extra covering, grinding down the walls by their united pressure, and pregnant with the elements of decay and destruction (the liability of timber to rot, and its fearful combustibility). Away also with the drafts from the Northern forests of Europe, and our dependencies upon a foreign supply of timber. The mines and rocks of our native country are our resource, and our manufactures are stimulated. Iron, in its best form, in its malleable state, not cast; tensile to a proverb, as we say, "tough as pinwire." Iron rods or cables; iron rafters; iron gratings and frames, to receive an impermeable cement, all secure, and next to imperishable.*

"This little model shows how, with a load equal to fifty, nay, a hundred times its proportioned weight of covering, the rods perform their duty. And I must stop at this point to tell you, that the power of wrought iron to bear suspended weight is such, is so astounding, as that a rod 1 inch square, will bear 80,000lbs. or 35 tons. The whole weight of the roof of the proposed Hall will be about 200 tons, including that which is to rest upon the piers and arches, therefore four rods of an inch square each would suffice to carry it; but I should have a power equal to forty such rods at least, or ten times the bare sufficiency."

Of the soundness of this new principle of construction we have not a shadow of doubt; and cannot help wondering that with so many striking examples before our eyes of its successful application in bridge building, no one should before now have thought of its still greater suitableness to roofs of buildings of large area. Doubtless the saving must be great from dispensing with the huge system of piers, pillars, posts, &c., now requisite in all cases, where roofs of large dimensions on the ordinary plan have to be sustained; and we are disposed to put every trust in the estimates on this head of a gentleman of Mr. Hansom's great practical experience; but we foresee other advantages as likely to arise from his suspension plan of roofing, which would make the cost a matter of comparatively small moment. Buildings may be raised on this plan on a scale of magnitude, as well in respect of height as of area, which could not be erected at all on any other plan; and with greater height than has been ever before witnessed in our public buildings, with one

• Referring to one exhibited in the Lecture Room.

or two rare exceptions, we may expect to see a vast improvement in all those architectural details to which height is auxiliary. Liability to fire will also be greatly diminished by the general substitution of iron for wood in building; we say general substitution, for once applied to roofing, that would infallibly lead the way to its adoption for floors, staircases, doors, windows, &c., to all of which it is equally applicable.

To the Music Hall, both for its own sake and for the sake of the new system of building likely to be identified with it, we cordially wish every possible success.

MESSRS. RENNIE'S NEW ENGINE. Sir,-Your remarks on Messrs. Rennie's engines seem to imply that the double cylinder engine has been shelved in London as well as by its "Cornish patrons." Now this is not quite correct, as these engines have long been the staple manufacture of the Messrs. Hall, of Dartford, who besides furnishing our French neighbours with a great number, who duly appreciate them on account of their small consumption of fuel, can point to many of their erection in London and its environs, as well as in different parts of the country. There is also the firm of Messrs. Easton and Amos, of the Grove, Southwark, who manufacture these engines to some extent, and have many of them doing their work exceedingly well in London and its neighbourhood. I believe there are two of 25-horse power each, of their construction, at Battersea flour-mills, and, I am told, so beautifully does the last erected perform, that in the dark, you would not know she is at work, but for the breathing of the steam through the passages. Besides the above, there are doubtless other parties who construct this kind of engine, and therefore the assumption "that the engineering world has been taken by surprise" by what is termed the "re-production in the metropolis of an engine upon this principle, by engineers of eminence" must be incorrect.

With regard to the consumption of fuel, I believe the Messrs. Hall guarantee 34lbs. per horse per hour; but 3lbs. I am given to understand is very seldom exceeded, and Easton and Amos's engines are equally successful, I apprehend, in this particular.

The Messrs. Rennie's engine is said not to consume more than 2.2lbs., but sufficient data to warrant such a conclusion have not yet appeared. The removal of three probably old crazy engines of the nominal power of sixty horses, is insufficient, as they were unlikely to perform more than thirty-five or

forty, and from the difficulty of ascertaining the real amount of duty in consequence of the nature of the work on which they were employed, I feel inclined, in the absence of further particulars, to take the power exerted by the new engine at 40 horses; divided by which, the consumption of fuel will agree with that required by the engines of the above experienced manufacturers, the Messrs. Hall, and Messrs. Easton and Amos.

