Mr. Editor,-In my last letter to you on this subject, I exhibited the saving of weights effected by the adoption of the screw, as amounting in the hull and engines of the Dauntless, when compared with her sister paddle-ship Avenger, as she now is, to 177 tons. I will now take the weight of this ship's engines as it would be if they were reduced to the same power as those of Dauntless, but retaining their own weight per horse power; and though this will reduce the aggregate weight saved in hull and engines to 131 tons, I am quite content to take the view of the question even as thus presented, (if to the satisfaction of any minds it may appear the fairest,) and describe the disposition of this smaller as I should do of the larger sum, by simply observing, that if the constructor of the Dauntless should be called on to work up into the hull and engines of his now completed ship, weights in wood and iron to either of the above amounts, in such way, that to the best of his extensive knowledge and experience, they should to the greatest practical degree ensure her instability; that in such case his ingenuity would be greatly taxed to contrive for them any other more admirably-adapted location for this purpose than that which in the Avenger they now occupy.

"But," say the advocates of the paddle," though it be true that very heavy weights are in paddle vessels placed very high, and therefore must have their due tendency to affect the rolling of the ship; yet this tendency is checked, nay, according to some, it is actually prevented, by the buoyancy of the wheels when in action, which are thus enabled to supply to the ship a counteracting support." Now as I am reviewing this whole question as it practically affects the royal navy, I will leave out of consideration the case of small lake or river steamers, and apply at once to Avenger herself as an ocean steamer to supply us with the means of testing, by a little analysis, the value of this claim in the case of her own paddle-wheels, which roughly for the purpose, I shall take to weigh 18 tons each. First, in the terms in which it is set up, and, secondly, as it is an opinion propounded by seamen, I cannot allow myself to think that it is pretended, that the instability produced by so unfavourable a disposition of so great weights, is in the least degree checked when the paddles

are not in action, which, in cruising menof-war steamers, is a period at least as long as when they are. Nor am I at liberty to suppose the claim of buoyancy is set up for either the iron, or heavilyironed heavy wood, of which the instruments themselves are composed. It is their action we are to look at, and in proceeding to the examination we will first take that of those immersed floats which labour behind or abaft the vertically central one; on resolving the pressure upon the after surfaces of which, into their horizontal and vertical elements, we find that the latter supply us with a certain amount in all, of depressing, not rising effort, that, in short, the power from these floats which goes not to drive the ship horizontally over the water, goes to drive her vertically into it,-and that here, therefore, we have found, instead of a remedy, an aggravation of the disease of rolling. A similar examination into the forces acting on the after surfaces of the equal number of floats which labour before the central one, will, however, afford an opposite result; here we find the vertical portion of the resolved force, an elevating— a buoyant on; and though it be not in extent all which for their purposes the advocates of the paddle should seek for, it is, I believe, all they need expect to find. To those who think it worth while, I would leave the investigation of how far this raising power from the foremost floats, in extreme cases not exceeding four inches, exceeds the depressing power from the aftermost ones, with the weight of back water on these to boot. But I think that most parties will agree with me that it cannot be large; and therefore, as the instrument when in action supplies the means of supporting, if any, but a very small proportion of its own dead weight,-that, to suppose it capable of supplying a support besides this, to prevent the rolling of the ship, is, with every deference to those who hold a contrary opinion, in my view an unreasonable fallacy. It must do surely the less before it can accomplish the greater, and the evil disposition of the weights necessarily connected with paddles must remain, as far as regards any power in the instrument itself to counteract it, an unmixed evil still; steadily but constantly working out its inevitable evil results. As

the employment of each ship may more or less develop them; in the instability, therefore want of ease, therefore want of durability of the whole frame. Nor ought any more than may, the paddle advocates claim as advantage the small modicum of support which the weight of the wheels may receive from the vertical portion of the resolved force acting on the foremost floats; because none more than themselves have denounced it as evil, or more assiduously plotted for its destruction. It is justly looked on by them as so much of the impelling power absolutely lost-it is the sin-the vitiation of the boasted direct action.-And Morgan and Seaward, and many more, have spared neither effort of head, nor means in money, to give us floats whose effort shall be free from all the impurity of vertical force.

