respondent will allow that there was no loss of power; his silence indeed on this head shows his inability to deny it. The fact that the 50 lbs. only moved 2.7 inches, proves, however, a great deal more than this, as it clearly shows that the leverage of the cross bar differed in the two different parts of his experiment for when the 28 lbs. moved 4 inches, the 50 lbs. moved 2 inches, but when the 37 lbs. moved 2 inches, the 50 lbs. can have only moved 0-7 of an inch, so that in the first case, the leverage was as one to two; in the second, as 7 to 20, or as 1 to 3 nearly. Hence, if "M.'s" expectations had been verified, and the 37 lbs. weight had been moved 6 inches, the 50 lbs. weight would have only moved 2+0.7 x 2 inches=3.4 inches, and therefore, the power expended would have been only 50 x 3'4=170; the same quantity of work would have been done as in the first instance, where 200 represented the power expended, and we should have had the power increased by the crank instead of being diminished. The same would have been the case, even if the 55 lbs. weight had been sufficient to draw the 37 lbs. 6 inches.

From these considerations, Sir, your correspondent will perceive, that in his future experimental career, he must be a little more careful in observing facts; lest he should innocently take for granted, the very thing he wants to prove, as he did in this case, where we see he never took the trouble to ascertain what the power expended really was! supposing that it was just what he expected it to be.

"M." seems inclined now to carry the war a little further into mechanics, and to attack some of its elementary principles, (considering the crank, of course, completely floored), for he tells us that, "although it is an elementary principle in mechanics, that increase of surface, the weight" (pressure) "remaining the same, will not cause increase of friction, nevertheless, such is not the case in these experiments. Now, in his experiment he certainly doubled the surface pressed, but the original pressure was not distributed over the whole surface thus increased, as is supposed in the above proposition; for the pressure on the lower side of the board continued precisely as it was before, and the additional surface sustained a pressure equal to that upon the lower side: whence it is evident, that the friction produced by this new surface, must have been equal to that still generated by the original surface, (supposing all the surfaces pressed to be alike), and therefore that the effect was just the same as if he had tied another board to the first, and loaded it with an equal weight; but was totally different from

that which would have been produced, by merely increasing the surface-putting a larger board instead of the smaller, for instance.

So much for "M.'s" original experiment. He now, however, gives us a new one, to which he requests particular attention. Before I examine it I shall just state what I understand "M." to mean by the expression "loss of power in the crank :" for if we were to misunderstand each other on this point, there would be no end to the discussion; and I presume, Sir, you would not consider that "a consummation de

voutly to be wished." By "loss of power," I take it for granted that he means an irreparable loss, consequent upon the use of the crank: a sort of destruction of force by it, when transferring the force from the piston to the machinery: otherwise it could not be rightly called a loss, and otherwise his experiments would have been utterly worthless, even in his own eyes. I must also take the liberty of reasserting and proving, that his apparatus does not at all represent a crank, except in the latter part of its action, after the bar comes against the stop; in the former part, the board is drawn by a constant force, acting upon it in a straight line for a definite time and distance; whereas, in the crank, the acting force is never constant for a moment, and never acts for any definite time in the same straight line, the motion being always rotary. Farther in his machine we may perceive, that the instant the bar touches the stop, the acting force it suddenly diminished to one-half, or less. Now, may I ask, in what part of a crank's course does this phenomenon take place? "M." very coolly tells us that he takes this sudden hop, from the force at 90° to that at 30°, to facilitate investigation! But, to return to his experiment. He now destroys all the momentum by using a steel spring, instead of the weight under the table; and by preventing the weight on the table from being moved (see vol. xxxv., p. 260). In this state of things he finds that the 37 lbs. weight does not move beyond the stop at all, and accordingly, in his usual vague way, he tells us, that he considered this "quite conclusive," without saying what his conclusions are. Here then, (although I suppose, I shall, as usual, be charged with "misrepresenting"-because I attempt to interpret his half-expressed, ill-digested notions, and that, because, forsooth, I "find a difficulty in refuting them!") I shall conclude, he means, 66 quite conclusive" in proving the loss of power in the crank. He then defies any body to prove his "facts or his reasoning to be erroneous." The former I admit, as there is not the same room for

