_To the Editor of the Scientific American_:
Although not a naval engineer, I wish to reply to some arguments
advanced by Capt. Giles, and published in the SCIENTIFIC AMERICAN of
Jan. 2, 1886, in regard to high speed on the ocean.
Capt. Giles argues that because quadrupeds and birds do not in
propelling themselves exert their force in a direct line with the plane
of their motion, but at an angle to it, the same principle would, if
applied to a steamship, increase its speed. But let us look at the
subject from another standpoint. The quadruped has to support the weight
of his body, and propel himself forward, with the same force. If the
force be applied perpendicularly, the body is elevated, but not moved
forward. If the force is applied horizontally, the body moves forward,
but soon falls to the ground, because it is not supported. But when the
force is applied at the proper angle, the body is moved forward and at
the same time supported. Directly contrary to Capt. Giles' theory, the
greater the speed of the quadruped, the nearer in a direct line with his
motion does he apply the propulsive force, and _vice versa_. This may
easily be seen by any one watching the motions of the horse, hound,
deer, rabbit, etc., when in rapid motion. The water birds and animals,
whose weight is supported by the water, do not exert the propulsive
force in a downward direction, but in a direct line with the plane of
their motion. The man who swims does not increase his motion by kicking
out at an angle, but by drawing the feet together with the legs
straight, thus using the water between them as a double inclined plane,
on which his feet and legs slide and thus increase his motion. The
weight of the steamship is already supported by the water, and all that
is required of the propeller is to push her forward. If set so as to act
in a direct line with the plane of motion, it will use all its force to
push her forward; if set so as to use its force in a perpendicular
direction, it will use all its force to raise her out of the water. If
placed at an angle of 45° with the plane of motion, half the force will
be used in raising the ship out of the water, and only half will be left
to push her forward.