puted, because there are no satisfactory experiments on the subject. It is an esti mate, and may be in excess.

The next element of the pressure to move the machinery, viz. 1.5 lb. per square inch, is a little over the mean of certain trials made by my direction, of which the diagrams are before me, the maximum being 1.63 lb., and the minimum 1.14 lb.

To this no reasonable exception can be taken.

The resistance to the rotation of the blades, 2.23 lbs., is calculated upon the basis of such experiments as I have access to on the friction of water on iron, and on the effective periphery of the screw.

For this element, also, more precise experiments are needed, and it must be considered an estimate only.

The slip, 1.22 lb., requires no elucidation, except that it is what remains after deducting the effect of the resistance to the rotation of the blades. The specific resistance, equal to 20.78 lbs. per square inch on the piston, or in the convertible terms of 1896 lbs. of effective power, may therefore be dealt with as a probable result, and if so, the power of the screw is needlessly in excess of any resistance the Erminia' is likely to offer; and I have explained why it is so, viz. because it was designed to produce a thrust at about 21 per cent. of slip, about 50 per cent. greater than the resistance of the vessel in smooth water, which resistance, viz. 2763.5 lbs., turns out to be 45 per cent. greater than the actual resistance.

Hence the screw is capable of a thrust nearly double of what the 'Erminia' requires in smooth water.

What, then, is the most suitable size and proportion of screw for this yacht?

I believe it will be found that the diameter should be as large as is consistent with its being sufficiently immersed, but no larger; and that the pitch should then be such as to produce a thrust to balance the resistance under ordinary conditions at sea with a moderate slip.

If the screw be 8 feet diameter, then, to produce a thrust of 1896 lbs. at 6 knots, the pitch must be 9.18 feet, and the slip about 13 per cent.

But, under ordinary conditions at sea, the resistance will be increased, and it is expedient to have a coarser pitch, in order that the thrust may balance the resistance without excessive slip.

If we assume the specific resistance at a mean between the two calculated results before described, or 2329.75 lbs., the pitch must be 11:27 feet; and when the thrust works up to this resistance, the slip will be about 20 per cent.

The next vessel to which I must invite attention is the yacht of the Duke of Sutherland, mentioned in my former paper.

Full particulars of the performance of the 'Undine,' at the measured mile in the Thames, on the 6th July, 1858, in Loch-Lochy on the 27th October, and in LochNess on the preceding day, have been laid before the " Steam-ship Performance Committee" by Mr. M'Connell.

Now, the specific resistance of the Undine' at 9.26 knots, estimated as the 'Erminia's,' by the empirical rule founded on Beaufoy's experiments, is 3809'4 lbs. By the synthetical method it is as under, viz.-—

The difference between 3805 57 and 3809'4 is not material in estimates such as this.

It will be seen, also, that there is a difference of 0'47 lb. per square inch in the aggregate pressure as compared with that given by the diagrams.

So far the two methods are in harmony; but now I have to show a screw that is not so tractable.

The direct thrust at 11.29 knots is 7469.3 lbs., and the slip being 17.91 per cent., the resultant is 5022.3 lbs., or more than 31 per cent. greater than the resistance of the vessel.

This, however, cannot be so, and the apparent excess must be accounted for.

I have already said that the screw must be sufficiently immersed, in order that its thrust may be that which is due to its diameter and pitch.

What is sufficient is yet an open question. The Erminia's' was 2 feet 6 inches, the Undine's' only 1 foot 8 inches.

I believe this to be the explanation of the anomaly.

The apparent thrust of 5022.3 lbs. was really an effective thrust of only 3805.57 lbs. in consequence of the rotation of the blade breaking up the surface of the water. This screw would produce, if sufficiently immersed, a resultant thrust of 3805.57 lbs. at 9.26 knots, with a slip of 1032, say 10 per cent.

The actual slip was 17.91 per cent.

We have now reached one of the most interesting of the investigations, which, in my former memoranda, I pointed out as worthy of the attention of the British Association. This is an investigation by experiment not difficult to accomplish, and yet I conclude it has not had the consideration of the naval authorities, as they continue to give screws to their ships, which are only immersed about one-fourth to one-eighth of their diameter, whereas the Erminia's' was immersed about one-third, while the Undine's' was about one-fifth. The due proportion of immersion will, I believe, be found to depend somewhat on the speed of rotation.