I turn then from this view of the effects, which must apparently be always expected from the disposition of the extra 131 or 177 tons (whichever pleases best) in the Avenger to consider that of the 320 tons which remain in the Dauntless; and as, in like cases, I have already stated that the hulls of screw as of paddle steamers require under-water-like support; the Dauntless has all she requires there, and above water requires none; this 320 tons is seen as that of her engines only; a glance at the plan of which will at once show that we are quite safe in taking the vertical position of the centre of gravity of their weight, as at least no higher than the screw shaft; seeing that their highest point falls 4 feet 6 inches below the water line. Now, the centre of the shaft is 9 feet above the under side of the keel-and have here, therefore, as opposed to the injurious but necessary disposition in Avenger, a mass of dead weight to the amount of 320 tons, so disposed that an equal amount of iron ballast distributed low along the flooring of the ship, could alone give us a more powerful element of stability by reducing to a greater degree the height of the general centre of gravity of the loaded ship.

I am, however, fully aware that abstract stability in a ship, instead of being in itself a desirable object to attain, would prove an unquestionable evil; because for that easy yielding motion, which in a seaway it is most essential for the ease of the fabric itself, as well as for all and everything connected with it, that it

should possess, it would substitute a violence of action and re-action, which would be at once dangerous and injurious, and render a ship of war most inefficient, by rendering any correct use of her artillery most impracticable. To those, therefore, who would properly object to such a disposition of so great a weight as in itself advantageous, I would beg to observe that previous to noticing such disposition there was exhibited a very great reduction of the weights themselves connected with the engines, and a yet more extensive reduction of the space occupied by them. So that on the same, or equal lines of displacement, we are now enabled to add very largely to the general efficiency of the equipment of a screwsteamer, and at the same time so to dispose of it, in the shape of increased armament, increased masting, &c., as to form stability, already supplied, in fixing the weight of an engine so low, the most efficient general balance for securing the ease, stability, and durability of the ship.

Nor is the action of the screw itself less advantageous in disposition than that of the weights connected with it. And a glance again at any elevation plans in which it is shown, will exhibit the centre of effort of its power as exerted in corresponding height, even to inches, with the centre of effort of the resistance to the ship. There is therefore, practically, no leverage in rolling, its action tending either to create or to continue lateral oscillation in the ship; and equally beautiful is its practical action, in refusing, as it were, in a head sea, to aggravate at such times the inevitable longitudinal oscillation-the pitching. As the bow is depressed, and stern raised, raising with it the screw, its effort is correspondingly relaxed; while, as the bow is raised and eased, the stern and screw are proportionally immersed, and a corresponding increased effort is then given off from it. When not in use, its light weight, and small surface, render it almost as nothing; and whether we regard the disposition of the instrument itself in action or out of it; the disposition of the weights directly connected with it; or the disposition of those of the general equipment which it permits of, we appear to have nothing further to desire in the adoption of the screw, as a means for securing the most easy formations and most efficient power for our ocean steam

ers.

How far these desirable objects have been attained, I am certainly at present unable experimentally to point out; and I believe, if for one cause more than another, because our first ship in which the securing of them has been from the first aimed at, has yet to be launched. But the testimony of all past knowledge and experience goes to assure us, that by such means, in the disposition of the necessarily great weights of every steamer, as the adoption of the screw now opens to us the advantages of, may they alone be expected; and as this ground is to a considerable extent experimentally supported by the very favourable performances in the bad weather first encountered by our Channel fleet, of the single, yet imperfect screw steamer, Rattler, when compared with her numerous and complete paddle competitors, I shall not hesitate to claim for the principle which I advocate the establishment of my first proposition in its favour; viz.," that its use does greatly reduce the dead weights of hull, of engines, and of expense of our steamers; while those which yet remain are so disposed as to increase the ease, the stability, and the durability, ay, and the general efficiency too, and therefore value, of the whole fabric."

I am, Sir,

E. P. HALSted.

EXPLANATION OF THE FLOATING OF COLD IRON IN MELTED IRON.

Sir,-The principal experiment brought forward by Dr. Prater to prove that heat is material, is the fact of cold iron floating in a mass of the liquid whence it was taken. It does not seem to have been recollected by any of your correspondents that metals of different temperatures exert a repulsive force on each other. For the details of the experiments on this subject I beg to refer your readers to Mr. Trevelyan's paper "On the Vibrations of Heated Metals," in the Phil. Mag., vol. II. (Nov. 1833,) and a continuation by Professor Forbes in the next volume for 1834. As a general example, if a piece of curved metal, A B, rest on a plane metallic surface, CD, and the metals have much difference of temperature, A B will vibrate to and fro on each side of P, the point of contact. It is obvious that it is almost impossible to place such a piece as A B upon CD, so that AP and BP shall be equal and equally dis

105

tant from CD; therefore, the repulsive power being different for the two portions, vibration follows. If, however, we take a sphere, since the surface is perfectly symmetrical, in whatever position it is placed, with respect to the plane CD, the repulsion would be the same for the two portions A P and BP; the horizontal parts of the force, therefore, destroy one another, and leave only a resultant vertical force, which, if CĎ were a liquid, would counteract gravity and prevent sinking.

