of piston rods are connected to one common cross-head, which acts through one connecting-rod on one crank attached to the propeller shaft.

Fig. 1 of the engravings represents a top plan of a pair of engines of this description as fixed in a steam vessel, and applied to a screw propeller; fig. 2 is a transverse elevation on the line a, b, of fig. 1; fig. 3 is an end view on the line c, d, of fig. 1; and fig. 4, a transverse view on the line e, f, g, fig. 1. A, B, are the two cylinders; a1, a2, the two rods of the piston of the cylinder A; and b1 62 the two rods of the piston of the cylinder B. Dis the cross-head, to which the two pairs of piston-rods are attached. One rod of each pair is placed above the cross-head, and the other below it, and the four rods are so placed in this respect, in relation to one another, that the two rods which are above the cross-head shall be diagonally the opposite of one another, and the two rods which are below it, the like. For example, if a', of one pair is placed above the cross-head, then b2 of the other pair must be also above it, and a2 and b be the two rods which are placed below it. The cross-head D, has a bearing in the middle of it for the connecting-rod E and prolongations, one at each end, which serve as a guide for the rod. F is the crank on the propeller shaft, having bearings, G G, in the frames, H H, which serve both to connect the two cylinders, and as guidesfor the crosshead. I I are side valves for regulating the inflow and outflow of the steam to and from the cylinders, which may either be connected together and worked by one eccentric, or may be worked independently, each by means of a separate eccentric. JJ are the condensers.

The air-pumps are also horizontal, and are fitted and worked in the manner shown on the left-hand side of fig. 1 and fig. 2; where the pump is represented as of the double-acting sort. Kis the barrel of the pump; L, the piston, or plunger; M, the piston-rod, which passes through a stuffing-box in the outer cover of the cylinder B, and is attached directly to the piston of the steam cylinder, so as to derive its motion therefrom. N is the lower foot valve at the end of the passage P, which communicates with the condenser, and leads to the space P, on one side of the piston or plunger. Q is another foot valve at the end of the passage R,

"Keeping this in view, that steam only loses half its power when expanded double the space, turn to the enclosed diagram of a cylinder: here I will consider the pressure at the top of the descent of the piston (as represented by Lardner in page 161) at 2,240 pounds; now as long as the steam continues to flow from the boilers or steamchest the pressure is constant and uniform, therefore continues equal to 2,240: but, after it has arrived at half its descent, a valve is closed by the working of the engine, which cuts off any further communication between the steam-chest and cylinder; all future power exerted must arise from the expansive quality of the steam.

"I have, however, stated above that it only loses half that power when expanded into double the space; consequently, when arrived at the bottom of the cylinder, it still exerts a power equal to half its original pressure, which in numbers shows 1,120; but that is the terminal pressure, not the mean, "to obtain which I will divide the second half of the cylinder into ten equal parts, and multiply 2,240 by 10, and divide it by 11, 12, 13, &c., this will give me the loss at each succeeding division of the descent; these added together and divided by 10 give 1,498 pounds as a mean pressure, which subtracted from 2,240 leaves 742, that being barely the third of the half; but the third of the half is the sixth of the whole; consequently I have saved half of the steam, therefore half of the fuel, and only lost onesixth of the effective pressure on the piston. I have, however, stated that half the quantity of steam in the cylinder, (called wire drawn') whose pressure is constant and uniform, and as such equal to one half, loses but of the velocity in accordance

with the law of the squares; then, should I require to know what one-sixth will lose, I have but to say as gives a loss of velocity of what will give? The answer is, 14 nearly.