error in this experiment, as in the original one, and I do not question his veracity; (although he is civil enough to hint that he doubts mine): but as to the latter, I really can find none, in either of his last letters-so of course I have nothing tangible to question. Perhaps, however, he alludes to his old statement, (ushered in with a "consequently,") that "the 37 lbs. weight should move 6 inches if there was no loss of power in the crank," which he seems to consider sound and available for this experiment also; I must therefore reiterate, that it is utterly inconclusive; as I have already shown, and as is, indeed, nearly self-evident in many ways. For instance, it may as well be stated thus: "We have therefore 56 x 2 in. + 28 × 4 in. =224"

74.6 x 3 in.; "a weight consequently," of 74 lbs. should be moved 3 inches; now this is just as legitimate a conclusion as "M.'s," but is manifestly an absurd one; for, by hypothesis, 56 lbs. is the greatest weight that can be moved by his apparatus. The true state of the case is, that 37 lbs. can be drawn 6 inches, or 74 lbs. 3 inches, by the power here expended, if a suitable apparatus be arranged; but to suppose that any machine which does not produce this full effect, is therefore to be thrown side as inefficient and a destroyer of power in all cases, would be scarcely less absurd, than if one was to object, that a smith's vice must necessarily be a useless inefficient instrument in all cases, because it has been found to be so, when applied to the operation of breaking stones for a Macadamised road, or of grinding corn, &c. Moreover, Sir, I think I might safely undertake, with the addition of another cross bar, (so as to suit it to the work to be performed,) to make your correspondent's own apparatus draw the 37 lbs. 6 inches, or even the 74 lbs. 3 inches, and thereby make it reproduce the lost power; but the description of this arrangement would be rather too complicated to insert here; besides, I dare say your correspondent will, himself, easily see how it may be done.

If" M." will turn to my first letter (page 389, vol. 34,) he will see that I there told him that the board with the 371b. weight on it would not have moved beyond the line gh, but for the accelerated velocity produced by drawing it back two inches; and that this was the case, "M." seemed to allow in one of his last letters, (page 259,) when he says that it was the increased momentum which he expected would have carried the 371b. 6 inches. Now in the experiment we are at present considering, he has confessedly destroyed all the momentum, and yet he blames the crank for not performing this work which in this same letter he tells us

could be done by the momentum and it alone. I must therefore take the liberty of asking him again, "with great simplicity," as he says, did he "expect an increase of power in the crank, in the latter part of its action, in the ratio 28 to 37, because in the former part," he "diminished its task in the ratio of 56 to 37?" And I hope he will give me a simple answer; he must, however, remember this time, that momentum is not to be brought forward in it. "M." seems anxious to know why there should be a difference with regard to the work done, in the two cases he gives, viz. with and without the cross bar; if he will try the following experiment, and explain it, he will find the explanation applicable to his own. Let his cross bar be removed altogether from the table, but the anti-momentum contrivance retained, and let the line be passed directly from the spring over the pulley to the board k: the spring is to be so arranged as to have only two inches play. We shall now suppose the 561b. weight placed on the upper board, and the under one (k) drawn back these two inches; of course it will again be drawn this distance by the spring; then, (the board being again drawn back), let the 561b. weight be replaced by the 37, and, (if all the momentum has been destroyed), we shall find that the board will still be only moved through the same space of two inches, and therefore that the same quantity of work will not be done; that performed in the first case being to that in the second as 112 to 74, or as 3 to 2: here he will have no crank upon which to throw the blame.

