independent of steam-power. In two trials the ships, under steam-power only, were made to tow each other, alternately, by the head; and, on comparing the results, it was found that the mean speed of the speed of the Rattler' exceeded that of the Alecto" by 13 knots per hour, Lastly, in one trial, the two ships were connected together stern to stern, and the engines of both were put in action with their maximum power; the result was still in favour of the screw-propeller.

52. In commenting on these experimental trips it is proper to remark that in heavy weather, and with a head-sea, a great deal of power was occasionally lost to the Rattler' by the screw being thrown quite out of the water. Again, the same vessel had, occasionally, a difficulty in obtaining steam, a circumstance which may have been caused by want of air in the engine-room, from some defect in its construction. The engines of the 'Rattler' were new, while those of the Alecto' had been five years in use. These trials are stated to have afforded some evidence of a greater amount of helm-power in the screw-ship over the paddle-steamer ; and this is what might be expected, since the screw is placed at the stern of the vessel, and its movement must cause a powerful stream of water to be forced against the rudder.

53. Some interesting trials were subsequently made between the 'Rattler' screw-ship, and the Prometheus' paddle-steamer, in which the engines of both ships had the same horse power, and the ships were laden to equal draughts of water, 11 feet 3 inches. The distance run was a measured mile in Long Reach; and in these the advantage was in favour of the screw-ship, whose speed was 10 miles per hour, while that of the paddle-ship was only 10 miles per hour.

54. In two trials made in the Channel, in 1849, between the 'Basilisk' paddle-ship and the Niger' screwsteamer (on board of which at the time was the author's son, now Capt. Douglas, R.N.), both vessels under steam and sail, the wind being a-beam during one trial, and aft during the other, the 'Basilisk' gained over

the Niger,' in the first case, 1796 fathoms, and in the second 3360 fathoms. The next day, both ships being under sail only, and on a wind, the Niger' gained over the 'Basilisk' in one trial 5756-6 fathoms, and in the other 5258 fathoms.

55. On another occasion the two ships were set to tow one another a-head, when, in two trials, the 'Basilisk' (paddle) had the advantage; the rates of towing and the consumption of fuel being as follow:

56. The ships were subsequently made to act against one another stern to stern; and in this operation the power of the 'Niger' (screw) was found to be the greatest this vessel drew the Basilisk' at the rate of 1.466 knots per hour, while the 'Basilisk' drew her at the rate of 1 knot only per hour.

6

In these trials the Niger' laboured under many disadvantages; her furnaces and boilers were defective, and the valves were frequently out of order. Her consumption of fuel was so great that she could not steam so far as the 'Basilisk' in the same time." It is stated that, whenever the 'Niger' could get the steam, she always beat the Basilisk.'

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57. The following remarkable experiment, which seems at first sight to disprove the supposition that the power of the screw is diminished by the eddy, and the cross actions of the water on the blades of a screw at the stern of a ship, was made by the 'Bee' steamer, which was furnished with wheel paddles and a screw, both of which were put in motion by the same engine at the same time. In the first trial the ship was impelled forward (the paddles being a-head and the screw a-stern), when the dynamometer registered 373 lbs. in favour of the paddles. Afterwards, the vessel was impelled

a Her consumption of fuel was about 52 tons, while that of the 'Basilisk' was only about 24 tons, in 24 hours.

in the contrary direction, the screw ahead and the paddles astern; the result was the same as before in favour of the paddles. Little dependence can, however, be placed on this experiment; and the cause of the agreement in the results of the trials is ascribed to the circumstance that the paddles required a smaller speed, and the screw one much higher, to develope their best effects; consequently the screw was always going much slower than it ought to go in order to do its work, and therefore did not do it faithfully. It should be added that the 'Bee' is too shallow a vessel for the purpose, experience having shown that ships deep in the water are more effectually acted upon by the screw than those of less draught.

