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Note: This is a sub-section of 1873 Institution of Mechanical Engineers
Visits to Works (Excursions) in the Cornwall area
In the afternoon an Excursion was made by the Members from Penzance by special train and steamboat to Penryn, Falmouth, Restronguet, and Truro.
At Penryn the Granite Works of Messrs. Freeman and Sons were visited, where the application of machinery to various processes of stone dressing was seen in operation. The shaping of columns, balustrades, and other stonework of circular section is done by turning in a lathe, the work having previously been roughly dressed to shape by hand labour and centred in the lathe. The turning tool, instead of being a fixed tool as in turning wood or metal, consists of a freely rotating circular disc of the hardest tempered steel, about 9 inches diameter, mounted upon a horizontal axis in the slide-rest, and held up obliquely with its edge against the work in the lathe, so that the work revolving causes the tool to revolve in contact with it, and the cutting edge of the disc flashes off continuously small particles of the stone.
In the case of the best descriptions of work, the lathe is used only for roughing out to the true circular form, and the work is afterwards dressed finally by hand labour to take out the tool marks. The smoothing and polishing of the granite is done by means of cast-iron rubbers, which in the finishing process are faced with flannel. The smoothing is done with sand and water, the sand being fed in by hand by the attendant; and the polishing is done in the same way, first with emery powder and water, and afterwards for finishing with putty powder and water. The work done upon the premises at Penryn is mostly of an ornamental description for architectural, monumental, and other purposes; and the stone employed is principally grey granite, very hard and close-grained, which is supplied from a large number of quarries in the neighbourhood of Penryn; red granite from Dartmoor is also worked, and another variety of grey granite from Lamorna quarry near Penzance. Ordinary plain stones for building purposes, dock works, &c., are dressed by hand labour at the quarries; complicated shapes, such as those having a curvature in two directions, are executed at the works in Penryn.
From Penryn the Members proceeded to Falmouth, where they were entertained at luncheon in the Polytechnic Hall by the invitation of the Royal Cornwall Polytechnic Society. They were then conveyed by special steamboat from Falmouth Docks to the Restronguet Tin Stream Works (a description of which forms the subject of a paper read at the Meeting), where the propeller knife huddle and Collom's self-acting jigging machine described at the Meeting were seen in operation.
From Restronguet the Members proceeded up the river to Truro, and returned from there by special train to Penzance.
In the afternoon the Members visited the Tin Smelting Works of Messrs. Bolitho in Penzance, where the whole of the process of smelting tin from the ore was seen. The tin ore (peroxide of tin) supplied from the mines under the name of "black tin" is charged by hand shovels into reverberatory furnaces, each charge consisting of 30 cwts. of black tin, previously mixed with 21 cwts. of "culm," which is the "small" of the South Wales anthracite coal and is free from sulphur. The charge remains in the furnace about six hours, the firing being moderate at first, and afterwards more intense; the charge is only stirred once, about half an hour before tapping, more frequent stirring being avoided because the draught of the furnace carries off a quantity of the finest particles of the tin ore, and deposits it in the chimney and on the roofs, whence it is only partially recovered by collecting the rain water in tanks.
The liquid metallic tin is run off from the furnace through the tap hole into a large cast-iron pan, from which it is ladled into moulds, the slag being skimmed off. The "coarse metal," as it is then called, containing about 95 per cent. of pure tin, is next refined by the two processes of " liquation " and "boiling." In the former, the blocks of coarse metal from the smelting furnace are piled up in a refining furnace, and the tin is melted out at as low a temperature as possible, and run off into a basin, called the boiling kettle; as pure tin melts at a lower temperature than the impure metal, that which runs into the kettle is purer than the blocks of coarse metal supplied into the refining furnace.
The kettle is heated by a slow fire to keep the metal just melted; a bundle of sticks of green wood in an iron cradle is then plunged into it to the bottom, and the steam given off from the wood rises up through the mass of melted metal, and any impurities that may be present are carried with it to the surface, where they form a scum; the violence with which the steam escapes gives the metal the appearance of boiling. About 10 tons of metal at a time are boiled in the kettle, and the boiling is continued as long as any scum continues to rise to the surface; the damper the wood, the quicker is the boiling completed, the time occupied being from three to four hours. Samples are taken from each charge to show the quality of the metal, which is then ladled into smaller moulds to form the slabs for sale. The dross remaining in the refining furnace is afterwards subjected to a much higher heat in another furnace, to melt out all the tin possible; and this then undergoes liquation in the refining furnace with a fresh charge of coarse metal, together with all the scum that is skimmed off the boiling kettle.