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In my last I suggested an experiment of which "M." has taken no notice, but to object that it is a limited one; referring to what Smeaton says in a spirit of sound philosophy; viz. that he looks upon experiments made on working models" to be "the best means of obtaining the outlines of mechanical science." Now may I ask "M." to tell me honestly, whether the plan of my apparatus, or of his, is most like a working model?-a working crank? and how his experiment is less limited than mine? The most general practical proof on the subject must include only some single crank, but at the same time we may observe, that any clear proof that there is no loss of power in any one particular crank, is to be considered as perfectly general for all others, for they can differ from each other only in size. For let us suppose the case of a crank, in the working of which there is no loss of power, and "M." has allowed (vol. 34. page 265) the possibility of such a case, for he says "there may be little or no loss of power in the crank of a railway locomotive"- -now if we alter the nature of the

work to be performed by it, would it not be absurd to suppose that there might be a loss, occasioned by this very crank? For this would imply, that the crank now stopped and appropriated a certain portion of the power which it formerly transferred, and applied with effect! If, therefore, "M." should at any time prove ever so clearly, any practical loss to exist in any engine or machine of his construction, he must look somewhere else than to the crank for the cause. But let him bear in mind that the unfitness of a machine to perform work, for which it is not adapted, is not to be taken as the proof of an irreparable loss. For instance, in the case of a steam engine working machinery without a flywheel, the varying nature of the force, which of course is owing to the crank, would undoubtedly cause a great inefficiency and waste of power; but as we know how to cure this sort of loss, we must, to describe it correctly, say it is owing to the want of a good flywheel, and not to the use of a crank: this is the sort of loss which "M." has fallen foul of, and which has so sadly puzzled him, and brought him into his "present troubles."

It is very easy, Sir, to sneer at the" blunders these mathematicians will make;" but, considering circumstances, I think I may fairly remind your correspondent of the old proverb, "those that live in glass houses should not throw stones." It is not however the mathematicians that make blunders, properly speaking, so much as the philosophers, who mingle mathematics and practice, not to discover the truth, but to carry out their own theories: nor are the blunders to be found in the mathematical parts of their calculations, but chiefly in the facts or statements with which as data they support those theories. It is then the experimentalists that are often the cause of error; they should therefore take peculiar care that they see clearly all the bearings of their experiments, and the conclusiveness of their arguments, before they found any theory on them, or bring them forward as controverting, long received principles.

I must now remark upon another misapprehension and misapplication of "M.'s" with regard to Dollond's (not Drummond's) well-known discovery of the achromatic lens. For Dollond "questioned" no "demonstrations of Sir Isaac Newton, that prince of mathematicians," as we are told he may justly be called, considering the age in which he lived!!! Of course if he lived now-adays, he would be but a prince of blunderers, not fit to hold a candle to some of our experimentalists and practicians. The case was this: Sir Isaac Newton found by his discoveries that an achromatic lens would

be procurable, if the relation between the dispersive and refractive powers should prove to be different, for different media: he accordingly tried the experiment between water and glass, and found the relation the same in each, and then unluckily gave up the hope of getting an achromatic lens, supposing that the same property obtained in all other media. Dollond fortunately questioned the correctness of this latter supposition, tried the experiment again, and found that his doubts were well grounded, and therefore that the achromatic lens was procurable. This however only showed that the glass and water Sir Isaac used (in which latter, I think, it is supposed he had dissolved some salts of lead, the refractive power of which was not then known) happened accidentally not to differ in their relative refractive and dispersive powers; but did not in the most distant way imply, that any of Sir Isaac's demonstrations were in the smallest degree incorrect, (as "M." would lead us to suppose) although they were arrived at with the assistance of that impotent science, mathematics.

Your Correspondent also shows the weakness of his side, by the analogy he attempts to strengthen it with; telling us, that we might as well attempt to give a formula for a daguerreotype sketch, or an electrotype deposit, as for the heating power of a boiler, or for the action of a crank, as exhibited in his experiments! Can anything, Sir, be more absurdly ridiculous than this? What connexion, analogy, relation, or similarity, is there between the crank (as much a mechanical power as the lever or the screw) and the actions of heat, light, or galvanism? In his next I suppose he will be defying us to settle mathematically the relation of the power to the weight in a lever of the first order.

