similar experiments to the above; but, with two exceptions, the details are not sufficiently ample to found any calculations upon. The two exceptions named are the furnaces at the Clyde, and at the Butterley Iron Works, when they were blown with cold air. Both these blowing machines are described as having been in use for several years; and it is, therefore, natural to suppose the various parts were more worn, and fitted less accurately than in those experiments already described. The experiments were also made with less care. They show a different result to those already detailed; as in these the calculated quantity of air appears to be less than the quantity which entered the blowing cylinders in about the same proportion as it exceeded it in the former cases. The difference, no doubt, arises from the imperfect fitting of the piston of the blowing cylinder, which, by allowing a portion of air to escape, would diminish the apparent pressure on the mercurial gauge placed at the further extremity of the apparatus, and thence the calculated rate of efflux would, of course, be diminished.

In the experiments at the Clyde Works, the quantity of air which was discharged into the furnace, when estimated by the quantity that entered the blowing cylinder, was 2827 cubic feet per minute. The pressure of the blast was equal to six inches of mercury, and the area of the tubes .0681 of a cubic foot. Calculating the discharge of air under this pressure, it amounts to 2450 cubic feet, being 13 per cent. less than the measured amount, supposing no loss to occur by imperfect fitting of the apparatus.

At the Butterley Works the quantity of air discharged into the furnace, estimated by the contents of the cylinder, was 2500 cubic feet per minute. The pressure of the blast was equal to five inches of mercury, and the area of the tubes .0681 of a cubic foot. The quantity, by calculation, appears to be 2235 cubic feet, being less by 101 per cent. than that shown by experiment. In both these last cases, however, there is but little doubt that the loss of air from the cylinder caused the pressure on the mercurial gauge to be less than it would have been, had the apparatus been perfectly tight; and a very small diminution in the observed height of the mercury would account for a much greater difference in the velocity of efflux than is here shown. We are fully warranted in the conclusion, from these experiments, that this method of calculation is as accurate as any theoretical determination of such a question can be; but, from the results so obtained, an allowance must always be made for friction, which will necessarily vary with the peculiar circumstances of each case.

The following table will exhibit the results of the preceding experi

ments at one view:

most accurate, as it is also the most simple, for general use. therefore, be concluded, that the method which gives this result is the culated amount to the quantity obtained by experiments. It may, of a reduction of 9 per cent. being made for friction to reduce the calhighest of these calculations is the nearest the truth, as it only allows were only 14 inch diameter, it would probably be considered that the periment-viz., 333 miles per hour-and also that some of the tubes charge of air, at the immense velocity which was obtained in this exConsidering the amount of friction which must result from the dis

Lond. Mining Journ.

Description of the new Dock Gates at Grangemouth, on the Forth. By JAMES THOMSON, ESQ., C. E., Glasgow.

Mr. Thomson exhibited a model and drawings of the dock gates recently constructed at Grangemouth, where the entrance lock, upon which are four pairs of gates, is 250 feet long, 55 feet wide, with a depth of 25 feet water over the sill into the new wet docks. The gates, which are wholly built of timber, consist of a double framework, the front framing, or that next the sill, being straight, and the back curved, both, of course, uniting together in the heel and meter posts. The back, or curved framing, is formed with arched ribs, composed of plank in three thicknesses of four inches, firmly bolted together, and to the corresponding front ribs; this double framework, being braced together with horizontal, and diagonal, tension rods of iron, is planked on both sides, and made perfectly water-tight. The gates constructed in this manner are extremely light and buoyant; and by the admission of more, or less, water, with additional balance weights, the buoyancy of the gate is so adjusted, that its whole weight is borne by the water, and, consequently, very little power required for opening and shutting, besides the great saving in tear and wear, thus reduced to a minimum; in proof of which it was stated, that instead of from ten to twelve minutes, with four men, the time and power usually required for opening, or shutting, gates of similar dimensions, these gates, by means of improved crab gearing, are opened, or shut, in three and a half minutes, with only two men; and with a longer allowance of time, even one man is able to work them. Thanks voted, and given from the chair.-Trans. Roy. Scot. Soc.

Civ. Eng. & Arch. Journ.

Observations upon Iron Lattice Bridges. By JAMES THOMSON, Esq., C. E., Glasgow.

Mr. Thomson illustrated his observations with numerous drawings and models of several lattice bridges already completed, or at present carrying into execution, by Mr. Macneill and himself. The principle of lattice bridges, applied in timber, has, for some time back, been in use in America; but the adaptation of the principle to iron bridges, as now so successfully applied by Mr. Macneill and Mr. Thomson, is likely to supersede, in a great degree, the use of timber, and will supply a desideratum hitherto felt in establishing internal lines of communication, &c., where the expense of stone bridges not unfrequently precludes their being carried into effect, while the objection to wooden structures, on the score of durability, is obviated by the substitution of iron. The first bridge of this kind, recently completed by Mr. Macneill, on the line of the Dublin and Drogheda Railway, and of which drawings were exhibited, is 85 feet in span, consisting of two latticework frames, or beams, one on each side, resting upon stone abutments; the lattice-work beams are composed of small bars of malleable iron, about 12 feet long, and only a quarter of an inch thick, placed VOL. VII, 3RD SERIES-No. 4.-APRIL, 1841.

