able amount of motion, and will, therefore, meet with less resistance than the fore part; the effect of this will be to twist the blade and increase the pitch. If the vessel be attached to a dynamometer, and we calculate the effect without allowing for this increase of pitch, we shall arrive at an erroneous result.

This increase of pitch must also take place when the vessel is under weigh, and will explain that strange anomaly "negative slip;" for if, while the screw goes one turn, the vessel goes the distance IH, we have a negative slip IG, but if we suppose the screw-blade to "spring" to the dotted line FA, we then have a positive slip FI.

A

H

To show that the power cannot move slower than the vessel, let us take the paddle-wheel, and let us suppose, if it were possible, that the velocity of the wheel was exactly equal to the velocity of the vessel, then, evidently, the paddles would be relatively at rest, and could have no propelling power. If they moved slower, it is equally evident that the paddles would retard the motion of the vessel. Now, if this be true, how is it possible that any vessel can move faster than the power that is moving it?

As regards the pitch connected with the negative slip of the screw, we shall take the Plumper steam-vessel. On December 1st, 1848, in Stokes's Bay, Portsmouth, the speed of the screw in knots per hour was 6-188, and the

speed of the vessel was 6-497, which would show a negative slip of 329. The diameter of the screw is 9 feet; the pitch, 5 ft. 7 in.; length, 1 ft. Revolutions, 112 per minute.

Now, if we suppose that, instead of the pitch remaining at 5 ft. 7 in., the screw is sprung so as to be 6 ft. pitch,

Then 112 x 6 x 60

40320 feet per hour.

which shows a positive slip of ·134.

From the various enquiries I have made of practical men, I find that, in screws of great stiffness, no negative slip is observed; but where the screws are comparatively slight, there appears to be a negative slip; in fact, it has been ascertained that some vessels have had the screws so slightly made that they have actually broken, which shows that previously to breaking the pitch must have increased, and would therefore indicate negative slip.

On so important a subject as the screw, too many observations and experiments cannot be made, so as not only to set the question of negative slip at rest, but many other questions which seem not as yet to have been fully elucidated.

Mr. Holland is a person well versed both in theory and practice, and possesses keen observing powers, therefore his opinion—to say the least—is worthy of respect; but I am quite sure that he is open to conviction, if any one can show him that his views are fallacious. He would rather court inquiry than shrink from it, and it has been beautifully observed by one of our English writers, that "inquiry is to truth what friction is to the diamond: it proves its hardness, adds to its lustre, and excites new admiration." I am, Sir,

Your obedient Servant,

King's College, March 4, 1853.

JAMES HANN.

Professor Woodcroft was the first to introduce the screw blade with a rising pitch, that is, with a pitch which varies at every point along the extremity of the screw blade.

He also has the merit of being the first to introduce another variation of considerable importance, which is to` turn the screw blades through an angle in opposite directions. This variation, which is an admirable expedient to overcome a great difficulty, he thinks will produce an effect in the velocity of the vessel, somewhat similar to that produced by increasing or diminishing the pitch in the constant pitch surface.

This variation is not useful in the screw propeller with a rising pitch only, but also in the one with a constant pitch. Suppose the blade of the screw to have its extreme helix making an angle of 20°, and Professor Woodcroft's apparatus to be applied in such a manner as to turn this helix through every angle to 15°; then, if we could persuade ourselves that the screw blade thus moved from its first position to its second fully coincided with a screw blade whose angle is 15°, the variation to which we allude would be a great improvement in the art of screw propulsion. If the angle through which this variation takes place be small, the screw blade, in its second position, will nearly coincide with a screw blade having the same angle; but generally this coincidence does not take place, and the relation of the screw blade to the axis of rotation in the one case is not the same as in the other.

In the screw the pitch is proportional to the tangent of the angle of the screw.

Rule.-Multiply the tangent of the angle of the screw by 3.1416 and by the diameter of the cylinder; the result is the pitch.

Example.-Given the radius of the cylinder 8 feet, the angle of the screw 17° 30′, to find the pitch.

Then, by the rule,

2 x 3.14156 × 8 × tan 17°.50

=50-26496 x tan 17°.50.

.. 50·26496 × 3152988 15.85 nearly.

In the same ship and the same screws, the horses power varies as the cube of the velocity of the ship.

In the same ship and different screws, the horses' power varies as the rectangle of the pitch and square of the velocity.

ON WINDING ENGINES.

In winding engines drawing coals out of a pit, where we intend them to go a given number of strokes in drawing a corf, we must ascertain the diameter of the roll at first lift. In this case, we suppose the engine to have flat ropes, such as are generally used, and which lie upon each other.

To find the diameter of a rope roll at the first lift, it is necessary to know the depth of the pit, the thickness of the rope, and the number of strokes which you intend the engine to make in drawing up a corf or corves.

Then, the thickness of the rope being known, and the number of strokes, we can determine the thickness of rope upon the roll, let the diameter of the roll be what it may. Thus, suppose the thickness of the rope to be 1 inch, and the number of strokes 10; then the radius of the roll is increased 10 inches, or the diameter is increased 20 inches, whatever that diameter may be.

Rule.-Multiply the depth of the pit, in inches, by the thickness of the rope, also in inches, for a dividend.

Then multiply 3·1416 times the thickness of the rope, in inches, by the number of strokes, for a divisor.

Divide the above dividend by this divisor, and from the quotient subtract the product, which is found by multiplying the thickness of the rope by the number of strokes, and the remainder will give the diameter of the roll in inches.

Example.-If an engine makes 20 strokes in drawing a corf up a pit, the depth of which is 100 fathoms, and the thickness of the rope 1 inch, what is the diameter of the roll at the first lift?

100 fathoms 7200 inches, and 7200 × 1 = 7200, which is the dividend mentioned in the rule.

And 3.1416 x 1 x 20 62-832, the divisor mentioned in the rule.

7200 62.832 = 112.8 nearly,

and 112.82092.8 inches 7 feet 8 inches.

It may be remarked here, that if an engine be drawing coals out of a pit with round ropes, and we wish to take the round ropes off and to put flat ones on, this rule will determine what diameter our roll must be at first lift, so that the engine may go the same number of strokes as before, when the round ropes were on.

Example.-If an engine goes 10 strokes in drawing a corf up a pit, the depth of which is 60 fathoms, with round ropes, where the round ropes do not lie upon each other, what must be the diameter of a flat rope roll, so that the engine may go the same number of strokes as before, the thickness of the rope being half an inch? 60 fathoms 4320 inches,

4320 × 2160, the dividend mentioned in the rule; 3·1416 × × 10 = 15·708,

the divisor which is mentioned in the rule;

2160 15.708 137.5.

And the product of the thickness of the rope and number of strokes is × 105.

Hence 137.5. 5 = 132.5 inches 11 feet 0 inch.

When an engine draws coals out of a pit, with flat ropes, the corves will not pass each other at mid-shaft, that is, half way between the top and bottom of the pit; for the corf which goes from the top of the pit will pass