My excuse, Sir, for trespassing at such a length upon your columns, on this subject, must be, that "M." seems to think his facts and statements "so perfectly conclusive, that really some of your unscientific readers might be misled by his confidence, and actually suppose that he had made out something of a case:" I repeat this from a former letter of mine, to correct an error in it, of which I forgot to send you a note at the time: "unscientific" was misprinted "scientific," whereas I assure you, Sir, I never intended for a moment, to libel your scientific readers, by supposing, that they could be puzzled or misled, by such facts or statements.

I am, Sir,

Your obedient Servant,
R. W. T.

PURIFICATION OF THE WATER SUPPLY OF THE METROPOLIS.

Sir, I have seen with the greatest satisfaction, in the Sun and other newspapers, mention made of the method of filtration, of which Mr. Stuckey is the inventor. I am heartily glad to see this matter at last taken up as it should be. I reside at Rotherhithe, and the water in our neighbourhood is dreadfully bad; people are obliged to boil it in order to destroy the flies and other insects with which it abounds, before it can be considered drinkable-it has a dirty look, and if put into a bottle there is speedily a deposit of mud at the bottom. If a large quantity is wanted for any purpose it must first be left to settle in large tubs-but water thus stagnant, and already mixed up with extraneous matter, some of which is already putrescent, will soon become really putrid, and unfit for use. I have had serious thoughts of changing my abode, in order to go to some other part of the town where the water is drinkable; I shall, however, now wait, and see if the invention is adopted, because, if it is, its grateful effects will be most gratefully received in all parts of London, and in none more than that where I am located.

Notwithstanding the large sums of money which have been expended on this object, it must be confessed that London does not shine in her water-works; the supply is plentiful, but in no instance whatever does it serve for ornament. Probably those who have the management of such things, are well aware that our London fountains would not pour fourth very silvery streams-consequently, they very wisely keep all their works under ground; though with half the money which has been expended we might have had aqueducts which would have vied with those of ancient Rome, and been an ornament to the country.

I am, dear Sir,

placed in a line athwart the vessel, and the three pistons are connected by oscillating rods directly to three cranks, forming angles with each other, which angles, it is to be presumed, are all equal, or 120° each. Now let us compare these engines with one of the many constructions now in use, say for instance the " Gorgon" engines, by the same makers. By the new arrangement we get rid of the weight, cost, and complexity of the cylinder-covers, piston-rods, and stuffing-boxes; a greater length of stroke can be obtained in a vessel of given depth, and the power of the engines throughout the stroke is more nearly equalised; but on the other hand we have three cylinders instead of two, and as the power of the engines in each case will be as the quantity of steam consumed by them in each revolution of the wheels, the area of the piston of the atmospheric engines must be to that of the "Gorgon" engines as 4 to 3, so that supposing the cylinders of a pair of "Gorgon" engines to be 50 inches diameter, the three cylinders of the atmospheric engine, to be of the same power, must each be of 57 inches diameter. There must likewise be three slide-valves instead of two; the intermediate shaft, too, must be cranked, and there is a double packing to each piston, and a guide-rod working in a stuffing-box. On a full comparison, therefore, of these different circumstances, I apprehend that the atmospheric engines will be found to be heavier, more costly, and not more simple than the "Gorgon" engines, while the former, at the same time occupy more room. With regard to the greater length of stroke which may be obtained, many persons, and amongst them Mr. John Seaward, are of opinion that no material advantage is thereby gained. The comparison, therefore, is, I think, rather to the disadvantage of the new plan, and the only point in which it can claim an indisputable superiority, is in the greater equalization of the power of the steam exerted upon the crank. Against this, however, there remain to be set two great disadvantages, either of which would, in my opinion, more than counterbalance the admitted advantage. The first is the loss of heat which the cylinders undergo from their interior being exposed to the atmosphere during one half of each stroke, whereby a considerable quantity of steam must be condensed without producing any mechanical effect. The second is, that whatever may be the pressure in the boiler, the pressure upon the piston during the effective stroke cannot exceed the pressure of the atmosphere. Thus in the account of these engines given by you, it appears that the pressure of the steam was 8lbs., which, adding, for the 14lbs. for the

THE ATMOSPHERIC MARINE ENGINE.