On the Manoeuvring of Screw Vessels.

By Admiral PARIS, C.B., of the Imperial French Navy.

The propelling properties of the paddles and of the screw are very different accord. ing to the form, mode of acting, and especially the position of the propellers in the ship.

The paddle acts at the surface of the water and pushes it in the direction of the keel, when working ahead. Thus the current produced by the resistance of the water is useless to the rudder, because it acts only on the upper part, where it presents no flat surface.

The screw acts on the water by a twisted surface, which, instead of pushing back the water in the direction of the keel gives it a whirling motion and projects it abaft in the shape of a cone, producing a current in the same way that the paddle-wheels do; but being below the surface of the water, and the propeller being just ahead of the rudder, the latter receives the impulse of this artificial current which acts before the ship has moved, because the inertia makes her resist, for a few minutes, the impulse of the propeller.

Hence a principle is deducible, viz. that the paddle-wheel ship cannot steer without moving, and that, on the other hand, screw ships steer before moving, and that even long after the propeller is at work, if any object offers resistance to its translating action.

Another difference arises from the action of the screw, because its blades are oblique to the length of the ship, and all of them are pushing the stern not only ahead or astern, but also sideways, so that if the water were equally resistant close to the surface and below it, the equilibrium of both vertical blades would make the screw act equally throughout its path. But this is not the case: the water being more resistant as the depth increases, the lower blade finds more difficulty in moving than the upper one; and the stern being acted on sideways by this difference in the resistance, the ship will not move straight ahead; and if the rudder does not balance this effect, she will always deviate to the same side when going astern. This effect will naturally be more or less energetic according to the immersion of the screw and the relative pitch; for if the screw shaft were at the level of the sea, and the pitch infinite-that is, should the blade be in the place of the axis, the stern will only be deviated and not propelled; consequently, in the actual state of things, the side action of the screw on the stern is a mixture of the propelling and of the lateral effect: this cannot be avoided, and is only lessened by a deeper immersion, or reduction of pitch; and the direction is according to the side of the thread; so that a right-handed thread deviates the ship to larboard when going ahead, and to starboard when going astern; it is the reverse for a left-handed thread.

From this it would appear, at first sight, that the paddle acts much better in making a ship steer well than the screw, and that the disturbances of the screw on the true shipway present obstacles to the management of the ship. But it is not so; and these properties of the screw can be used in such a way as to make various manœuvres, impossible with paddles.

Thus if a ship is required to turn short at the moment before leaving her anchorage, the paddle vessel will want ropes, or at least sails, if the direction of the wind permits, and her propeller will be used only to resist the wind or to act in the direction of the keel. The screw, however, enables her to turn round on the same place when in a calm; for if the ship has a little more cable out than the depth of water, so that the anchor will still offer a small resistance, and she moves her screw slowly, the anchor holding on, prevents the ship from going ahead, whilst at the same time the screw throws water on the rudder and makes it steer the ship as though she were under way this is well known; and many vessels are handled in this way to give them the proper direction without moving ahead; and when at the proper point of the compass, they weigh anchor and go ahead.

If she is not at anchor, a screw ship can also turn herself by her own inertia : thus, if the screw backs, the ship will begin to turn her head to starboard, and when she has gone about half her length, reverse the engines, and work them quicker with the helm a-port-the ship will go ahead but turn on the same side; so by repeating several times the same reversing operation, the turn of the horizon will be made much more quickly than would at first be supposed, and the space required to turn in may be lessened at pleasure by shortening each period of the operation.

If there is any breeze the sails can be employed to accelerate the evolution, either by their oblique action, as with the gib or the mizen sail, or by being used only to resist the impulse of the propeller, in order to give it a more energetic oblique action. So with the wind ahead, and the main-top sail bearing on the mast, a stronger current 1859. 16

is produced on the rudder's surface; and when the wind is abaft, the same sail being full, a greater speed may be given to the screw in order to make its oblique action stronger and let the ship turn quicker. In the intermediate positions between the head and the back wind, the main-top sail is directed in such a manner that it is always acting against the screw. These manœuvres of screw ships have been executed several times, and have enabled ships to enter crowded roads and to pass through spaces where ordinarily it would have been impossible to pass.