The slag remaining in the smelting furnace after tapping is raked out at the back, and solidifies in large masses, which are broken up first by hand hammers and then by crushing rolls; the coarser portions are returned to the smelting furnace with a fresh charge of ore, and the finer after having been washed . and jigged are stamped by ordinary tin stamps; the slimes from the stamps are allowed to settle in "strips" or tyes, and are washed on a hand "frame," the metal recovered being sent to the refining furnace, while the waste is thrown away. The tin ore or "black tin" from the Cornish mines yields on an average 66 per cent. of metal or "white tin," 1 ton of metal being obtained from 12 ton of ore. Stream tin ore is richer, whether Cornish or Australian, and yields up to 72 and even 75 per cent. of metallic tin.
The Members were entertained at the works by Mr. Thomas S. Bolitho at a luncheon of steaks cooked upon the hot slabs of freshly run tin standing in the moulds to cool.
Excursions were then made by the Members in two parties, by special conveyances, to visit Botallack and Boscaswell Downs Tin and Copper Mines near St. Just, and the Longships Lighthouse near the Land's End.
Botallack Copper and Tin Mine
At Botallack Mine, where the Members were received by the Purser, Mr. S. Harvey James, the Diagonal Shaft extending under the sea at an inclination of 32.5° to the horizon (Plate 34) was descended in a safety break-carriage lowered by the winding engine with wire rope, as far as to the 190 fathom level, the water being in the bottom of the mine up to that level, from which a small quantity of tinstone of poor quality is at present being raised. The water from the 190 fathom level is hauled in a barrel by the winding engine up to the 180 fathom level, along which it runs back to Button shaft sunk to that level, and is raised by the pumping engine in Crowns shaft, by means of flat rods taken along a higher level from one shaft to the other. The 165 fathom level is the one that has been driven furthest out to sea on this lode, called Crowns lode; but it has now been abandoned, the end being very poor.
The most promising part of the mine at present is at Wheal Cock in the northern portion, where a lode containing good yellow copper ore and good tinstuff has been discovered, and is being opened upon; a timber framework 95 feet high is erected at the top of the shaft to carry the head gear for winding the stuff, in order to develope this part of the mine rapidly. The dressing operations are carried on in accordance with the description given at the meeting in the paper on Ore Dressing Machinery; in the part of the mine visited there are altogether sixty-four heads of ordinary stamps, and fifteen circular huddles all convex, and a large circular frame.
The slimes are dressed by fifteen of Zennor's machines, each of which consists of a circular revolving table, 17 feet diameter, having a slightly conical surface rising about 6 inches to the centre; the centre spindle on which the table wolves is canted at such an inclination as to raise the edge of the table on one side just level with the centre. The slimes are delivered upon the table at the centre, but only over the quadrant immediately preceding the lowest point of the edge; and during the slow rotation of the table, which is at the rate of one revolution in three minutes, a continuous gentle stream of pure water is delivered upon it, which washes off the waste over the edge at the lower side, while the richer tinstuff deposited upon the table and thus cleaned from the waste is carried round to the higher side, where a stronger stream of pure water washes it off into a separate receptacle. The small proportion of tinstone that contains mundie is calcined in a small reverberatory furnace stirred by hand, the quantity not being sufficient to require the employment of a self-acting calciner.
The Members were also invited by Capt. Richard Williams to visit Boscaswell Downs Mine, and witness the working of the semi-portable winding engines employed there, as described in the paper read at the meeting.
The Longships Lighthouse, visited through the special invitation of the Resident Engineer to the Trinity Board, Mr. Michael Beazeley, stands on a rock of killas at a distance of 13 mile from the Land's End, and has been built from the designs of Mr. James N. Douglass, Chief Engineer to the Trinity Board, to supersede an old lighthouse erected on a pinnacle of the same rock by private enterprise in 1795.
The new tower is built entirely of granite, and is 106 feet high, the total height including the lantern being 132 feet. The solid basement of the structure, containing fifteen courses of 2 feet thickness, is constructed of very fine-grained granite from Dinan in Brittany; the upper portion is of granite from the Lamorna quarries near Penzance, chiefly in 1.5 foot courses; and the thickness of the walls diminishes gradually from 7.75 feet at the bottom to 2.25 feet at the top.