Sir,-May I trouble you with the following remarks, which, coming from one who is in no way connected with engineering, directly or indirectly, may be looked upon as arising from no interested motive? They have reference to the atmospheric engines placed on board their new river steamer by the respectable firm of Seaward and Co.

It has always been considered as of the greatest moment that, in steam-engines, the loss of heat by radiation or otherwise, should be as little as possible, both as to economy, and also as a matter of comfort to the engineers and stokers. Boilers and steam-pipes are felted, cylinders are surrounded by wood, and even charcoal, as a highly non-conducting substance, has been frequently employed. Every cubic inch of heated air arising in an engine-room is considered as a loss.

Having then been taught this by the most eminent engineers, what are we to think of an engine which every hour during its work, inspires upwards of two hundred thousand cubic feet of cold air, and after warming it, vomits it forth again in the expanded form of about two hundred and fifty thousand cubic feet of heated air-for an expansion of onefourth is a moderate calculation? Are we to consider this as a legitimate use of coals? Is it necessary that a few tons of this expensive material should be burned occasionally for the sole purpose of heating the engine room, and distressing, by the suffocating and tallowy effluvia, the engineers already sufficiently heated by this high summer temperature? I think most engineers will pause before they give an answer in the affirmative. They will say, on the contrary, that there is an enormous and wasteful expenditure of heat and comfort.*

However, I am sure it would be interesting to your readers to know the actual consumption of coals in the atmospheric plan per horse-power, and also the temperature of the engine-room. If these are not very great-great virtue in that if-then only we may have some hopes of its success.

I remain, sir, your obedient servant, Blackwall, July 4.

W.

Three cylinders 47 inches, each of 3 feet stroke -pistons making 34 strokes per minute.

MR. VALLANCE'S PLAN FOR NAVIGATING THE NIGER IN SAFETY, OR OF CONSTANTLY MAINTAINING A SALUBRIOUS ATMOSPHERE ON BOARD OF SHIPS IN HOT CLIMATES.

16, Brook street, West-square. 19th July, 1842. Sir, The erroneous view which Colonel Macerone's letter, in last week's Number of your Magazine, gives of one of two plans of mine, rendering a rectification necessary, I have to solicit the favour of your giving publicity to the accompanying letter relating to the last subject mentioned by the gallant colonel.

The method of producing ice, and consequently "cold," therein referred to, is not "by the ministration of carbonic acid gas;" and its object is to make ships in hot climates salubrious rather than "delightfully cool."

I have the honour to be, Sir,
Your most obedient servant.
JOHN VALLANCE.

Letter to Captain Trotter, R.N.
16, Brook-street, West-square.
16th June, 1842.

Sir,-A letter which I have received from Mr. Buxton recommending that I should address you, causes me to solicit the favour of your attention on a subject which may, possibly, prove not utterly unimportant in preserving health, and saving life, during the purposed ascent of the river Niger, by the expedition which is to be under your command.

You are aware that the Americans annually send cargoes of ice from Boston to Calcutta. The proofs given by these vessels make it known, that the caloric rendered latent by the liquefaction of about a hundred weight of ice per hour, is sufficient to keep a place containing 200 tons of ice-and equal, consequently, in cubical capacity, to a room 60 feet long, by 19 feet wide, by 7 feet highcool, even when those ships are crossing the line."

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It follows that if (allowing me to use such an idiom) the expenditure of the "cold" produced by a hundred weight of ice per hour, will keep the holds of these vessels cool on the equator, your ships may be kept so on the Niger, provided we can furnish them with means of (again using an à propos, though incorrect expression) creating "cold" to a sufficient degree.

The object of my addressing you is to submit that we can (once more conveying a meaning, by using an incorrect idiom) create "cold" to any extent which you may require.

Above 20 years ago I discovered a prin