“You are not quite correct,” said Mr. Seymour; “the water does 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; the line which a body will always describe under such circumstances, is called a _tangent_, because 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: but I will render this subject more intelligible by a diagram. “Suppose a body, revolving in the circle, was liberated at _a_, it would fly off in the direction _ab_; if at _c_, in that of _cd_; and if at _e_, in that of _ef_; and so on. Now, if you draw lines from these several points to the centre of the circle, you will perceive that such lines will form, in each case, a right-angle. In the experiment which you have just witnessed, the surface of the water must have formed, during its revolution, a right-angle with the string, and consequently could not have fallen out of the wine-glass. A knowledge of this law,” continued Mr. Seymour, “will explain many appearances which, although familiar, I dare say, have never been understood by you. You may remember accompanying me to the pottery, to see the operation of the turning-lathe; it was owing to the centrifugal force produced by the rotation of the wheel, that the clay, under a gentle pressure, swelled out so regularly; from a similar cause, the flour is thrown out of the revolving mill as fast as it is ground; and I shall presently show you that you are indebted to this same force for the spinning of your top and the trundling of your hoop. But let us quit this subject for the present, and pursue the stone in its course after it is liberated from the sling. Louisa has justly observed that it described a curve; can you explain why it should deviate from a straight line?” [Illustration: Fig. 8. Diagram for centrifugal force.] “Let me see,” said Tom, thoughtfully; “it would be acted upon by two forces, one carrying it forward in a right line, the other bringing it to the earth; it would, therefore, not obey either, but describe a diagonal: but why that diagonal should be a curve I cannot exactly explain.”