The outline of the tower is an elliptical curve, the diameter being 35 feet at the base, and 17 feet at the top below the gallery course, which spreads out to 21 feet diameter. Advantage was taken of the form of the rock to leave a solid cone, round which the first ten courses are built as a casing; as these do not extend round the entire circle, the end stones of each course are notched into the solid rock, which is accurately benched to receive the several courses. In the first entire course each stone is bolted to the top of the cone of rock by strong bolts of Muntz metal, and each stone of the first five entire courses is bolted to those below in the same manner. The remainder of the courses are secured to those immediately below by a circular dovetailed band and groove, by which they are so bound together as to become virtually one solid mass; the individual stones in each course are likewise dovetailed together, so that none of them can be detached without disturbing the whole course. The floors are "through" stones, united by a centre stone dovetailed into all the others, thus binding the whole structure together at the level of each floor, and avoiding all outward thrust upon the tower from vaulted voussoirs.
The whole work is entirely set in Portland cement, which was carefully worked into all the joints by means of thin iron "swords" until every crevice was filled. There are seven rooms in the lighthouse, and a large fresh-water tank in the basement beneath the first room; the windows are protected by strong gunmetal shutters and frames, fixed flush with the outside of the tower, and the entrance is closed by thick gunmetal doors.
The framing of the lantern is constructed of steel, and is carried upon a cast-iron pedestal; it was first fitted with the catoptric apparatus from the old tower, which will be replaced by a first order dioptric light apparatus; the elevation of the light gives it a sea range of 16 miles distance.
A fog bell is suspended from a bracket on the gallery, and is rung by machinery during thick weather. For landing the stones and supplies a wharf was constructed on the rock, provided with a powerful crane, the winch and jib of which were lowered down into a protecting chamber when not in use; and on the top of the rock was placed a steam winch for raising the stones by means of a jib to the top of the work, where the setting was done by a second shorter jib, without interfering with the lifting jib. These two jibs were fitted to a central wrought-iron mast, which was gradually raised by hydraulic jacks as the work advanced; the mast being turned perfectly true in the lathe for its whole length, and accurately adjusted in the centre of the tower, formed a true centre for a trammel by which the stones were set.
All the stones above the basement were dressed at the Trinity wharf in Penzance, and each successive floor and room was completely fitted together there, and then taken apart and conveyed to the rock, 13 miles distance, in barges towed by a steam tug. In consequence of the very exposed position of the rock, the work could only be proceeded with in calm weather, and frequently an interval of many days passed, in which no landing upon the rock could be effected.
The operation of preparing the rock having been commenced in 1869, the foundation stone of the tower was set on 7th August 1871; and on 21st August 1872, just over a year afterwards, the last stone at the top of the tower was set. The lantern was then fixed, the old tower taken down, and the pinnacle of rock on which it had stood was carefully removed by blasting, with continually diminishing charges of powder so as not to injure the solid rock near the base of the new tower.
The total cost of the lighthouse when completed will be £34,000. The number of stones in the tower is 965, measuring 22,900 cubic feet and weighing 1,700 tons; and the total number of hours occupied in landing and setting the stones was 640, the entire tower being thus placed on the rock in 64 working days of 10 hours each, without the occurrence of any accident during the whole progress of the work.
In the evening the Members and their friends dined together in Penzance, in celebration of the first Cornwall Meeting of the Institution.
On Thursday, 31st July, an Excursion was made by the Members by special train from Penzance to visit Dolcoath and Carn Brea Tin and Copper Mines, near Camborne; and at Hayle, Messrs. Harvey's Engineering Works, and Wheal Lucy Tin Mine.
At Dolcoath the Members were received by Capt. Josiah Thomas, who showed sections of the mine and described the nature and extent of the underground operations. The present working was commenced in 1799, the mine having stood idle for thirteen years after the previous working, during which it had been drained entirely by a Newcomen engine, the bottom being only 150 fathoms below the adit; but on the working being resumed a Watt pumping engine with 60 inch cylinder and 7 feet stroke was erected to drain the mine, afterwards a second with 70 inch cylinder and 7.5 feet stroke, and in 1869 a modern Cornish engine with 85 inch cylinder and 9 feet stroke.
