, 1883-06-07
RFFECTS OF GRATTrY ON PRO-
JECIILES.

JKCIILES
When any body-a stone, for example
is thrown straight upward into the
air, it does not. in reality. go up or come
down vertically. II 1t did, it would
come down at q great distance from US.
Suppose it takes two seconds for it to go
up and to reach the ground. II we
stand at the equator. in that two seconds
We move from the point where We threw
up the stone nearly 3, 200 feet eastward:
and, therefore, if the stone rose and fell
vertically, it would fall 5000 feet west
ward of US. Why, instead of this, does
1t fall at our feet? Because when thrown
into the air 1t not only has the upward
motion given by ihe hand, but also the
forward motion of the earth. 1t is
case similar to that of @ man on board
of a steamboat, who, though the vessel
move fifteen miles an hour, tosses up his
ball or orange and catches it as we'll as
if he were on land. This he could not
have the same forward motion as the
boat. II man fall from masthead,
he reaches the deck at the foot of the
mast when the vessel is sailing rapidly
just IS if it were lying still at the wharf.
If he did not by inertia retain the for-
ward motion which he had in common
with the vessel, he would fall at some
distance behind the mast. The air being
held to the earth by attraction, it has a
motion in common with the earth. 1t
revolves with the earth Just as the tire
of a wheel revolves with wheel. This
being so, our winds are nothing but
slight variations of this constant rapid
whirl of the aerial coating of the earth.
If the atmosphere were suddenly to stop
whirling round with the earth, we should
move through it with velocity of 1,500.
feet second; and the destructive effect
upon US would be the same as if the
earth were standing still while the air
moved over its surface with this fearful
velocity. A thoughtless man, not re-
hecting that the atmosphere moved with
the earth, proposed rising in q balloon
and waiting till the country to which he
wished to go should pass under him, and
then to descend to the earth. II We
consider the connection between the
motion of the earth and the course of q
body thrown into the air, and ascertain
its actual path, We find that it forms a
peculiar curve. The path which it fol-
lows is that of q parabola. Suppose O
stone be thlown by a man, it will deviate
from straight line on account of the
attraction of gravity, and actually de-
scribe the parabolic curve. OF course,
it is underctood that, besides the propel-
ling force of the arm and the attraction
of gravity, thIrd force acts upon the
stone, viz. the resistance of the air; but,
this being in direct opposition to the
first, it only retards the motion, and
does not tend to turn it from its straight
course. II the stone be thrown hori-
zontally, 1t also describes q parabola. Ii
the propulsive force be very great. as in
the case of bullet discharged from q
gun, the path will appear to be straight;
but this is not so. The force of gravity
pulls the bullet toward the ground from
the instant that it leaves ths gun. This
deviation is very slight, however, and
for short distances the bullet may be
considered as moving in straight line.
When, however, marksman shoots at
long range, ne must make allowance for
this bending down of the motion. Ac-
',ordingly. for the sake of precision, a
double sight is provided in modern guns.
This arrangement secures the pointing
of the gun a little above the level OF the
object simed at.