of the perpendicular, and let it fall against the other, E, the reaction of the latter, on account of its not being elastic, is not sufficient to destroy the motion of the former; only part of the motion of D will be communicated to E, and the two balls will move on together to _d_ and _e_, which is not so great a distance as that through which D fell. Observe how useful reaction is in nature. Birds in flying strike the air with their wings, and it is the reaction of the air, which enables them to rise, or advance forwards; reaction being always in a contrary direction to action. _Caroline._ I thought that birds might be lighter than the air, when their wings were expanded, and were by that means enabled to fly.