The theoretical effect of this wheel is the same as that of the common wheel, and experience alone, the result of

Fig. 134.

which has not yet been obtained, can prove its efficiency. The number of bars, or separate parts into which each paddle-board is divided, has been very various. When first introduced by Mr. Galloway each board was divided into six or seven parts: this was subsequently reduced, and in the more recent wheels of this form constructed for the government vessels the paddle-boards consist only of two parts, coming as near to the common wheel as is possible, without altogether abandoning the principle of the split paddle.

(226.) To obtain an approximate estimate of the extent to which steam-power is applicable to long sea-voyages, it would be necessary to investigate the mutual relation which, in the existing state of this application of steam-power, exists between the capacity or tonnage of the vessel, the magnitude, weight, and power, of the machinery, the available stowage for fuel, and the average speed attainable in all

Field did not persevere in its use at the time he invented it. It has, however, been more generally adopted since the date of Galloway's patent.

weathers, as well as the general purposes to which the vessel is to be appropriated, whether for the transport of goods or merchandise, or merely for despatches and passengers, or for both of these combined. That portion of the capacity of the vessel which is appropriated to the moving power consists of the space occupied by the machinery and the fuel. The distribution of it between these must mainly depend on the length of the voyage which the vessel must make without receiving a fresh supply of coals. If the trips be short, and frequent relays of fuel can be obtained, then the space allotted to the machinery may bear a greater proportion to that assigned to the fuel; but in proportion as each uninterrupted stage of the voyage is increased, a greater stock of coals will be necessary, and a proportionally less space left for the machinery. Other things being the same, therefore, steam-vessels intended for long sea-voyages must be less powerful in proportion to their tonnage.

It will be apparent that every improvement which takes place in the application of the steam-engine to navigation will modify all these data on which such an investigation must depend. Every increased efficiency of fuel, from whatever cause it may be derived, will either increase the useful tonnage of the vessel, or increase the length of the voyage of which it is capable. Various improvements have been and are still in progress, by which this efficiency has undergone continual augmentation, and voyages may now be accomplished with moderate economy and profit, to which a few years since marine engines could not be applied with permanent advantage. The average speed of steam-vessels has also undergone a gradual increase by such improvements. During the four years ending June, 1834, it was found that the average rate of steaming obtained from fifty-one voyages made by the Admiralty steamers between Falmouth and Corfu, exclusive of stoppages, was seven miles and a quarter an hour direct distance between port and port. The vessels which performed this voyage varied from 350 to 700 tons measured burden, and were provided with engines varying from 100 to 200 horse-power, with stowage for coals varying from 80 to 240 tons. The proportion of the power to the

tonnage varied from one horse to three tons to one horse to four tons. Thus the MESSENGER had a power of 200 horses and measured 730 tons; the FLAMER had a power of 120 horses, and measured 500 tons; the COLUMBIA had a power of 120 horses, and measured 360 tons. In general it may be assumed that for the shortest class of trips, such as those of the Channel steamers, the proportion of the power to the tonnage should be about one horse for every two tons; but for the longer class of voyages, the proportion of power to tonnage should be about one horse-power to from three to four tons measured tonnage. These data, however, must be received as very rough approximations, subject to considerable modifications in their application to particular vessels. We have already stated that the nominal horse-power is itself extremely indefinite; and if, as is now customary in the longer class of voyages, the steam be worked expansively, then the nominal power almost ceases to have any definite relation to the actual performance of the vessel. It is usual to calculate the horse-power by assuming a uniform pressure of steam upon the piston, and, consequently, by excluding the consideration of the effect of expansion. The most certain test of the amount of mechanical power exerted by the machinery would be obtained from the quantity of water actually transmitted in the form of steam from the boiler to the cylinder. But the effect of this would also be influenced by the extent to which the expansive principle has been brought into operation.

