mn
of air, an inch square, and of the atmospheric density, would require to
be, to weigh 15 lbs. The height would be 27,818 feet, and the velocity
which the fall of a body from such a height produces would be 1,338 feet
per second.

VELOCITY OF FALLING BODIES AND MOMENTUM OF MOVING BODIES.

15. _Q._--How do you determine the velocity of falling bodies of different
kinds?

_A._--All bodies fall with the same velocity, when there is no resistance
from the atmosphere, as is shown by the experiment of letting fall, from
the top of a tall exhausted receiver, a feather and a guinea, which reach
the bottom at the same time. The velocity of falling bodies is one that is
accelerated uniformly, according to a known law. When the height from which
a body falls is given, the velocity acquired at the end of the descent can
be easily computed. It has been found by experiment that the square root of
the height in feet multiplied by 8.021 will give the velocity.

16. _Q._--But the velocity in what terms?

_A._--In feet per second. The distance through which a body falls by
gravity in one second is 16-1/12 feet; in two seconds, 64-4/12 feet; in
three seconds, 144-9/12 feet; in four seconds, 257-4/12 feet, and so on. If
the number of feet fallen through in one second be taken as unity, then the
relation of the times to the spaces will be as follows:--

Number of seconds | 1| 2| 3| 4| 5| 6|
Units of space passed through | 1| 4| 9|16|25|36| &c.

so that it appears that the spaces passed through by a falling body are as
the squares of the times of falling.

17. _Q._--Is not the urging force which causes bodies to fall the force of
gravity?

_A._--Yes; the force of gravity or the attraction of the earth.

18. _Q._--And is not that a uniform force, or a force acting with a uniform
pressure?

_A._--It is.

19. _Q._--Therefore during the first second of falling as much impelling
power will be given by the force of gravity as during every succeeding
second?

_A._--Undoubtedly.

20. _Q._--How comes it, then, that while the body falls 64-4/12 feet in two
seconds, it falls only 16-1/12 feet in one second; or why, since it falls
only 16-1/12 feet in one second, should it fall more than twice 16-1/12
feet in two?

_A._--Because 16-1/12 feet is the average and not the maximum velocity
during the first second. The velocity acquired _at the end_ of the 1st
second is not 16-1/12, but 32-1/6 feet per second, and at