feet, with an elevation of 100 feet from the level of high water to the bottom of the bridge. This could not have been accomplished by the ordinary applications of iron, such as cast-iron arches or chain bridges; the former not giving sufficient height above the water-level, and the latter from their flexibility being unsuited for the support of railway trains. It was ultimately conceived that huge wrought-iron elliptical tubes, supported by chains, through which the trains should pass, might meet the demands of the Admiralty. But before this conception was adopted, a laborious series of experiments was instituted, which pointed out the defects of the elliptical tube, gave the true principle on which such a structure should be designed, determined the formulas for calculating its strength and proportioning its parts, and thus established an entirely new system of construction. The Britannia Bridge was then erected, and with its companion at Conway remains with hundreds of others composed of the same material and upon the same principle of construction, memorials of the accuracy of the investigations which led to the introduction of wroughtiron in this and other systems of adaptation. The total length of each tube of the Britannia Bridge is 1524 feet; height in the middle, 33 feet; and width, 14 feet 8 inches. The total weight of iron amounts to the enormous quantity of 10,570 tons. The Conway Bridge is of smaller dimensions, consisting of one span of 400 feet, crossed by two tubes, each 424 feet long, 25 feet 6 inches in height at the middle, and 14 feet 8 inches wide. The weight of iron amounts to 2892 tons. Since the erection of these bridges wrought-iron has been extensively employed for girder bridges of all spans up to 500 feet, and it is capable of being extended if necessary to spans of 1000 feet. PRIME MOVERS.-From 1784 to 1815, the cotton trade, with some fluctuations, made considerable progress; but the steam-engine was comparatively little appreciated or applied. In the earlier stages of manufacturing industry, the mills were erected on streams with waterfalls of sufficient power to turn their machinery. Hence the distribution of the mills over the country-as at Cromford, Belper, Bakewell, and Darley. At the commencement of my own career this was the condition of some of the largest cotton works in the kingdom. At the present time water, as a motive power, is only of secondary consideration, nineteen-twentieths of the mills now in existence being driven by the steam-engine. I well recollect the skill, beauty, and solidity with which the water-wheels were constructed when water was generally the motive power. The best of these wheels were made entirely of iron, on the principle of suspension, the arms not exceeding two or two and a quarter inches in diameter, and the power being taken off from the periphery by a pinion on the loaded side of the wheel. Mr. T. C. Hewes is justly entitled to the merit of these improvements, and was assisted by the late Mr. Strutt, of Belper. The most important of the recent improvements of the water-wheel are, however, the ventilation of the buckets and the use of cotters in the place of nuts and screws for fixing the arms and braces to the centre flanges of the wheel. Fig. 35 represents a large water-wheel of iron, of the best modern construction, and erected at Gefle, by W. Fairbairn and Sons, near Stockholm, in Sweden. This wheel is 40 feet in diameter, and 20 feet broad, or 18 feet between the shrouding plates. It is supported on the large cast-iron axis, a a, by means of the small wrought-iron arms, cc, and braces bb, on which the wheel is in fact hung or suspended. The arms are attached to the shrouding plates ss, and at the other end to the main centre, to which they are fixed by gibs and cotters. similarly attached diagonally between the main centre an The braces are the middle ring of cast-iron. Around the periphery of the wheel is fixed an internal spur-wheel, d, cast in segments, into which gears the pinion e, so as to afford the highest velocity direct from the water-wheel; this again is further increased by the spur-wheel ƒ and pinion g, and bevel wheels k and h, which transmit the power to the upright shaft i of the mill. There is therefore (1.) the water-wheel segment, d, with 324 teeth of 43 inch pitch, and 14 inches width, giving 2 revolutions per minute, geared into, (2.) Pinion e, 6 feet 1 inch diameter, giving 12.3 revolutions of cross shaft. (3.) Spur-wheel ƒ, 20 feet diameter, geared into pinion 9, 5 feet 6 inches diameter, giving 43 revolutions per minute to second cross shaft. (4.) Bevel-wheel h, 7 feet 8 inches, geared into bevel wheel k, 4 feet 1 inches, giving 80 revolutions of upright shaft i of mill. The power of this wheel is equivalent to 150 horses. Figs. 36 and 37 illustrate the construction and details of two pairs of condensing beam engines of 400 total nominal horses power, employed in driving the machinery for preparing spinning and weaving alpaca fabrics in the extensive mills of Titus Salt, Esq., of Bradford. They may serve as types of the present development of steam prime movers. These engines are arranged in two large enginehouses on either side of the front entrance to the mill buildings, and they are supplied with steam from ten boilers placed in a boiler-house beneath the surface of the ground, and a short distance in front of the mills. Fig. 36 shows a side elevation of one of these engines, giving a general view of the arrangement of the parts, and fig. 37 a cross section. The power generated in the cylinder e, and transmitted through the working beam b b to the large fly-wheel w, 24 feet in diameter, is taken direct from its circumference by the pinions pp, which give it off at the required velocity to the shafting of the mill. This arrangement has become very general for factory engines, and is the most effective and economical plan for generating at once the high speed which they require. The valves are of a peculiar construction, being a modification of the double heat or equilibrium valve invented |