Ships are sometimes required to remain in one position without dropping anchor; with sails, as with paddles, there is always lee way, and the ship cannot keep the same position unless with a beam wind. It is also difficult to take another ship in tow, as large ships want much time to send their heavy tow ropes on board, and have generally to drop anchor and weigh again when the second one is in tow; this is a very long operation, and may be readily avoided by making use of the properties of the screw when the wind is ahead or astern. Suppose, for instance, that a ship is intending to take in tow another lying at anchor. She will sheet and hoist her mizentop sail and gallant sail according to the wind, and place herself a short distance ahead of the other, and make her engine work slowly. Thus as the backing force of the mizen sails would be compensated by the heading force of the propeller, the ship acted on by these two equalized and opposite forces will be motionless, but she will steer as well as if making way, on account of the artificial current before alluded to, and may change her direction or remain quite motionless, regardless of the direction of her head, as long as may be desired. This I have done several times when ordered to take ships in tow, and once remained nearly twenty minutes in almost exactly the same position.

This combination of both propelling powers, the sails and the screw, may also be used to maintain the ships with an oblique direction of the wind, two or three points, for example, by bracing properly the mizen-top sail; but when there is a slight lee way, and if the wind blows in the direction of the beam, it is the common condition of sailing or paddle vessels standing on.

Condensed Abstract of a First Set of Experiments, by Messrs. Robert Napier and Sons, on the Strength of Wrought Iron and Steel. By W. J. MACQUORN RANKINE, C.E, LL.D., F.R.SS. L. & E.

The experiments to which this abstract relates form the first set of a long series now in progress by Messrs. Robert Napier and Sons, the details being conducted by their assistant, Mr. Kirkcaldy. The whole results are now in the course of being printed in extenso, for publication in the 'Transactions of the Institution of Engineers in Scotland' for the session 1858-50*.

The present abstract is all that it has been found practicable to prepare in time for the meeting of the British Association; and, notwithstanding its brevity and extreme condensation, it is believed that the results which it shows will be found of interest and importance. It gives the tenacity and the ultimate extension, when on the point of being torn asunder, of the strongest and the weakest kinds of iron and steel from each of the districts mentioned. Each result is the mean of four experiments at least, and sometimes of many more.

The detailed tables, now being printed, will show many more particulars, and especially the contraction of the bars in transverse area along their length generally, owing to "drawing out," and the still greater contraction at the point of fracture. The experiments now complete were all made with loads applied gradually. Experiments on the effect of suddenly applied loads are in progress.

Note.-The strongest lengthwise is the weakest crosswise, and vice versa.

Note. The strongest and weakest lengthwise are also respectively the strongest and weakest crosswise.

On the Comparative Value of Propellers. By JOHN ROBB.

Robertson's Patent Chain Propeller. By PETER SPENCE.

The peculiar principle of Mr. Robertson's invention is, that he applies the power by dragging the vessel from a fixed point; and its great ingenuity is, that the fixed point is at the same time a moveable one, a constantly fixed point in relation to the power exerted by the engine in propelling the vessel, and a constantly changing point in relation to the course on which the vessel is being propelled. The construction of the propelling apparatus is as follows:-At or near the bows of a boat, say 70 feet long, is placed a steam-engine, the main shaft of which crosses the bows of the vessel at or about the level of the deck; a fixed pulley is attached to each end of this shaft, these pulleys projecting over the sides of the vessel; they are three feet or more in diameter, and on their periphery have a hollow or groove to receive the chains which are to run over them; they are also so constructed as to take a firm hold of the chains as the power is exerted in dragging the chains over the pulleys. On the other or the stern end of the boat are two pulleys, also projected over, one on each side; these are loose, so that the chains merely run over tl.em. Friction rollers are also placed along each side of the vessel, to carry the chains as they pass from the stern to the bows of the vessel; the chains, which are endless, pass or are dragged over fixed pulleys at the bow of the vessel; and falling down, lie along the bottom of the canal, and thus become the fixed point or lineal anchor on which the power acts; the action of the engine in dragging the chain over the loose and fixed pulleys being necessarily to drag or propel the boat forward.