The engines manufacted by the above firms, expand, I believe, only four times, which gives an advantage to the Messrs. Rennie, as their engine expands five; but much will depend upon the construction of the boiler employed by each party, as a difference in that respect may give an advantage with so small a consumption as to cause a variation of perhaps 25 per cent.

I have taken occasion to remark long ago in the Mechanics' Magazine, and other publications, that though engines had attained a very high degree of perfection, boilers generally were still very rude, unsafe, inefficient, and without principle, either in construction, the application of heat, or the exposure of the water; and I am glad to see Mr. Williams has in your last Number also denounced the existing boiler practice in similar terms.

I have also urged in your pages, and elsewhere, that the kind of engine in question is the only one by which the greatest possible effect can be obtained, and therefore the fittest for steam navigation-not, however, that steam exerts a greater power by expanding in two cylinders, than it does in one, but because the use of two cylinders "equalizes" the power more nearly; that is to say, it reduces the difference between the commencement and termination of the stroke to a smaller amount than it is possible to effect in one cylinder, thus producing a more equable motion, a more convenient and manageable application of the principle, and thereby affording the means of carrying it out to the utmost profitable extent of which it is capable, but which it has not yet attained. This is the advantage of employing two cylinders, though the Messrs. Rennie were of opinion, not long since, that there is no more advantage in expanding in two cylinders than there is in one. It seems, however, they have found reason to become converts, and I hope they will oblige us with the result of the experiments said to be in progress with their new engine.

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RECENT AMERICAN PATENTS. [Selected and abridged from the Franklin Journal.] IMPROVEMENT IN DRY DOCKS; Charles F. Johnson. This dock operates on the principle of those which raise the vessel by having inverted boxes under the cradle, into which air is introduced to exclude the water, and the patent is granted for a mode of preserving the equilibrium of the cradle and prevent rocking, which is effected either by means of vertical screws attached to each side of the cradle, and passing through nuts on each side of the dock, or by racks with ratchets into which palls work, or by ropes passing around pulleys attached to the sides of the cradle and dock.

LAYING VENEERS; Casper Kittenger. A board of any given length and width, to be governed by the size of the article to be veneered, passes through mortices made in two pieces of hard wood, one placed at each end. A mortice or slot is made in each of these pieces of wood, parallel with the one through which the board passes, to receive two rods of iron having a screw and nut on each end, to prevent them from falling out of the slot, but not screwed tight enough to prevent them from sliding along the length of the slots. To these rods is attached a band or sheet of iron, leather or cloth, reaching nearly the whole length, and hanging loose from one to the other, so as to form about a half cylinder. The board is provided with any given number of band screws, which pass through it and act upon the back of the article veneered, thus forcing the veneered surface against the band which adapts itself to the form of said surface. The claim is to the combination of the screws and bands, as described.

IMPROVEMENT IN THE MODE OF PACKING ROTARY STEAM ENGINES; John D. Aikin. This improvement is only for the packing of those parts which present two or three faces. The part to be packed is provided with a groove, into which the metallic packing is fitted. This packing consists of two metallic plates put together by halving the pieces where they meet, in the manner of a square-shouldered splice, and two screws, with conical ends, are employed to spread and force out the two halves, the conical end of each screw passing in between the end of one piece and the shoulder of the other; so that as the screws are advanced the packing will be forced out and spread endwise. This kind of packing is described as applied to the steam heads and sliding valves of a rotary steam engine. The claim is to the mode of packing "by means of the countersink, halved plates, and conical screws." IMPROVEMENT IN SPRING SADDLES ; Orren M'Cluer. The leather of the saddle

seat, instead of being attached to the head of the tree, is attached to a spring which is fixed to the head of the tree, or to the straining web, which connects the spring and the back of the saddle. The skirts of the saddle lap over the covering of the seat sufficiently to allow the play of the spring, without allowing the edge of the covering of the seat to be drawn from under them. This arrangement allows full play to the spring without straining the covering of the seat.