The mine being situated exactly upon the junction of the granite and killas, the upper portion of the principal lode, known as Dolcoath Main Lode, is in the killas, in which it has but little underlie; it passes into the granite at a depth of about 150 fathoms, and then underlies south about 18 inches per fathom. Down to about 200 fathoms depth the lode was very rich in copper, of which £3,000,000 worth was raised in the earlier part of the present working from a length of about two thirds of a mile on the run of the lodes between the boundaries; it was principally yellow copper ore in the rich part of the lode, but very little copper ore is now being raised; only a small quantity of tin was met with in the killas.
In the next 30 fathoms the lode was poor, and was in a transition state, containing a little tin and copper mixed, but not enough of either to pay for working; below that depth however it became richer in tin, averaging 2 per cent. of black tin, and containing also a little yellow copper ore.
The present deepest level is 314 fathoms below adit, or about 350 fathoms below surface, and the lode is there very rich in tin, averaging as much as 8 per cent. of black tin; the lode in the bottom of the level is 12 feet wide, and worth £120 per fathom, (that is 1 fathom forwards, 1 fathom high, and the width of the lode,) equivalent therefore to £60 per cubic fathom. The highest temperature experienced in the deepest levels has been 85° Fahr. The shaft that is sunk to the bottom of the mine is used both for pumping and for winding; it is sunk vertically till it meets the lode at 160 fathoms depth from surface, and then follows the underlie of the lode to the bottom; the pump rods are turned from the vertical into the inclined direction by a radius bob at the angle; the plunger lifts are each about 240 feet height. At this shaft the stuff is raised in a kibble, about 21 feet diameter and 41 feet high and containing about a ton of stone, a steel wire rope 1 inch diameter being used; there are no guides in the shaft, and in the lower inclined portion the kibble and rope rub on the timber lining of the shaft.
The time occupied in drawing the kibble to surface from the depth of 350 fathoms is about 8 minutes, giving a mean speed in the shaft of about 3 miles an hour; at the two deepest shafts about 10 tons per hour are raised, which is as fast as the stuff can be got in the workings; from the deepest level nearly the whole of the stuff broken is being sent to surface, the lode being there remarkably rich. During the last twenty or thirty years skips working between guides have been gradually introduced; and the other winding shaft at Dolcoath is provided with guides and skip.
The levels are driven 12 fathoms apart, to prove the lode, which where soft is stoped overhand, that is, in the roof or back of the levels; but if bard, the stoping is done underhand, in the bottom of the levels. The dressing floors are very extensive, comprising as many as 232 heads of ordinary stamps, worked by three engines; there are 30 buddies, all convex, which with other portions of the dressing floors are roofed in, and lighted with gas for continuing the dressing operations by night. The calcining is done by three of Brunton's horizontal calciners, and the consumption is about 1 ton of coal to 12 tons of "whits," which yield about 3 tons of black tin. About 1200 persons are employed on the mine, of whom 500 are engaged underground; the quantity of material raised annually is about 70,000 tons.
Carn Brea Mine
At Carn Brea Mine the Members were received by Capt. William Teague. The two principal lodes at this mine are producing both copper and tin, but a larger quantity of the former than of the latter at the present time, though the tin is becoming more abundant as greater depth is attained. The deepest level is 266 fathoms below surface, and is in granite, the junction of the killas and granite occurring here at about 110 fathoms depth; there is about a mile length on the run of the lodes between the boundaries.
Three pumping, five winding, and three stamping engines are at work on the mine; the latest of the stamping engines, erected by Messrs. Harvey in 1871, is one of the most recent examples of a good rotary Cornish engine, having a cylinder of 34 inches diameter and 9 feet stroke, working with 55 lbs. steam cut off at 1-12th of the stroke; the usual vacuum is about 12 lbs. per square inch. This engine is at present driving 80 heads of ordinary stamps, being capable of driving 20 more also.
Steam is supplied to it by three Cornish boilers, each 30 feet long and 5.75 feet diameter with 3.25 feet flue, which are fired with fine slack, the firing being done from two to four times per hour; the whole of the ashes are preserved, and mixed with the slack for burning over again.
There are altogether 208 heads of stamps at work, and the dressing floors, which have recently been newly laid out upon the most modern plans, contain 23 convex and 40 concave huddles. Two of Oxland and Hocking's calciners are employed, the cylinders being 32 feet long, placed at a slope of 1 in 24, and making one revolution in eight minutes. A pair of crushing rolls and a set of hand jigging machines dress the copper ore raised at the mine.
