THE principal difference between this hammer and pile-driver of Messrs. Hammond and Souter, and that of Mr. Nasmyth, is that the former is worked by air and the latter by steam.

Fig. 1 and 2 are two sectional elevations of this instrument,-fig. 1 being on the line AB of fig. 2. Fig. 3 is a plan on the line CD of fig. 1, and fig. 4 another plan on the line EF.

A is an air reservoir, which is con

A new firm, which this invention must serve to introduce with favour to the knowledge of the engineering public. Both the members of it have long been leading hands in the eminent establishment of Messrs. Seaward and Co., and in setting up for themselves, carry with them much practical skill acquired in one of the first of schools, and (what is somewhat rarer,) the friendship and best wishes of their late employers. We subjoin, in corroboration of what we have stated, a paragraph which appeared in the Dispatch of last week, and should be glad to have oftener occasion to record such instances of cordial understanding between employers and their workmen :

"Presentation of a Silver Tea Service to the Manager of Messrs. Seaward and Co.'s Factory. at Limehouse, by the Workmen in their Employ. On Monday and Tuesday last, Messrs. Seaward and Co., the celebrated engineers, at Limehouse, entertained the whole of their workmen, upwards of 1,000 in number, together with their wives and families, with a trip to Gravesend, where an excellent dinner was provided for them, consisting of every substantial delicacy of the season. Adams's new cricket-ground, in the Wrotham-road, was the

nected by a pipe with an air-pump (not shown in the engravings ;) E is a working cylinder; F piston; G piston-rod; HH passages through which the air passes to and from the cylinders; J, K, L, and M, valves which open and close the passages, HH; N is the hammer; O is the anvil; PPPP, columns which support the cylinder and guide the hammer; Q is a lever which turns upon a stud attached to the hammer, and one

spot selected for the treat. Tables were laid out for the immense assemblage in the Grand Stand. The scene was a truly gratifying one, alike pleasing to the employer as to the employed. After the cloth was removed, nature was refreshed with copious libations, in which the healths and happiness of the founders of the feast (among the many loyal and frindly toasts drunk) were not forgotten. The most gratifying portion of the day's enjoyment was the presentation of a splendid Silver Tea Service to Mr. Hammond, the manager of the factory at Limehouse, by the workmen, as a token of their esteem for the universally kind feeling evinced towards them. The present was received with the same feeling it was given, and responded to in a neat and sensible speech. After this interesting ceremony, the services of the excellent band in attendance was put in requisition for the exhilarating enjoyments of the dance, and so earnest were its votaries, that, notwithstanding the thermometer indicated 80°, it was with extreme reluctance that the announcement of Ladies and gentlemen, the boat will not wait,' told the happy hundreds that the finale of the enjoyment had arrived."

end of which projects below the hammer, when it is descending.

When the blow is struck, the lever Q is forced up to the level of the bottom of the hammer, and the rollers SS, at the other end of it, move the rod T, which gives motion to the levers attached to the spindles of the valves, J, K, L, M, so as to open J and L, and to shut K and M, upon which the piston and hammer begin to ascend again.

U is a rod attached to the lever Q, and projecting above the top of the hammer; V is a stud which can be moved up and down one of the columns by the screw W, and fixed at any point so as to give the requisite length of stroke to the hammer. When the hammer has moved up the requisite distance, the top of the rod, U, strikes against the stud, V, and is thereby forced downwards, closing the valves J and M, and opening the valves L and K by means of the train of levers described before.

X and Y are a catch and spring, which prevent the rod, U, moving, until it is necessary to reverse the motion of the hammer.

Z is a spring, to the upper part of which the rod T is attached, and which allows the motion of the gear-work to be continued after the valves are closed.

SCREW PROPELLING-MR. GACHET'S
EXPERIMENTS.

Sir, Mr. Gachet, in the description he has given in No. 1189, p. 396, of his improved screw propeller, should have stated the comparative speed obtained by the four men with his screw, when put in competition with the speed obtained by four men with oars, in the same boat; because, from the statement as it stands, nothing can be learned as to the success of the experiment, except that with four men the speed was but slow, and that with six men, (all the boat could stow,) the speed was slower still. To be sure, your correspondent states, that the experiment "nevertheless proved successful, so far as the limited power applied to it would allow;" but we all know, "that every man's own goose is a swan," and being an amateur conjurer myself, I can truly say, that this is frequently my own case, and therefore know, from sad experience, that it is necessary, to save

future expense and disappointment, to sift a statement that recommends an innovation, even if it appear much more feasible than the one in question, which to me appears to have insurmountable defects. As I have before stated in one of your former Numbers, I think the friction of the water passing through a tube from stem to stern of a vessel must inevitably very much outweigh any advantages that can possibly be obtained by this plan; for the friction will be upon the whole length of the shaft as well as upon the tube, and supposing the inventor to carry out his idea of making the after ends of the tube larger than the fore end, and succeeds by this in reducing the friction, he could only gain this profit by another loss.