20

so as to cross each other at right angles, and forming a net, or latticework, riveted at every intersection; the lattice frames, so constructed, and stiffened with angle iron, support the roadway by means of light transverse beams, also of malleable iron, secured to the lattice-work at each end. This bridge, which altogether weighs only fourteen tons, sustained a load across its centre of twenty-four tons, under which the deflection amounted only to 3-10ths of an inch.* A viaduct 230 feet in length, with a central span of 140 feet, is now being constructed by Mr. Macneill, over the Royal Canal in Ireland, for heavy locomotive traffic. This viaduct, of which Mr. Thomson exhibited drawings, has a third lattice frame in the centre, and is composed of malleable iron bars half an inch thick. Mr. Thomson described a very useful application of this principle to the widening of the roadways of existing bridges; and exhibited a prettily constructed model of one of the arches of a stone bridge, about 400 feet long, with the addition of footpaths on each side, as at present executing under his direction, the old width of roadway being only 17 feet, while, with the new footpaths, supported by iron lattice-work, the width will be increased to 30 feet. The appearance of these bridges, which may be either perfectly straight, or slightly curved, as circumstances require, is light and graceful, combining, as they do, great strength with the least possible quantity of materials, and seem to be admirably adapted for crossing wide and deep valleys, rivers, &c., at a small expense, as also for ornamental bridges in parks, or approaches to gentlemen's seats, &c. The expense of construction, Mr. Thomson estimates at less than half the cost of stone; but he stated that he was at present making an investigation into their principle, with experiments upon a different arrangement of the lattice-bars, which he hoped would lead to a considerable saving, both in the quantity of material and workmanship.-Trans. Roy. Scot. Soc.

Ibid.

Description of a Cast-Iron Bridge over the Avon, near Tewkesbury, on the line of the Birmingham and Gloucester Railway. By Capt. W. S. MOORSOM.

The principal novelty of this work, which was proposed, and its execution superintended, by Mr. Ward, of Falmouth, is the mode of constructing the two piers, which were externally of cast-iron, in the form of caissons, each weighing about twenty-eight tons; the plates composing each caisson were put together on a platform erected upon piles over the site of the pier: the bottom of the river being leveled by a scoop-dredger, the caisson was lowered, and, some clay being thrown around the exterior, a joint was formed, so nearly water-tight, that two small pumps drained it in six hours. The foundation being then excavated to the requisite depth, the caisson, which sank as the excavation proceeded, was filled with concrete and masonry; cap plates were then fixed for supporting eight pillars, with an entablature,

The cost of this bridge, including the stone abutments, was £510; the lattice bars are 21, the lattice frames 10 feet deep, and the floor line is 5 feet above the bottom.

to which was attached one end of the segmental arches, 57 feet span, with a versed sine of five feet two inches. There were three of these arches, each formed of six ribs of cast-iron, and two such piers as have been described, the land abutments being of stone work, joining the embankment of the railway. It was stated that this mode of construction was found to be more economical, in that peculiar situation, than the usual method of fixing timber coffer-dams, and building the piers within them, the total cost of the bridge being only 10,1927, and the navigation of the river was not interrupted during the progress of the work. The paper was illustrated by eighteen drawings by Mr. Butterton.-Trans. Inst. C. E.

Lond. Athenæum.

The Screw Propeller-Steam Navigation.

At the last meeting of the Liverpool Polytechnic Society, the president, John Grantham, Esq., C. E., in the course of his annual address, said, that finding that he had but few observations to make on the state and prospects of the Society-so even had been the tenor of its way through all the changing scenes of the times-he should introduce to their notice a topic of public interest, suited to the character of their meetings: the subject he alluded to was the present state of steam navigation. After some introductory observations, as to the failure of the science as a profitable mercantile speculation, he called their attention to the screw propeller, as a substitute for paddlewheels-an improvement which he had great hopes would do much to place steam navigation on a firmer foundation. Several short notices of the screw propeller had appeared in scientific publications, [see Mining Journal of the 28th October, for a detailed description, with diagram,] but they were very imperfect, and little could be gleaned from them. It had, however, been referred to more satisfactorily, in a paper written by Mr. Elijah Galloway, the patentee of paddle-wheels, in an appendix to Tredgold's work on the steam engine. But the author had not formed a decided opinion on the question, and did not establish its superiority. The French claimed to be the original inventors of the screw propeller, and few would dispute with them the honor on this point-though they also claimed the steam engine, which was due to the English. The lecturer here referred to a French paper detailing the performances of the French war steamer Napoleon, which were certainly satisfactory, and next noticed a number of instances in which the screw had been employed, even from the year 1699. It was also tried by different parties in 1743 and 1763. In 1802, the Doncaster transport, which had been becalmed, was worked into harbor, at Malta, at the rate of one and a half mile per hour, by eight men at a spell. She went seven leagues with a screw, and the parties seemed to have contemplated every kind of propeller since patented by others. In 1825, the screw was applied to a vessel in the Thames. In 1828, a patent was taken out for a screw by Mr. Charles Cummerow. In 1832, M. Sauvage also