From the reported performances of the larger class of steam-ships within the last few years, it would appear that the average speed has been increased since the estimate above mentioned, which was obtained in 1834; and on comparing the consumption of fuel with the actual performance, it would appear that the efficiency of fuel has also been considerably augmented. No extensive course of accurate experiments or observations have, however, been obtained from which correct inferences may be drawn of the probable limits to which steam-navigation, in its present state, is capable of being extended. The jealousy of rival companies has obstructed the inquiries of those who, solicitous more

for the general advancement of the art than for the success of individual enterprises, have directed their attention to this question; and it is hardly to be expected that sufficiently correct and extensive data can be obtained for this

purpose.

(227.) Increased facility in the extension and application of steam-navigation is expected to arise from the substitution of iron for wood, in the construction of vessels. Hitherto iron steamers have been chiefly confined to river-navigation; but there appears no sufficient reason why their use should be thus limited. For sea-voyages they offer many advantages; they are not half the weight of vessels of equal tonnage constructed of wood; and, consequently, with the same tonnage they will have less draught of water, and therefore less resistance to the propelling power; or, with the same draught of water and the same resistance, they will carry a proportionally heavier cargo. The nature of their material renders them more stiff and unyielding than timber; and they do not suffer that effect which is called hogging, which arises from a slight alteration which takes place in the figure of a timber vessel in rolling, accompanied by an alternate opening and closing of the seams. Iron vessels have the further advantage of being more proof against fracture upon rocks. If a timber vessel strike, a plank is broken, and a chasm opened in her many times greater than the point of rock which produces the concussion. If an iron vessel strike, she will either merely receive a dinge, or be pierced by a hole equal in size to the point of rock which she encounters. Some examples of the strength of iron vessels were given by Mr. Macgregor Laird, in his evidence before the Committee of the Commons on Steam Navigation, among which the following may be mentioned: An iron vessel, called the ALBURKAH, in one of their experimental trials got aground, and lay upon her anchor in a wooden vessel the anchor would probably have pierced her bottom; in this case, however, the bottom was only dinged. An iron vessel, built for the Irish Inland Navigation Company, was being towed across Lough Derg in a gale of wind, when the towing rope broke, and she was driven upon rocks, on which she bumped for a considerable time

without any injury. A wooden vessel would in this case. have gone to pieces. A further advantage of iron vessels (which in warm climates is deserving of consideration) is their greater coolness and perfect freedom from vermin.

Iron steam-vessels on a very large scale are now in preparation in the ports of Liverpool and Bristol, intended for long sea-voyages. The largest vessel of this description which has yet been projected is stated to be in preparation for the voyage between Bristol and New York, by the company who have established the steam-ship called the Great Western, plying between these places.

Several projects for the extension of steam-navigation to voyages of considerable length have lately been entertained both by the public and by the legislature, and have imparted to every attempt to improve steam-navigation increased interest. A committee of the House of Commons collected evidence and made a report in the last session in favour of an experiment to establish a line of steam-communication between Great Britain and India. Two routes have been suggested by the committee, each being a continuation of the line of Admiralty steam-packets already established to Malta and the Ionian Isles. One of the routes proposed is through Egypt, the Red Sea, and across the Indian Ocean to Bombay, or some of the other presidencies; the other across the north part of Syria to the banks of the Euphrates, by that river to the Persian Gulf, and from thence to Bombay. Each of these routes will be attended with peculiar difficulties, and in both a long sea-voyage will be encountered.

In the route by the Red Sea it is proposed to establish steamers between Malta and Alexandria (eight hundred and sixty miles). A steamer of four hundred tons' burden and one hundred horse-power would perform this voyage, upon an average of all weathers incident to the situation, in from five to six days, consuming ten tons of coal per day. But it is probable that it might be found more advantageous to establish a higher ratio between the power and the tonnage. From Alexandria the transit might be effected by land across the isthmus to Suez- — a journey of from four to five days — by caravan and camels; or the transit might be made either