A vessel intended for anything like speed is tapered off, (at the depth he proposes to place the tube,) for near twothirds of her length, until at her sternpost she presents nearly a knife edge, and therefore the ends of the tube would operate in the same manner as if the vessel was towing a round table, by a rope made fast to its pillar, which, in proportion to the tube employed in the experiment, would be 41 feet diameter to 100 feet in length, and this without enlarging the after end. As to the supposed advantages under sail, by "closing the fore ends of the tube," if this were ever carried into practice it would be found to retard the vessel much more than if the tube were left open, for the suction would be so great, when she attempted to move ahead, and thus leave the water in the tube behind her, that I am confident no screw, ay, not even a dozen of screws, of the present construction, would be a tithe of the obstruction.

I am, Sir, your old subscriber,

P.S. Upon looking over Mr. Gachet's letter, I am inclined to think that I have made a mistake in supposing he intends to increase his screw, and the diameter of the tube in proportion to the length; therefore, my supposition of 41 feet as the diameter is incorrect: but even 7 feet circumference is no joke, and if I Now understand his intention, he must see that his experiment with a 6 feet tube can be no guide to the performance of one of the same diameter, and upwards of sixteen times the length.

T.W.

Again, in reference to iron, Berthier, from his own experiments, considers that oxygen combines with this in the proportions 6, 7, 8, 9, and that in Mosandre's experiments a new oxide is formed.* The opinion that oxidation takes place gradually, is by no means inconsistent with the fact that bodies combine by preference in definite proportions; and that affinity has a term of saturation in such proportions. It may be, indeed, in consequence of a greater facility of combination in such definite proportions, that the intermediate states of oxidation are rarely or never met with in nature, and require express experiments for their production, as in Mosandre's or Berthier's, or my own experiments explained on the views taken in this paper. I do not, however, presume to say, that the experiments I have brought forward tend to establish this point decisively, but they certainly seem to render it probable, particularly when viewed in connection with the fact, that to uphold the theory of multiple proportions as a general principle, it is often necessary to have recourse to gratuitous suppositions; such, for instance, as when the relation of the radical and oxygen is as 3 to 4 or 3 to 5 (Berzelius, Traité, vol. iv. p. 593.) Besides, combination is known sometimes not to be in definite proportions, as in the case of solutions in water, and in the case of sugar, or gum, in water especially, which-probably in consequence of being organic matters,-combine in still more unlimited proportions, and apparently with no other limits than those imposed by the mixture actually becoming solid when the gum or sugar is in too great proportion. And here, if it be said there is no new substance formed, this cannot be asserted when an alkali or an earth dissolves in an acid. Here is a case of solution which cannot be separated from the idea of chemical affinity,

*Berzelius, Traité, tom. iii. op. cit.

* Albumen and gelatin, have also more or less of this peculiar property of combining with water in such very great proportions. This excessive solubility in water, and power of absorbing it (as it were,) is a property often, no doubt, of very great importance in both animal and vegetable life.

properly so called; and Berzelius calls solution "an unknown modification of affinity," (Op. cit. p. 585.) I have merely taken a cursory glance at this matter as somewhat connected with the present enquiries respecting the oxidating effect of heat in close vessels, to which I was led by doubting at first whether any change of weight took place in the metal tarnished under such circumstances. I therefore leave it for others to prosecute by more numerous experiments; for though Dr. Dalton's theory may be true as a general principle, it by no means follows that it should be received as true in all senses and all circumstances, or that there are not many exceptions to it.

Being anxious to approximate still nearer to the truth as to the possibility of oxidation by the specific power of a certain degree of heat, the following experiments were made: some iron filings and iron wire in an open tube, were found very visibly tarnished by exposure to a heat below redness.

Experiment H H.-A piece of thick bright iron wire was put into a glass tube with iron filings (a few copper turnings being also included.) The tube was hermetically sealed by the blowpipe, being about two-thirds full of filings, and thus allowing as little air as possible. Heated in the flame of a spirit lamp, and subsequently put in a crucible full of sand over a fire for seven or eight hours, the bottom of the tube had become quite blue by the heat, but had not given way. It had not increased in weight, but by comparison with some of the same filings in another part of the same tube, it was evident that oxidation had taken place, in the iron filings, and also in the end of the iron wire; the middle did not appear oxidated, probably from not having been exposed to so great a degree of heat. The copper turnings were not at all blackened or tarnished. Exposed now to the heat of nearly red hot sand, the tube burst with a slight explosion into a thousand pieces. On putting part of it, which had been blued by the heat already mentioned, into hot muriatic acid, the stain was not re