A man-engine is employed for taking the miners to and from their work in a shaft sunk vertically a depth of 200 fathoms below surface; the rod has a stroke of 12 feet, and is worked at a speed of four double strokes per minute, by a 26 inch cylinder engine with 8 feet stroke making four strokes to one in the shaft; this man-engine was seen at work and partly descended by the Members.
The oldest of the pumping engines is a vertical combined-cylinder engine on Sims' plan, erected in 1841, having cylinders of 90 and 50 inches diameter and 9 feet stroke, the high- pressure cylinder being placed on the top of the low-pressure cylinder on the same piston rod; it is probably the last specimen of this class of engine still continuing at work in Cornwall, and is about to be replaced shortly by a modern Cornish pumping engine.
The Members were entertained at luncheon at Carn Brea by Capt. Teague, who also presented each of them with specimens of the tinstone raised at Carn Brea mine, and of the dressed "black tin" and the metallic tin obtained from it.
Harveys of Hayle
At Hayle the Members were conducted through the Engineering Works of Messrs. Harvey and Co. by Mr. William Husband; and large Cornish pumping and winding engines were seen in course of construction, one having a cylinder of 80 inches diameter and 10 feet stroke. Two of the large pumping engines for draining Haerlem Lake were made at these works, with annular cylinders of 144 inches diameter and 10 feet stroke; and marine engines have been made up to 60 inch cylinders, the different shops being furnished with machinery suitable for turning out the heaviest descriptions of work. An iron screw steamer of 650 tons burden, for navigating shallow waters, was seen in course of construction, 172 feet long and 27 feet beam, to be fitted with compound engines having cylinders 33 and 18 inches diameter and 1 ft. 9 ins. stroke.
At Wheal Lucy Tin Mine the Members were received by Capt. William Harris, who showed the working of Husband's pneumatic stamps described at the meeting, of which a pair of heads have been successfully working there for fifteen months, driven from the rotary double-acting pumping engine.
On the return to Penzance in the evening by special train, the Members visited St. Michael's Mount and Castle, through the kind invitation of Sir John St. Aubyn, Bart., M.P.; and they were conducted by the Steward, Mr. Joseph Thomas, round the base of the rocks, to see the interesting illustration afforded there of the junction of the granite and killas, the south-west portion of the Mount being composed of granite, and the north-east of killas. The granite, which forms the greater part of the hill, has the appearance of leaving been forced up through the killas, into which several small veins of granite penetrate; and the junction of the two formations is distinctly seen on the rocks at the base of the Mount, where also at low tide veins of tin and copper ore are discernible. The Members returned by water to Penzance.
On Friday, 1st August, an Excursion was made by the Members by special train to the West of England China-Clay Company's Works at Drinnick Mill, near St. Austell, where they were received by the Managing Director, Mr. Edward Stocker.
The clay is worked in extensive open cuttings, the largest of which is 100 feet deep; and streams of water being directed down the sloping face of the workings upon the lines of the richest clay deposit wash the clay down, together with the sand and mica, which are here mixed with it in the proportion of about 8 tons of sand and mica per ton of china-clay. The several streams of water meeting at the bottom of the cutting are well stirred with hand shovels, to facilitate the separation of the sand from the clay; and the whole of the liquid mixture is run into a large reservoir at some distance, where much. of the mica becomes deposited. The remaining liquid, containing nearly all the clay, is then pumped up into a series of long settling channels, called "micas," about 32 feet wide and 9 inches deep and from 100 to 200 feet long and upwards; these have a very slight fall of 1 inch in about 40 feet, and are interrupted by dams or weirs at about every 20 feet, so as to give the stream of clay solution a succession of checks, whereby the heavier particles of mica are deposited.
From the lower end of these channels the clay solution passes off through a fine grating, to strain out as much as possible of the remaining mica; and in order to prevent the grating from becoming clogged with particles of mica and clay, it is subjected to the constant blows of a light hammer worked by a small waterwheel, giving about 30 blows per minute; the slight jarring action produced is sufficient to prevent the stuff from settling upon the grating. The clay solution is then conducted into large tanks or "pans," about 40 feet long, 50 feet wide, and 6 feet deep, where the clay now freed from mica is allowed to settle, and the water is gradually run off as the thickness of the deposit increases.
