hip.

599. _Q._--What is the nature of Beattle's screw?

_A._--Beattie's screw is an arrangement of the screw propeller whereby it
is projected beyond the rudder, and the main object of the arrangement is
to take away the vibratory motion at the stern,--an intention which it
accomplishes in practice. There is an oval eye in the rudder, to permit the
screw shaft to pass through it.

600. _Q._--When the diameter of the cylinder of water projected backward by
a screw, and the force urging it into motion are known, may not the
velocity it will acquire be approximately determined?

_A._--That will not be very difficult; and I will take for illustration the
case of the Minx, already referred to, which will show how such a
computation is to be conducted. The speed of this vessel, in one of the
experiments made with her, was 8.445 knots; the number of revolutions of
the screw per minute, 231.32; and the pressure on each square foot of area
of the screw's disc, 214 lbs. If a knot be taken to be 6075.6 feet, then
the distance advanced by the vessel, when the speed is 8.445 knots, will be
3.7 feet per revolution, and this advance will be made in about .26 of a
second of time. Now the distance which a body will fall by gravity, in .26
of a second, is 1.087 feet; and a weight of 214 lbs. put into motion by
gravity, or by a pressure of 214 lbs., would, therefore, acquire a velocity
of 1.087 feet during the time one revolution of the screw is being
performed. The weight to be moved, however, is 3.7 cubic feet of water,
that being the new water seized by the screw each revolution for every
square foot of surface in the screw's disc; and 3.7 cubic feet of water
weigh 231.5 lbs., so that the urging force of 214 lbs. is somewhat less
than the force of gravity, and the velocity of motion communicated to the
water will be somewhat under 1.087 feet per revolution, or we may say it
will be in round numbers 1 foot per revolution. This, added to the progress
of the vessel, will make the distance advanced by the screw through the
water 4.7 feet per revolution, leaving the difference between this and the
pitch, namely 1.13 feet, to be accounted for on the supposition that the
screw blades had broken laterally through the water to that extent. It
would be proper to apply some correction to this computation, which would
represent the increased resistance due to the immersion of the screw in the
water; for a column of water cannot be moved in the