Analogous experiments were previously known. In Brandé's Journal of Science for Oct. 1827 (p. 461,) and in the vol. of 1828, experiments of Mr. Perkins are given. In these, water being introduced into iron cylinders in which was perforated a hole, the surface round the hole being raised to a very high temperature, when the water was converted into steam and the hole opened, no steam issued: on lowering the temperature, it issued with a deafening noise," which was heard half a mile off. The safety-valve was loaded to 50 atmospheres on the square inch, and yet, whilst the steam issued from the valve, none issued from the redhot orifice.

I should explain on the same principles the extraordinary experiment of making ice in a red-hot crucible, which many of your readers have no doubt seen at the Polytechnic Institution. The liquid introduced is equally repelled on all sides at first, and cannot therefore expand. On lowering the temperature of the crucible, a portion comes into contact with the crucible by its weight, and the whole "flashes off" into vapour, producing cold by its rapid change into the gaseous form.

Similarly a drop of water rolls about on a red-hot iron plate, but is suddenly converted into steam when the plate becomes sufficiently cool.

These effects may be considered with reference to either of the theories of heat. A rapid projection of the particles of caloric must be looked upon as the cause in the one theory, and a rapid vibratory motion of the ethereal medium in the other. So far as a test of theory, therefore, little perhaps can be concluded. The same effects can be just as well produced by the impulses of successive particles (of caloric) as by the successive impulses of the same particle (of the elastic medium called ether.)

As a natural consequence of these views, I would suggest to Dr. Prater and the other experimenters to try the effect on metals of different forms, taking care to have as few points of contact as possible,-just on the same principle as it takes more force to rock a wide-bottomed cradle than one with a narrower bottom. If vibrations are observed, the explanation will be confirmed. At the same time, since it is extremely difficult to place a solid in a fluid, so as to rest in it with few points of contact, I should not expect any very decided result.

Yours respectfully,

A. H.

CATALYTIC FORCE, OR ATTRACTION OF SURFACE CONCERNED IN THE DIFFUSIVE POWER OF GASES-AN OCCULT ENERGY OR POWER IN SATURATED SALINE SOLUTIONS. BY H. PRATER, M.D., &C.

66

I have already made known to the English reader the admirable generalization of M. Mitscherlich, as regards attraction of surface"-(Mech. Mag., vol. xliv., p. 476, 1846.) My present object is to add another case to those he has so well distinguished. The case alluded to is the "diffusive power of gases." Although I believe that motion is the essential condition of gaseous matter, still if, as seems probable, the diffusion takes place sooner when certain kinds of gases are mixed than when others are mixed-as, for instance, when carbonic acid is mixed with hydrogen sooner than when mixed with any other gas, it will be evident that "attraction of surface" must go for something, as regards the cause of this motion. I write this as a note to my essay on "Inherent Activity, &c.," published formerly in the Mech. Mag. I have lately been making a few experiments on the remarkable power of common salt, especially to gradually ascend on the sides of a glass, when this is half filled with the same, and the solution allowed to evaporate. I believe Berzelius discovered this fact; but Fusinieri has written a very large work on the subject in Italian. As I have not read this work, I can only say I believe he attributes the motion to the " expansive power of matter," which to me is not very intelligible. I find this upward motion takes place even when the sides of the glass above the fluid are PERFECTLY DRY, the saturated solution of salt, for instance, being so carefully

poured in, as not to touch the sides above the fluid. Hence it seems very doubtful whether capillary attraction has anything to do with the motion. Heat much increases it, by increasing evaporationespecially heat in the sun. But I shall make fresh experiments to see if this acts more powerfully than other heat. The above experiments were made with wine-glasses, which slope gently upwards; but the effect I found to be the same in tumblers, the sides of which are quite perpendicular. Sulphate of copper succeeded as well as common salt. I shall try a variety of other soluble matters, crystallizable and not, and perhaps consult Fusinieri again; but I confess his opinions and mine on physics generally do not agree. I think he is right to have so much studied this point, for it seems to me calculated to upset most of the book theories of physics.