a

58. Though in almost every experiment the screw appears to have advantages over the wheel in respect of the speed with which it moves a ship, the advantages, even in this respect, were not sufficiently decisive to obviate a doubt whether they might not have been due to accidental circumstances, particularly to the want of equality in the powers of the engines employed, and in the moving qualities of the ships themselves. To eliminate these causes of uncertainty it would be necessary not only to repeat the trials, but to use greater precautions in selecting for the trials, ships which might be more equally matched. Even were the superiority of screw-steamers more clearly proved, it would not be proper to supersede the wheel-steamers entirely. The disadvantages of these last, with respect to gunnery power, do not exist in vessels intended for mercantile and packet service; and for these services large and powerful wheel-steamers have been constructed, which, properly armed, would form most important vessels for the purposes of war. In making future trials, the points particularly to be attended to should be the comparative capabilities of the wheel and screw to contend with heavy gales, their relative values with

.

'Niger' and 'Basilisk' were as far as possible sister ships, with the same nominal horse power.

D

respect to steerage power in the ships, and as auxiliaries to the sail.

59. It is certain that a screw, except when the ship is small and has much pitching motion, is never exposed above water to an enemy's shot, but it may be doubted whether, from the propinquity of the screw to the sternpost and rudder, the damage the ship might receive would not be more injurious than that which would be produced by a shot striking a wheel; the whole stern of a screw-steamer forms a broad target exposed to fire, which, supposing good gunnery on the part of the enemy, might, by the stern-post being disabled or carried away, cause the screw to be put out of service, and perhaps the shaft broken by the overhanging weight of the steerage apparatus. In small screw-steamers, the propelling shaft may be made to bear on a collar in the main stern-post, instead of passing through it, and by this construction any damage done to the outer sternpost would not much affect the action of the machinery; but with the larger screws, weighing from four to eight or ten tons, a bearing on the outer stern-post becomes indispensable. It is remarkable

that, as far as present experience goes, the paddlewheel escapes, in a wonderful manner, the action of shot; and it remains to be proved whether or not the sterns of screw ships, in which the rudder, ruddercase, trunk, yokes for steering, the inner and outer sternpost, all lie in close propinquity, will be equally fortunate.

The

60. In order to avoid the necessity of a screw-steamer going into dock whenever an accident happens to the screw itself, all the screw-ships in Her Majesty's service are now provided with trunks or quadrangular apertures, through which a screw may be hoisted up and repaired, or even replaced by a spare screw. trunk at T, fig. 6, p. 68, being in a situation which prevents the employment of an ordinary tiller, it becomes necessary to provide a particular apparatus for steering the ship; this consists of a yoke A C B, fig. 6, fixed to the stem D of the rudder, to which the requisite

motions are given by means of tackles from the ends A and B of the yoke, passing through pulleys inserted in a beam astern of it, leading on each side of the trunk or propeller-well T, to the steering wheel on the quarter-deck.

Fig. 6 a represents an elevation of the upper part of a rudder, and shows the positions of two yokes, EF and IK, one on each deck; one of these is called a preventive tiller, and is intended to be used in the event of the other being destroyed.

61. Messrs. Maudslay and Field have lately made for H. M. S. Marlborough' a screw with blades of the ordinary form, bolted by flanges into the central axis, the bolt holes through the flanges being elongated, so as to admit of the obliquity of the blades to a vertical plane being altered. When the bolts are slackened, the blades may be turned round as far as may be required, and the bolts may then be tightened in order to fix the blades fast in their altered position. A comparatively slight alteration of the angle is sufficient to make a considerable difference in the speed of the vessel, and the blades may be set to that obliquity which is found under all circumstances to give the greatest speed to the ship. The peculiar advantages of this description of propeller are as follow:

1st. To enable a vessel fitted with it to proceed under canvas alone, without the necessity of the screw being raised out of the water, and without the immersion of the screw offering any resistance to the onward progress of the ship.

This is accomplished by placing the faces of the blades fore and aft, in a vertical plane passing through the keel.

2nd. An alteration of the angle of the blades at pleasure, to suit the varied circumstances under which the screw may be employed, which is of especial advantage in long voyages, when sailing and steaming are combined; it also admits of a great saving in the consumption of fuel, and a high speed of vessel being maintained.