“Certainly; the former of these forces is, therefore, termed the _centripetal_, because it draws the body towards the centre, while the latter is called the _centrifugal_ force, since its influence disposes the body to fly off from the centre. In circular motion, these two forces constantly balance each other; otherwise it is evident that the revolving body must either approach the centre or recede from it, according as the one or the other prevailed. When I whirled round the sling, I imparted a projectile force to the stone, but it was prevented from flying off in consequence of the counteracting or _centripetal_ force of the string; but the moment I let go my hold of this, the stone flew off in a right line: having been released from confinement to the fixed or central point, it was acted upon by one force only, and motion produced by a single force is, as you have just stated, always in a right line.” “But,” observed Louisa, “the stone did not proceed in a straight, but in a curved line: I watched its direction from the moment it left the sling till it fell to the ground.” “You are perfectly correct,” replied Mr. Seymour, “it described a curve, which is called a _parabola_; but that was owing to the influence of a new force which came into play, viz. that of gravity, the effect of which I shall have to explain hereafter.” “I cannot understand,” said Tom, “why the stone should not have fallen out of the sling when you whirled it round over your head.” “Because, my dear, it was acted upon by the _centrifugal_ force, which counteracted that of gravity: but I will render this fact more evident, by a very simple and beautiful experiment. I have here a wine-glass, around the rim of which I shall attach a piece of string so as to enable me to whirl it round. I will now fill it with water, and although during one part of its revolution it will be actually inverted, you will find that I shall not spill a single drop of water.” Mr. Seymour then whirled round the glass, and the young party were delighted with the confirmation thus afforded to their father’s statement. “I see,” said Tom, “how it happened: when the glass was inverted the water could not fall out, because it was influenced by the centrifugal force which opposed gravity.” “Exactly. Have you ever observed what happens during the trundling of a mop? The threads which compose it fly off from the centre, but being confined to it at one end they cannot part from it: while the water which they contain being unconfined, is thrown off in right lines.” “I have certainly observed what you state,” said Louisa; “the water flies off in all directions from the mop.” “Yes,” added Tom, “the water was not acted upon by the _centripetal_ force as the threads were, and consequently, there was nothing to check the _centrifugal_ force, which carried the water off in a straight line from the centre.”