"Here then is perceived the immense advantage to be derived from working steam 'expansively.' i

"Let me now recapitulate how these proportions bear to each other; by cutting off at half stroke, I lose but one-sixth of the effective pressure on the piston; that equals one-fourteenth of velocity; but half the steam has been saved, which equals half the fuel, in accordance with the law' that the quantity of steam generated, is the direct proportion of fuel consumed.' Suppose then the maximum velocity of a steamer to be eleven miles per hour, the loss of onefourteenth would only reduce the rate of steaming of a mile; that would still leave the velocity 10 knots; so, if the expenditure of coal without the 'expansive gear' was 20 cwt. to the hour, I shall obtain for 10 cwt. the above named velocity, 10 knots. But it is evident, if the law of the squares be true, I can reduce half upon this again with only a sacrifice of two miles.

"Then I come to this inevitable conclusion, that, by cutting off at half stroke' and keeping in view the law of the squares, I obtain 32 instead of eleven miles for every ton of coals; and, supposing such velocity possible to be averaged, the time in which a thousand miles would be traversed stands 125 to 91 hours, with an expenditure of 31 instead of 91 tons of coal, with the use of one boiler for 125 hours in lieu of four boilers for 91, which is as 125 bears to 364. "Throughout, I have considered the steam cut off at half stroke, merely to keep together a connected chain of reasoning; but a still greater saving will take place by cutting off at a quarter.

"Great confusions of ideas seem to be in some men's brains with respect to expansive steam. They certainly have laboured under the delusion that it is essential to raise steam to a high pressure to work it expansively. The loss is relatively the same, viz., less than one-sixth of the power, whatever pressure the steam may be raised under; for steam loses less than one-sixth by calculation of mean pressure, and no more when expanding into double the space. I have said less than one-sixth by calculation; for by the Indicator,' that beautiful invention, and only sure test, it is found, that, 'while half the quantity of steam is saved, the loss of power by the engine is less than an eighth;' this was the result of an experiment on board the Commodore, a fine 700 ton steamer.-Robinson on Steam, p. 161.

"Of all the beautiful discoveries of the great Watt, none I think more astonishing than that of the expansive system.' I arrive at this conclusion from perceiving the difficulty which even men of ability show, 22 years after the decease of this wonderful man, in defining what he has discovered; nor do I clearly perceive by what chain of reasoning he arrived at so happy a discovery; all other parts of the engine are described with the greatest facility, but this brings us fairly to a stand. If you question my judgment on this head, read Lardner attentively, whenever he treats on the 'expansive system,' more particularly in his seventh edition, page 235. Capt. Otway flounders most delectably in the vain endeavour to make that intelligible to others which he does not understand himself; in his definitions and illustrations he is equally unhappy. I would be understood only to make these affirmations when either seeks by words to explain their ideas; for it is remarkable enough, that they both are sure to negative their own calculations; as their definitions ever contain something more than is proved by the figures. A few simple facts oftentimes speak more clearly to the understanding than the most astute reasoning upon them. Sixty-one years have elapsed since the discovery of the expansive action of steam, yet we find Government have but three vessels fitted with the necessary gear out of the thirteen steamers in the Mediterranean; these three are Stromboli, Vesuvius, and Hecate. I deliberately stated in our mess, that the Captain of the firstnamed ship did not understand the advantage to be derived from the expansive system; and I arrived at this conclusion from the perusal of his own work. I need hardly tell you that my opinion on this head gathered but little strength from a fact which I subsequently, discovered on visiting the engine-room of the Stromboli. When in conversation with the engineer, I said, 'You are not fitted with expansive gear, I suppose? Yes,' he replied, we are.' Indeed!' I rejoined, now I shall learn what I want. How many revolutions in smooth water do you obtain, when cut off at half stroke; and what velocity does that number of revolutions give you?' 'I don't

6

know,' was his answer, we tried it but for an hour, and then Capt. Williams gave it up.' I said nothing more, for I had heard enough.

"The Vesuvius, I have also been told, has

I have since discovered I am wrong as to the number; but, were I to revise this paragraph, I could demonstrate to the satisfaction of all parties, hat I have not made the most of my case.

never been able to make it work. The Hecate has, but only for two months out of fourteen, though she has rarely been in harbour. My informant for this fact is the officer that keeps the steam log. If it be pleaded that the gear is out of order, why is it not properly represented? for it would amply repay Government to send a fit person from England to set them all in order. But the truth I conceive to be, is, that its value is not clearly understood, consequently not justly appreciated.

