Curve pitching is a scientific fact, the practice of which preceded the
discovery of its principle. For a long time after its existence was
familiar to every ball-player and spectator of the game, there were wise
men who proclaimed its impossibility, who declared it to be simply an
"optical delusion," and its believers the victims of the pitcher's
trickery. It was only after the curve had been practically demonstrated
to them, in a way which left no room for doubt, that they consented to
find for it a scientific explanation.
The discovery of the curve itself was purely an accident. During the
years from 1866 to 1869 the theory was held by many pitchers that the
more twist imparted to a pitched ball, the more difficult it would be to
hit it straight out. It was thought that even if it were struck fairly,
this twist would throw it off at an angle to the swing of the bat. One
writer on the game declared strongly against this practice of the
pitchers on the ground that, though this twist did do all that was
claimed for it, it at the same time caused the ball, when hit, to bound
badly, and thus interfered with good fielding. Of course, both of these
theories become absurd in the light of the present, but it was doubtless
the belief in the former that led to the introduction of the curve. In
1869 Arthur Cummings, pitching for the Star Club, noticed that by giving
a certain twist to the ball it was made to describe a rising, outward
curve, and his remarkable success with the new delivery soon led to its
imitation by other pitchers, and finally to the general introduction of
curve pitching.
The philosophy of the curve is, in itself, quite simple. A ball is
thrown through the air and, at the same time, given a rotary motion upon
its own axis, so that the resistance of the air, to its forward motion,
is greater upon one point than upon another, and the result is a
movement of the ball away from the retarded side. Suppose the ball in
the accompanying cut to be moving in the direction of the arrow, B C, at
the rate of 100 feet per second. Suppose, also, that it is rotating
about its vertical axis, E, in the direction of I to H, so that any
point on its circumference, I H D, is moving at the same rate of 100
feet per second. The point I is, therefore, moving forward at the same
rate as the ball's centre of gravity, that is, 100 feet per second, plus
the rate of its own revolution, which is 100 feet more, or 200 feet per
second; but the point D, though moving forward with the ball at the rate
of 100 feet per second, is moving backward the rate of rotation, which
is 100 feet per second, so that the forward motion of the point D is
practically zero. At the point I, therefore, the resistance is to a
point moving 200 feet per second, while at D it is zero, and the
tendency of the ball being to avoid the greatest resistance, it is
deflected in the direction of F.
In the Scientific American of August 28th, 1886, a correspondent gave a
very explicit demonstration of the theory of the curve, and, as it has
the virtue of being more scientific than the one given above, I append
it in full.
"Let Fig. 3 represent a ball moving through the air in the direction of
the arrow, B K, and at the same time revolving about its vertical axis,
U, in the direction of the curved arrow, C. Let A A A represent the
retarding action of the air acting on different points of the forward
half or face of the ball. The rotary motion, C, generates a current of
air about the periphery of the ball, a current similar to that caused by
the revolving flywheel of a steam engine.
"If, now, at a point on the face of the ball we let the arrow, R,
represent the direction and intensity of this rotary current of air, and
if at the same point we let the arrow, A, represent the direction and
intensity of the retarding action of the air, then we will find by
constructing a parallelogram of forces that the resultant or combined
effect of these two currents acts in the direction indicated by the
dotted arrow, T. In other words, we have a sort of compression, or force
of air, acting on the face of the ball in the direction indicated by the
arrow, T. This force, as we can readily see, tends, when combined with
the original impetus given to the ball, to deflect or cause time ball to
curve in the direction of the dotted line, B P, instead of maintaining
its right line direction, B K. If the ball rotate about its vert axis in
the opposite direction, the curve, B N, will be the result."
To the above demonstrations it is only necessary to add an explanation
of one other feature. The question has arisen why it is that the ball
apparently goes a part of its course in a straight line and then turns
off abruptly. One might suppose at first thought that the greater speed
at the beginning would create the greater resistance and consequently
cause the greatest deflection. This, however, is not true. The
difference between the resistance upon opposite points of the ball in
the circumference of its rotation always remains the same, no matter how
great the force of propulsion, and therefore the increased force of the
latter at the beginning has no effect on the curve. But while the force
of the twist itself is not affected by the rate of the forward movement,
its effect upon the ball is greatly nullified. The force of propulsion
being so great at first, drives the ball through the air and prevents it
from being influenced by the unequal resistance. It is only when the two
forces approach one another in strength that the latter begins to have a
perceptible effect. As soon, however, as it does, and the course of the
ball begins to change, the direction of the dotted arrow, T, begins to
change likewise. It follows the course of the ball around, and the more
it curves the more this resultant force tends to make it curve, and this
continues until the ball has lost either its twist or its forward
motion.
Having established the fact that a ball will curve in the direction of
the least resistance, it is only necessary to alter the direction of the
axis of rotation in order to change the direction of the curve. Thus, if
in the cut first given the ball were rotating in the direction of D H I
instead of I H D, the ball would curve, not toward F, but to the right.
So, also, if the axis of rotation is horizontal instead of vertical, and
the greatest resistance is made to come on top, the ball will curve
downward, or "drop." And in the same way, by imparting such a twist that
the resistance falls on some intermediate point the ball may be made to
take any of the combination curves known as the "outward drop," the
"rising out-curve," and so on through the entire category.