raction in so far, at least, as
that they are not drawn thereby to total destruction.

It is so desirable that the student should understand clearly how a
central attraction is compatible with revolution in a nearly circular
path, that we give an illustration to show how the moon pursues its
monthly orbit under the guidance and the control of the attracting
earth.

[Illustration: Fig. 36.--Illustration of the Moon's Motion.]

The imaginary sketch in Fig. 36 denotes a section of the earth with a
high mountain thereon.[12] If a cannon were stationed on the top of the
mountain at C, and if the cannonball were fired off in the direction C E
with a moderate charge of powder, the ball would move down along the
first curved path. If it be fired a second time with a heavier charge,
the path will be along the second curved line, and the ball would again
fall to the ground. But let us try next time with a charge still further
increased, and, indeed, with a far stronger cannon than any piece of
ordnance ever yet made. The velocity of the projectile must now be
assumed to be some miles per second, but we can conceive that the speed
shall be so adjusted that the ball shall move along the path C D, always
at the same height above the earth, though still curving, as every
projectile must curve, from the horizontal line in which it moved at the
first moment. Arrived at D, the ball will still be at the same height
above the surface, and its velocity must be unabated. It will therefore
continue in its path and move round another quadrant of the circle
without getting nearer to the surface. In this manner the projectile
will travel completely round the whole globe, coming back again to C and
then taking another start in the same path. If we could abolish the
mountain and the cannon at the top, we should have a body revolving for
ever around the earth in consequence of the attraction of gravitation.

Make now a bold stretch of the imagination. Conceive a terrific cannon
capable of receiving a round bullet not less than 2,000 miles in
diameter. Discharge this enormous bullet with a velocity of about 3,000
feet per second, which is two or three times as great as the velocity
actually attainable in modern artillery. Let this notable bullet be
fired horizontally from some station nearly a quarter of a million miles
above the surface of the earth. That fearful missile would sweep right
round the earth in a nearly circular orbit, and return t