"In the fairness of argument it is necessary for me to state an objection that has been raised against me, for the same is likely to strike your mind. It had occurred to me previously, but I could not at that period find any way of getting rid of such difficulty; and therefore adopted my old plan of falling back on the fact which was adverse to this apparent deduction of reason. The objection is this, that when I run a steamer on half fuel, I throw upon that half all loss by condensation, friction, working the pumps, &c. Captain Otway was not the man to help me in this difficulty; for he quietly jumbles them all together, and then estimates it at half the entire pressure of the steam raised in the boiler; so does Lardner, 7th edition, page 293. A pretty loose way this is for estimating the effective pressure on the piston; but the 'indicator' once more comes to my aid and solves all doubt.

"These are the results of two experiments, the first, the Commodore; second, the Wilberforce. 1st. In this instance, the mean effect of the force is, for steam 4.52 lbs., and vacuum 11.80 lbs. ; total, 16.32 lbs. Total of the force in the boiler and condenser being 20 lbs., and the maximum force exerted in the cylinder being 19 lbs.'

"2nd. The total force in the boiler and condenser in the Wilberforce was 19 lbs. per square inch, and the mean effect was 15.35 lbs.; the maximum force in the cylinder being 17 lbs per square inch.'-Robinson, page 159.

"I see that the last-named gentleman estimates friction and working pumps at 2 lbs. the square inch; this may be well enough when working at a maximum; but I object most strenuously against the same estimate for my poor half; as I have always been led to understand that friction decreases considerably at the lesser velocity. Does not this prove how cautious we ought to be, ere we oppose a fact by a theory?

"I had stated, some months previous to the Great Liverpool's piston being broken, that, were she to steam from Malta to England on half power, seven miles an hour ought to be her average. I deduced this

not from her maximum velocity, but from the time she had absolutely taken to perform the voyage from Malta to England, and vice versa; the distance is done, in nineteen instances out of twenty, in ten days; which gives, on an average, nine miles to the hour. My reason for taking the Great Liverpool into consideration, arose from the remarkable precision with which her voyages, both out and home, have generally been performed.

It

"I wish, however, to remind you, Sir Graham, that any vessel steaming but with one piston does so at a mechanical disadvantage; for there is a loss of power in passing over the dead points. She would have steamed faster working both cylinders, with valves half closed; for, the cranks being placed at right angles to each other, when one is at the maximum as to power, the other is at the minimum; this proves that, had she not laboured under this disadvantage, she would have performed the voyage in less time than I deduced from the average. was denied to me previous to this case that the law of the squares held good at different velocities; some admitted that it might be true enough in smooth water, but not otherwise. This assertion on their part showed an entire ignorance of the laws of motion; but all reasoning was vain; I failed to con-. vince when once an error has taken full possession of some men's minds, the very absurdity of the doctrine seems to confirm them in the belief. Should they hold to it now, I simply ask whether they mean to assert, that the Great Liverpool steamed neither more nor less than 7 knots 2 fathoms every hour in the twenty-four for thirteen days? as I conceive the most strenuous opponent will hardly answer in the affirmative, I therefore state that it appears to me evident that she must have occasionally fallen below and risen above it. Her maximum velocity tells me what her half power could not have exceeded, but I cannot say without an inspection of her logs what was her lowest speed; I trust, nevertheless, I have established enough to satisfy even the most incredulous.

"Following up a rule, which I have laid down for myself of not considering any point proved, without I have brought some strong facts in support of my theory, I shall now produce two powerful examples of the advantage to be derived from working steam expansively; these cases are well worthy of consideration, for as yet they appear to have excited no particular attention, except it be to have had their accuracy pretty distinctly called in question by a certain worthy gentleman, who, finding them a little above his comprehensive