MODE OF STRETCHING CLOTH IN THE PROCESS OF FULLING; B. D. Whitney, and G. W. Lawton. The cloth is carried round a series of rollers, one of which has its surface cut into a right and left-handed screw, each commencing in the middle and running out to the end. As this roller or double screw revolves with the cloth drawn over it, it will stretch the cloth widthwise. The same object is more effectually attained by making the surface of the roller in sections, sliding from the middle towards each end; the surface being seared or grooved so as to take hold of the cloth. The sections or segments project at each end beyond the body of the roller, and the part which projects is provided with a roller which fits into a spiral groove in a stationary cylindrical block at each end, and the grooves are so arranged that as the segments move round when they come to that part of their circuit in which they meet the cloth, (which is only in contact with a portion of the circumference,) the segments move outwards, stretching the cloth from the middle towards each edge, and when they leave the cloth they are drawn in.

IMPROVEMENT IN GRIST MILLS; Edward Gray. This improvement consists in placing a pair of small stones in the eye of the runner to prepare the grain before it passes to the large stones. The under one of the two small stones rests upon the driver of the large runner, and revolves with it, and the upper one is driven in the opposite direction by a driver attached to a small shaft, which takes the place of the damsel; its lower gudgeon working in the upper end of the main spindle. The grain is fed into the eye of the small upper stone, and after being partially ground, passes to the large stones to be ground into flour or meal. The defects in the common grist mill, which the patentee says he has overcome, are, "1st. The slowness of the grinding performed around the eye of the common stones, owing to the slow movement of the runner at this part of it, and the consequent insufficient supply of prepared grain for flouring, or being ground into flour, which is accomplished by the surfaces of the stones near the circumferences thereof, where the movement is quicker.

And 2nd, the introduction of too much cool air between the stones through the eye of the runner."

IMPROVEMENT IN BREAST AND PITCH BACK WATER-WHEELS; Edward Robbins, Jr., and William Ashby. This improvement is designed to admit of the escape of the air which remains in the buckets after they have received the water from the flume, and allowing it to escape through openings in the sole or lining of the wheel, as the buckets, in their descent, are being immersed in the back, or tail race water. The buckets are either radial, or inclined to the radii, and do not reach the sole or lining of the wheel, but leave an open space between the two, which is closed by a flap either hinged to the back of the bucket, or turning on pivots in the shrouding, so as to operate as valves or shutters, the back or inner edge closing against the sole or inner lining directly above the hole made through it for the escape of the air in the bucket below. As the bucket descends, the back of the flap comes in contact with the water in the tail race, which opens it, and passes into the bucket following, forcing the air contained in the bucket, above the water, out through the hole in the sole or lining. Were it not for this arrangement, the patentee says the "air would be carried down, and be forced to descend with the buckets through the back water. This carrying down of the air, it must be manifest, would offer a considerable resistance to the motion of the wheel, and that it does so, is well known to us from most satisfactory experiments which we have made. What we claim, is the employment of valves in buckets of waterwheels, such valves having the position herein described, and being used in combination with the openings through the soling of the wheel, that is to say, said valves forming an angle of 130°, more or less, with the radiating buckets, or with radii of the wheel, and closing against its soling immediately above the opening, for the escape of air."

PREPARING WHITE LEAD PAINT; James N. Trovillo. This improvement consists in incorporating with a given quantity of white lead, equal, or nearly equal, quantities of linseed oil and of pure water, preparatory to grinding the paint, by first carefully incorporating the water and lead, and then adding the oil, and also in subsequently reducing it to a proper consistence by the addition of the same materials in the same proportions. The patentee says "I do not claim to be the first to have incorporated linseed oil and water together in the preparation of paint, with a view to economy in the use of the former article, this having been done by the