ERY TWENTY-FIRST EVENING.

P. True. That is a consequence of the universa:
law in nature, that bodies attract each other in pro-
portion to their bulk. So a very small thing in the
neighbourhood of a very large one always tends to go
to it, if not prevented by some other power. Well—
you know J told you that the sun was a ball, a vast
many times bigger than the ball we inhabit, called the
earth ; upon which you properly asked, how, then, 1t
happened that the earth did not fall into the sun.

i. And why does it not?

P, That I am going to explain to you. You have
seen your brother whirl round an ivory ball, tied to the
end of a string which he held in his hand.

ZL. Yes; and I have done it myself, too.

P, Well, then—you felt that the ball was con-
tinually puiling, as though it tried to make its escape.

L. Yes; andone my brother was swinging did make
its escape, and flew through the sash.

P. It did so. That was a lesson in the centrifugal
motion, or that power by which a body thus whirled
continually endeavours to fly off from the centre round
which it moves. This is owing to the force or impulse
you give it at setting out, as though you were going
to throw it away from you. The string by which you
hold it, on the contrary, is the power which keeps the
ball towards the centre, called the centripetal power.
Thus, you see, there are two powers acting upon the
ball at the same time; one to make it fly off, the other
to hold it m; and the consequence is, that it moves
directly according to neither, but between both; that
is, round and round. This it continues to do while
you swing it properly ; but should the string break or
slip off, away flies the ball; on the other hand, if you
cease to give it the whirling force, it falls towards your
nand.

£, I understand all this.

P. 1 will give you another instance of this double
force acting at the same time. Do not you remember
seeing some curious feats of horsemanship ?