THE MAGNETIC TELEGRAPH IN AMERICA.

The following extraordinary dramatic scene, we will venture to say, has never had its parallel on this earth, and is the legitimate offspring of that remarkable invention, the magnetic telegraph,-an invention to which the public attention at this moment is so much and so universally attracted.

On Saturday evening, June 6, Professor Morse, the inventor and superintendent of the magnetic telegraph, and his assistant, Mr. Vail, in their office at Washington, wished to test the integrity of the telegraphic line the whole distance from Washinton to New York, a distance of no less than 260 miles. The better to understand the singularity of the scene we are about to record, the reader must imagine four individuals, one at the office in Washington, one at Baltimore, 40 miles distant, one at Philadelphia, 108 miles farther, and one at Philadelphia, (or rather Jersey City, opposite New York,) 112 miles farther. telegraphic line passes through the instru ments at the offices at each of these places, and a communication despatched from any one of these places is written and understood instantly at all the others. We shall designate the operators by the names of the places at which they are stationed:

The

Washington.-Baltimore, are you in connection with Philadelphia?

Baltimore.-Yes.

Phila.-Pretty well. Is that you, Washington?

Wash.-Ay, ay; are you connected with New York?

Phila.-Yes.

Wash.-Put me in connection with New

York.

Phila.-Ay, ay; wait a minute. (After a pause.) Go ahead. Now for it. Wash.-New York, how are you? (New York does not answer.) Phila.-Hallo, New York, Washington is talking to you. Don't you hear him? Why don't you answer?

New York. I don't get anything from him.

Wash.-I get that from New York. Phila.-New York, Washington says he gets that from you.

Balt. How is it that Washington hears from New York, and New York does not hear from Washington?

Phila.-There's where I'm floored. Balt. What is the reason, Washington? Wash.-Because New York has not properly adjusted his magnet.

Phila. I have been hard at work all day. I feel like bricks. Had no supper. I have had a stiff evening's work; there has been so many messages to-night-one alone that gave us seventeen dollars. I want to go.

Wash.-Wait a little.
Balt.-Go it ye cripples.
Phila.-Who is writing?
Wash.-Don't talk all at once.
Balt.-Mary Rogers are a hass, so are
Sally Thompson;

General Jackson are a hoss, so are
Colonel Johnson.
Phila.-Who is that? I will discuss that

107

And so ends this curious scene; not an imaginary one, but one of actual occurrence. Let any one reflect upon the fact, that all these questions and answers occurred in a space of time but very little longer than that in which this unique drama has been related.-Union.

MR. GACHET'S SCREW EXPERIMENTS.

Sir, I am sorry that my letter, inserted in your Number of June 13, should have given Mr. Gachet any cause of complaint; but the fact is, that whenever a working model is mentioned, it always occurs to me that the model is in all its parts an exact copy of the machine, except in size,-that is, that every part of the model is in proportion to the other parts of the same,-as they would be in the machine itself. This must be my apology for the error I committed in reference to the proposed tube.

Mr. Gachet confesses that his statement of the experiment may have reasonably disappointed me, and then enters into particulars of the disadvantages that he had to contend with, which, no doubt, were considerable; but as he did not previously mention them, of course I could not make any allowance for them; for I have never been at Coggeshall, although I have frequently heard of the place. But I still think that he is mistaken as to the comparative speed to be obtained by four oars (not four pairs) and his screw; for a man at an oar is not always pulling; he must make the back stroke as well as the forward one, and therefore, upon the reasoning followed by your correspondent, the power exerted would only be equal to half a man; but it appears to me that Mr. Gachet's arrangement must have been very, very defective, to leave only one-fourth of his power available. Mr. Gachet says, "my chief object was to ascertain the effect of a screwpropeller inclosed in a tube, and seeing that it had the same power of propulsion thus fitted as being in free water, I there ended my experiment." Allow me to ask him how he ascertained this fact; for I remember no experiment described by him that bears him out in this supposition. Remember, the experiment to prove this must be made with tube, &c., of the same dimensions in proportion to the other parts of the machinery, the vessel included.

In answer to the question respecting the "flat-headed" stopper, I should calculateas Brother Jonathan would say-supposing the plan carried out, that the head of the stopper should be upon a plane, and quite flush, at all parts, with the bows of the vessel. As to the effect of the stern aperture when left open, I think Mr. Gachet would find himself much disappointed, and that