Admiralty steamers between Falmouth and the Mediterranean. These vessels have, for several years, departed from Falmouth monthly, and touching at Gibraltar, have proceeded to Malta and the Ionian islands, and more recently to Alexandria, and occasionally to some of the ports of Syria.

Within the last year, considerable public attention has been directed to the question of the practicability and advantage of establishing a line of steam communication between Great Britain and the United States; and various projects have been started, and companies formed, for the construction of vessels for that purpose-several of which are already in a state of some forwardness. At the meeting of the British Scientific Association' held at Bristol last September, one of the topics which engrossed a large share of interest, was the question of the practicability of a steam voyage across the Atlantic,' raised in the mechanical section. The statement laid before that section, by Dr Lardner, obtained such publicity at the time through the press, that it would be superfluous now to recapitulate its arguments. The conclusions, however, at which he arrived, were briefly these :-That, in the present state of the steam-engine, as applied to nautical purposes, he regarded a permanent and profitable communication between Great Britain and New York by steam vessels, making the voyage in one trip, as in a high degree improbable; that, since the length of the voyage exceeds the present limits of steam power, it would be advisable to resolve it into the shortest practicable stages; and that therefore, the most eligible point of departure would be the most western shores of the British isles and the first point of arrival the most eastern available parts of the western continent; and that, under such circumstances, the length of the trip, though it would come fully up to the present limit of this application of steam power, would, nevertheless, not exceed it; and that we might reasonably look for such a degree of improvement in the efficiency of marine engines as would render such an enterprise permanent and profitable.

Mr Field, of the firm of Maudsley and Field, whose experience in marine engines must justly give great weight to any opinion expressed by him on this subject, assented generally to the principles and positions laid down by Dr Lardner; but he objected, that the data from which that gentleman deduced his conclusions, were obtained from the performances of steamers antecedent to 1834; whereas since that period, considerable improvements having been made in the arrangements of these vessels, data obtained from their more recent performances would have furnished juster ground of reasoning as to the present prospects of the success of long steam voyages.

The great interest which this discussion excited in most of our ports and indeed throughout the whole commercial community, has induced us to re-examine the question, on grounds to which the objections raised by Mr Field will not be applicable; and we have accordingly investigated the performances of a vast number of steamers, more especially those of the Admiralty, from which we have been enabled to obtain data, upon the accuracy of which the utmost reliance may be placed; and we propose, in the present article, to found upon these data an enquiry into the question of the practicability of establishing a permanent and profitable line of steam communication across the Atlantic.

There cannot be a more fallacious inference, than that which is founded upon an assumed analogy between the limited coast and channel trips, to which steam has been hitherto applied, and the extensive voyages now contemplated. It will be easy to show that the circumstances which confer certainty, regularity, despatch, and safety, on the former, have no existence in the latter, and, indeed, are to a certain extent reversed. Those, therefore, who desire that these enterprises should be founded on just reasoning, deduced from practical data, will pause before they proceed; and investigate the question more carefully and dispassionately, than many projectors, actuated by more zeal than knowledge, have hitherto done. We shall endeavour to point out a few of the many nautical and mechanical difficulties which will obstruct such a project as an uninterrupted steam passage from the shores of Great Britain to those of the United States; and we shall consider ourselves as doing a public service, if we shall be the means of inspiring among capitalists that wholesome degree of caution, which, without extinguishing the spirit of enterprise, will limit its operation to objects which afford a fair probability of profitable accomplishment.

To be enabled to arrive at any sound practical conclusion on this question, it will be necessary to explain some preliminary matters which are more or less involved in it.

The moving power of steam applied to a ship is rendered available for propulsion by causing it to give rotation to a shaft which traverses the vessel near the middle, at right angles to its length. Upon the extremities of this shaft, and outside the planking of the vessel, are fixed two wheels, usually constructed like undershot water-wheels, furnished with flat boards, which, as the wheels revolve, strike the water, driving it towards the stern of the vessel. The reaction upon these paddle-boards gives the vessel a corresponding forward impulse. The paddle boards are placed upon the arms of the wheels, so as to diverge either exactly or very nearly from their centres. The

edges of the boards are therefore always presented towards the axis of the wheel. The nature of this mechanical arrangement is such, that each paddle-board is a perfect instrument of propulsion only at or near the lowest position which it assumes as the wheel revolves; and it is therefore necessary that the wheel should not be submerged to a greater depth than the breadth of one of the paddle-boards. If the wheel be immersed to a greater depth, the paddle-boards, on entering and leaving the water, act obliquely in such a manner that the mechanical power expended upon them, or a considerable part of it, either is wasted, or may even have the effect of retarding the course of the vessel

So long as a steamer works in smooth water, such as river or lake navigation, a regular immersion of the paddle-boards is possible; and, if a proper trim be given to the vessel, there will be no material waste of the propelling power; but this can never be the case with sea-going vessels. A vessel in the open ocean is at all times liable to the action of waves of greater or less height, and will pitch or roll so that the wheels will be immersed to varying depths, as the waves pass them. A waste of power must ensue, and will be greater according to the increased depth of immersion of the wheels.

