fected through the bridge, one-the northern-will be first completed, requiring the erection of two of the four main tubes. And the first of these two tubes erected has been that at the west end of the bridge, or between the Anglesea and Britannia towers. At this time (Nov. 1849), this one only of the tubes has been erected and completed; and the follow

ing sketch of the proceedings connected with the floating of the tube on the 27th of June last, written by an eye-witness, is sufficiently interesting and authentic to be quoted. After the preliminary arrangements for letting go had been completed, Mr. Stephenson and other engineers got on the tube, as also Captain Claxton, R.N., famed for releasing the 'Great Britain' from her dangerous imprisonment in Dundrum Bay, to whom the management of the floating was intrusted. "Captain Claxton was easily distinguished by his speakingtrumpet, and there were also men to hold the letters which indicated the different capstans, so that no mistake could occur as to which capstan should be worked; and flags, red, blue, or white, signalled what particular movement should be made with each. About half-past seven o'clock in the evening, the first perceptible motion, which indicated that the tide was lifting the mass, was observed, and, at Mr. Stephenson's desire, the depth of water was ascertained, and the exact time noted. In a few minutes the motion was plainly visible, the tube being fairly moved forwards some inches. This moment was one of intense interest; the huge bulk gliding as gently and easily forwards as if she had been but a small boat. The spectators seemed spell-bound; for no shouts or exclamations were heard, as all watched silently the silent course of the heavily-freighted pontoons. The only sounds heard were the shouts from Captain Claxton, as he gave directions to 'let go ropes,' to 'haul in faster,' &c., and broadside on :' the tube floated majestically into the centre of the stream. I then left my station and ran to the entrance of the works, where I got into a boat, and bade the men pull out as far as they could into the middle of the straits. This was no easy task, the tide running strong; but it afforded me several splendid views of the floating mass, and one was especially fine; the tube coming direct on down the stream,-the distant hills covered with trees,-two or three small vessels, and a steamer, its smoke blending well with the scene,-forming a capital background; whilst on one side, in long stretching perspec

tive stood the three unfinished tubes, destined ere long to form, with the one then speeding on its journey, one grand and unique roadway. It was impossible to see this imposing sight and not feel its singleness, if we may so speak. Anything so mighty of its kind had never been before; again it would assuredly be; but it was like the first voyage made by the first steam vessel,-something till then unique. At twentyfive minutes to nine o'clock the tube was nearing the Anglesea pier, and at this moment the expectation of the spectators was greatly increased, as the tube was so near its destination; and soon all fears were dispelled as the Anglesea end of the tube passed beyond the pier, and then the Britannia pier end neared its appointed spot, and was instantly drawn back close to the pier, so as to rest on the bearing intended for it. There was then a pause for a few minutes while waiting for the tide to turn; and when that took place the huge bulk floated gently into its place on the Anglesea pier, rested on the bearing there, and was instantly made fast, so that it could not move again. The cheering, till now subdued, was loud and hearty, and some pieces of cannon on the shore gave token, by their loud booming, that the great task of the day was done." ."* When in its position, the tube is made to settle down upon a bed of timber on its bearings at the feet of the towers, by opening the valves in the pontoons, and thus sinking them sufficiently to free them from the tube.

Lifting the Tubes with the Hydraulic Presses.-If there is one part in the design of these stupendous bridges which evinces boldness greater than another, it is in the first idea of raising a weight of 1,800 tons, through an elevation of 100 feet, over a rapid stream of 460 feet in width, and utterly without scaffolding of any kind over the opening. The power to be employed for this gigantic purpose, and the manner of employing that power, are two problems of startling novelty, and which threatened to involve immense practical difficulty.

*Correspondent of the Illustrated London News, June 30, 1849.

The happy adaptation of the buoyant power of water, so successfully realised in the floating of the tubes, promised no assistance in the raising of them; yet with the aid of simple machines, actuated by this same liquid, which by a law of its action multiplies to an almost unlimited degree the minimum. of power applied to it, these tubes are raised with the utmost facility, and with all the regularity and safety of motion which characterise mechanical operations upon a smaller scale.

These machines, known as Hydraulic, or Hydrostatic Presses, are adapted for gaining great power; acting, however, through a limited space. The invention of the apparatus belonged to the late Mr. Joseph Bramah, who, on March 31, 1796, obtained a patent for it, under the title of "certain new methods of producing and applying a more considerable degree of power to all kinds of mechanical apparatus and other machinery requiring motion and force, than by any means at present practised for that purpose." The operation of this machine is founded upon the elementary principle in hydrostatics, that "when a liquid mass is in equilibrium, under the action of forces of any kind, every molecule, or part of the mass, sustains an equal pressure in all directions." The consequence of this principle is, that a pressure exerted on any portion of the surface of a confined mass of fluid is propagated throughout the mass, and transferred, undiminished, to the entire surface in contact with the water. In the middle of the 17th century, Pascal suggested the application of this principle to the operation of a press, but to Bramah is due the credit of first realising this suggestion in a practical form. The hydraulic press has been employed to a considerable extent in pressing goods for packing, expressing vegetable oils, and other similar purposes. By its aid, moreover, the performance of experiments upon the strength of various materials has been much facilitated. In the testing of iron girders, anchors, and other similar productions intended to sustain great weights and strains, this powerful apparatus has been usefully engaged for many years in the English dockyards

and the establishments of iron-founders and manufacturing engineers. But its most recent and distinguished employment is in the elevation of the tubes for the railway bridges over the river Conway and the straits of Menai.

Hydraulic presses consist of two essentially distinct parts, viz. the press, or machine, in which the acquired force is applied, and the pumping apparatus, by which the water is forced into the press; these two parts, constituting the entire apparatus being connected only by a pipe through which the water passes from one to the other. The press consists mainly of the cylinder, into which the water is admitted, and which is solid at one end, and open at the other to receive the ram, plunger, or piston, which is solid and cylindrical, and turned to fit the bored opening in the cylinder. This opening is enlarged at a few inches from the face, so that, although the ram fits it closely along these few inches, an annular space is left within, between the ram and the cylinder, and into this space the water is forced by the pump. The pump needs no detailed description here, being of the ordinary kind used for forcing liquids, and is varied in its parts, form, and dimensions, according to the particular applications of the apparatus. The pump is usually worked by manual labour, with a leverhandle, and the rule for finding the increase of power commanded by the pump is derived, first, from the ratio of the areas of cross section of plunger of pump and ram of press; and, secondly, from the ratio of the leverage of the pumphandle. Thus, suppose the plunger to be inch, and the ram 10 inches in diameter, and the arms of the lever or handle as 1 to 6, the power will be thus found:

and thus a power equal to 20 lbs. applied on the end of the pump-handle will produce a pressure equal to 21,333-20 lbs. on the ram, or 9 tons 10 cwt. 3 qrs. 1.20 lbs.