When sufficiently solidified, the deposited clay is removed in cubical blocks, which are dried either in the open air or in kilns, according to the weather; in wet seasons, air drying cannot be resorted to, as the clay would be washed away again.
The drying kilns are 120 feet long, each capable of drying 4000 tons of clay per year, and each charge requires about 24 hours to dry at the hot end of the kilns. About 23,000 tons of clay per year are being sent out of these works, which are at present in a very early stage of their development, and are being greatly extended by opening additional workings at several different points. The sand separated from the clay is used for making excellent firebricks, and mixed with a certain proportion of coarse clay it also makes good and durable building bricks of white colour; the bricks are hand-pressed, the manufacture being at present only on a small scale, and are then dried on drying floors with flues running beneath them, and are afterwards burnt in kilns.
A quantity of china-stone is also quarried, and sent away to be used for glazing pottery. Some parts of the works are kept constantly going, by night as well as by day; and the total yield of the various products from these works at present amounts to 30,000 tons per annum. The Members were entertained at luncheon at the works by the Company.
The train then proceeded to Saltash where, by the special invitation of the Chief Engineer of the South Devon, Cornwall, and West Cornwall Railways, Mr. Peter J. Margary, the Members were conducted over the Royal Albert Bridge, and were enabled to ascend into the tubes, under the guidance of the District Engineer, Mr. William Wright.
The Royal Albert Bridge at Saltash carries the Cornwall Railway over the River Tamar, which divides Cornwall and Devon. The river is required to be kept navigable for large vessels of war, and the bridge crosses it with two spans of 433 feet clear opening, and 100 feet clear height from high water. The bridge is of peculiar construction and was designed by Mr. Brunel, assisted by Mr. Brereton, to meet the special requirements of the case; the ironwork was executed by Messrs. Hudson and Male.
Each span is formed of an arched tube, with the ends tied together by suspension chains in a corresponding inverted curve, and the roadway suspended below for a single line of railway. The tubes are of wrought iron, of z to 4 inch thickness, of oval section 12 feet deep and 161 feet wide and having a sectional area of metal of 433 square inches; they are made with an internal stiffening ring at every 20 feet of the length, and longitudinal stiffening ribs. The curve of the tubes has a rise of 29 feet, and the chains have a drop of 27 feet, giving a total depth of truss of 56 feet centre to centre, amounting to about one eighth of the span. The chains are double, consisting altogether of four rows of fourteen and fifteen links alternately, each 7 inches by 1 inch section in the fourteen links, giving a total sectional area of 392 square inches.
They are connected to the tube by a series of vertical struts, strengthened by diagonal ties; and the transverse girders carrying the roadway are suspended from the chains. Each tube with roadway complete weighs about 1100 tons.
The ends of the tubes are flattened at the sides, with extra plates and struts inserted for the attachment of the ends of the chains; and the outer end of each tube rests upon four rows of 31 inch rollers, each 3 feet long, which move in roller boxes filled with oil and planed true at the bottom, to allow of the expansion and contraction from change of temperature. The greatest change observed was between the summer and winter of 1860, from 117° down to 20°, being a range of 97° of temperature, and the variation caused in the length of the tubes was 21 inches.
The outer ends of the tubes are carried on brick standards, built upon granite piers 29 feet by 19 feet, and the inner ends rest upon a cast-iron standard erected upon four octagonal cast-iron columns 10 feet diameter and 89 feet height, which are put together in 6 feet lengths, and stand upon a solid granite pier 34 feet diameter, built in the middle of the river, with its base 90 feet below high water. The erection of this centre pier in so great a depth of water was the greatest obstacle to be overcome in the whole structure, and specially showed the originality of Mr. Brunel. The depth of water, 64 feet at high water, precluded the use of a cofferdam; and a preliminary examination was made by Mr. Brereton by means of a wrought-iron tube 6 feet diameter, with which rock was found at 21 feet depth below the bed of the river, or 85 feet below high water.