This disadvantage has been attempted to be diminished by the use of feathering wheels. The mechanism by which the feathering motion is given to the paddle-boards is usually one which causes their arms to revolve round a centre different from that of the shaft driven by the engine; the paddle-boards being thus moved independently of the framing of the wheel to which they are fixed. Such a position is or ought to be given to them, that as they enter and leave the water, either none or but little of the moving force shall be expended upon them, and while they are passing through the water, they are caused so to shift their position, that they shall successively throw themselves into those attitudes which are best for propulsion.

These

The advantages of the common paddle-wheel are its simplicity and strength; from which two important consequences follow; 1st, That it is less liable to be deranged or broken; 2dly, That, being deranged or broken, it may generally be repaired by such means and with such skill as may be commanded at sea. advantages must be placed against the waste of fuel, which is the necessary consequence of the imperfect action of the paddleboards, whenever they become too deeply immersed. The disadvantage, on the other hand, of the feathering wheels, is their greater mechanical complexity and inferior strength, from which follow two corresponding consequences, 1st, The greater liabi

lity to be deranged or broken; and, 2dly, If deranged or broken, the difficulty or impossibility of repairing such complex mechanism by the means and skill which can be commanded at sea, subject to the great impediments presented by the motion of the vessel.

It will be perceived that the advantages of the common over the feathering wheels will be greater for long sea voyages than for channel and coast navigation; because, if a derangement should occur in the latter case, there may be hope of coming into port, to refit; but, if it occur in the former case, without the means of repair on board, the vessel must become irreparably disabled. Yet it is precisely in the long sea voyages that the common wheel is most injurious and the feathering wheel most necessary.

The operation of the steam-engine is maintained by the conversion of water into steam, which takes place in the boiler, and the alternate reconversion of that steam into water after it has worked the pistón. The steam coming from the cylinder is reduced to water, by conducting it to a vessel called the condenser, in which there constantly plays a jet of cold water. The steam mixing with the jet is instantly reduced to water, and forins a mass of hot water, which is pumped into a cistern reserved for feeding the boiler. It is by this warm water that the waste of the boiler by evaporation is replaced. The cold water admitted through the jet in a marine boiler is necessarily sea-water; inasmuch as a sufficient quantity of fresh water could not be carried. The cistern of warm water by which the boiler is fed, is therefore chiefly sea-water, and the contents of the boiler, even though at starting they should be fresh water (which ought always to be the case),* must eventually become sea-water.

Sea water, as is well known, holds in solution certain salts and alkaline substances, the principal of which is muriate of soda or common salt. There are also present, but in much smaller proportions, sulphate of soda, muriate of magnesia, and muriate of lime. The heat by which the water is converted into vapour does not vaporize these substances; and consequently, as the evaporation proceeds in the boiler, the proportion of salt in it must be constantly increased; but as sea water is not a saturated solution, and is therefore capable of dissolving a greater

Whenever a steamer comes into port, the boilers should be discharged of their contents, and refilled with fresh water. This is too frequently overlooked.

quantity of salt, the process of evaporation at first only renders the water in the boiler more salt, or brings it nearer to the point of saturation. If, however, the process of evaporation be continued, the quantity of salt in the boiler will at length be all that the water is capable of holding in solution; and any further evaporation must necessarily precipitate the alkaline substances which have already saturated the water, and cause them either to form a loose deposit, or yield them in a hard crust to the surface of the iron, to which they have a tendency to attach themselves.

If the process of evaporation were continued without resorting to any remedy for the removal of this deposit and incrustation, the final consequence would be that the boiler would be filled with the solid matter thus deposited or incrusted.

This inconvenience is attempted to be removed by a process called blowing out. As the water becomes salt, but before it arrives at the point of saturation (a large feed of water being previously supplied to the boiler), the over-salted water is allowed to escape into the sea by a pipe provided for that purpose. By such a process, if it were regularly observed, the water might be prevented from reaching the point of saturation.

Admitting for a moment this method to be effectual, it is attended with considerable disadvantage. The salted water which is continually blown out at a high temperature, carries away with it a proportional quantity of the effect of the fuel, while the water which replaces it, being at a low temperature, must receive from the fuel a corresponding increase of heat before it will be prepared for evaporation. In effect, the waste of fuel proceeding from this cause will be proportional to the quantity of water blown out, multiplied by the difference between the temperatures of that and the water which takes its place. The waste of fuel, however, is not the only disadvantage attending this process. The production of steam is necessarily checked by it; and the rate at which the boiler can supply the engine with moving power, is diminished. To produce a given effect, the boiler, therefore, will require to be of proportionally greater magnitude, occupying an increased space in the vessel, and in a part of the vessel where space is most valuable.

But with all the other disadvantages attending it, this process of blowing out attains its end but imperfectly; it retards, but does not prevent the collection of incrustation; and we have seen on boilers, where the process has been strictly attended to, an incrustation as hard as marble, varying from one to five inches in thickness. The substances forming this crust being practically non