A wrought-iron cylinder 37 feet diameter and 95 feet height was then sunk down to the rock in the required position, for the purpose of building the pier within it, pneumatic pressure being employed for keeping out the water while excavating and constructing the foundations; and in order materially to diminish the quantity of work that would have to be done under pneumatic pressure, the bottom of the cylinder was made with an internal cylindrical lining of 27 feet diameter and 20 feet height, thus leaving an annular chamber of 5 feet width all round between the lining and the cylinder. This annular chamber was closed at top by an air-tight diaphragm, the work being performed within it under pneumatic pressure; and by this air pressure any water leaking into it from the outside was pressed into the central compartment of the cylinder, which was left open to the atmosphere and from which the water was pumped. With this arrangement the only work required to be executed under pneumatic pressure was an annular wall of masonry of 5 feet thickness and 37 feet diameter, during the construction of which the drainage was maintained by pumping from the central open compartment.
On the completion of the annular foundation wall to the height of 20 feet, the pneumatic apparatus was removed and the central core built up to the same height, the leakage of water being found to be no more than the pumps could keep down; after which the building of the rest of the pier within the cylinder was completed in open air up to the top. In sinking the cylinder for the pier, it was loaded partly with cast-iron ballast, and partly with water by means of a water-tight lining carried down from the top of the cylinder to the top of the annular pneumatic chamber, leaving a central space of 10 feet diameter for access to the bottom of the cylinder; on completion of the foundations this lining was removed for building up the rest of the pier. The total weight of the cylinder and inner linings was about 360 tons.
The two arched tubes were erected complete with their chains and roadway on the bank of the river, and floated upon pontoons into their places one at a time at high water; the pontoons then sinking with the tide left the tubes resting upon the piers just above high-water level; from which they were raised by three hydraulic presses at each end up to the permanent level of 100 feet above high water. The floating of the tubes to their places was effected with complete success, each tube being accurately guided whilst floating by means of hawsers from five vessels moored in the river and four crabs on the shore, all the movements of which were regulated by signal flags from the floating tube.
The bridge when completed was tested by the passage of a pair of heavy engines with a long train, and the deflection produced was very minute. The approaches to the bridge are formed of seventeen openings altogether, of from 70 to 90 feet span, crossed by wrought- iron girders. The bridge was erected in 1857; and twice since then, in 1861 and 1867, the outside has been scraped and repainted, the last time with two coats of Torbay paint; and once, in 1864, the inside of the tubes was scraped and tarred; the total area of painting is 71 acres. The condition of the whole structure as regards durability is apparently as good as when first erected.
The total weight, including the approaches, is 2600 tons of wrought iron, and 1200 tons of cast iron; and the total cost was £223,220, amounting to £102 per foot run for the total length of 2190 feet.
The Members then visited Keyham Steam Yard, Devonport, by kind permission of the Admiral Superintendent, Sir William King Hall; and were conducted by the Engineer-in-Chief, Mr. John Trickett, over the large foundries, forges and smithies, boiler shops, turning, fitting, and erecting shops, provided with all the appliances requisite for turning out the machinery of the largest vessels for the navy. The double-turret ship "Hecate," recently built, was visited, as well as other vessels in various stages of progress.
The Members proceeded to Plymouth, remaining there that night.
On Saturday, 2nd August, the Members visited Devonport Dockyard, by kind permission of the Admiral Superintendent, and saw the very extensive machinery employed in the preparation of hemp and the manufacture of ropes and cables of all sizes for the purposes of the navy; the largest cables are as much as 25 inches in circumference, and are made in lengths up to as much as 200 fathoms. The cable-testing arrangements and the wood-working machinery were also seen; and the boat house containing a number of screw launches, with portable engines ready for application to any one of them.
A special steamer then conveyed the Members to H. M. S. "Cambridge " Training Ship lying in Hamoaze, through which, by kind permission of Commander the Hon. H. Holmes a Court, they were conducted by Captain Herbert, who showed the arrangements made for giving a course of instruction in practical gunnery to the officers and men of the Royal Navy.
The Plymouth Breakwater, and the new armour-plated Fort erected in its rear, were then visited, by kind permission of Major General Sir Charles Staveley; and the Excursion was extended to the mouth of the Yealm River, noted for its fine coast scenery, and to the Eddystone Lighthouse at 13 miles distance; and afterwards up the River Tamar to Saltash, for affording the Members a view of the Royal Albert Bridge from the water.
In returning from the Cornwall Meeting several of the Members availed themselves of Mr. Froude's kind invitation to visit the experimental works adjoining his residence at Chelston Cross, Torquay, where he showed the working of the machine used for shaping ship-models, as described in the paper at the meeting, and also the rest of the interesting and ingenious apparatus employed in carrying out the very extensive experiments upon the forms and resistances of ships.