BOOKS.

CLARK 0.N THE BRITANNIA AND CONWAY TI7BiTLA.R

BRIDGES.*

Tim Chester and Holyhead Railway was designed to consolidate the " union " of Great 13ritain and, Ireland. (and. to pay a dividend to the shareholders) by shortening the time between London and Dublin ; and, like the Union itself it has had to contend with a series -of .natural and man-made difficulties. The mountains of Wales, the river Conway, the Menai Straits, the owners of the land, the navigators of the water, opposed themselves to its con- struction; while the Government offices of Admiralty and Woods and Forests added to the troubles-

" Chaos umpire sits,

• And by decision more embroils the fray."

Difficulties began from its leaving Chester, not to cease till the Menai was crossed, or rather till the railway reached its final des- tination, Holyhead.

"A series of works of unrivalled magnitude eharacterizes its whole length of 84i miles. It emerges from Chester through a tunnel in the red sand- stone 405 yards in length; a viaduct of forty-five arches leads to the bridge by which it crosses the Dee. Following the embanked channel of this river and the level shores of its estuary, it crosses the river Foryd by a pile and swing bridge, and continues its course along the shore through the Rhyddbui Marshes, and through the limestone promontory of Penmaen Rhos, by a tunnel 530 yards long, until stopped by the bold headlands of the Great and Little Orme's Head. It now for the first time leaves the coast, and, passing through the narrow valley that separates these headlands from the main- land, crosses the river Conway beneath the castle-walls, by means of the tubular bridge. Passing through the town of Conway and under the walls by a tunnel 90 yards long, it again reachea the coast at the Conway Marshes, and continues its course along the shore through the greenstone and basaltic promontories of Penmaen Bach and Penmaen Mawr, the terminating spurs of the Snowdon range, by tunnels 630 and 220 yards long respectively ; being carried for some distance after leaving Penmaen Mawr on a cast-iron girder viaduct over the beach. The sea-walls and defences, on the one hand, along this exposed coast, are all on a .magnificent scale ; whilst, on the other, a timber gallery, similar to the avalanche galleries on the Alpine roads, pro- tects the road-line from the debris that rolls down from the lofty and almost overhanging precipices above it.

"The Ogwen river and valley are then crossed by a stone viaduct 246 yards in length ; and between this and the 13ritminia Bridge the line passes through three ridges of hills perforated by tunnels, 440, 920, and 726 yards in length, through slate, greenstone, and primary sandstone ; the river

Ce with its beautiful valley, being crossed by a viaduct 132 yards long and 57 feet high. The line thence continues rising to the level of the Bri- tannia Bridge, and entering Anglesey, passes across the Maldraeth Marsh, and through a tunnel, in slate, rock, and clay, 550 yards in length. To enter the island of Holyhead, use is made, to a certain extent, of the em- bankment of the Holyhead Road Commissioners, called The Stanley Sands Embankment ' ; for which the Company are required, as at Conway, to make A yearly payment to her Majesty's Commissioners of Woods and Forests. The amount in this case is 106/.

"It is the object of the present volume to describe two of the most im- portant works in this magnificent catalogue—the passage of the Conway Elver and of the Menai Straits."

All the land difficulties could be overcome by money, or over- ridden by act of Parliament. The water obstacles were not sci manageable by mere outlay ; and the necessity for a free naviga- tion enforced by the Admiralty, interposed obstacles beyond what Nat-Ulu herself had placed -upon the passage. The essential diffi- culty to be encountered—a bridge of enormous span without in- termediate support—was much the same at the Menai Straits and the Conway River. As the Menai Tubular Bridge, though the same in principle, and not very much more difficult to erect than the bridge across the Conway, more impresses the mind from the daringness of its conception, the dangerous-looking grandeur of its-place, and its somewhat longer span, it has consequently re- ceived more public attention. We shall therefore confine our notice to that structure; endeavouring to present an idea of the difficulties encountered, the way in which they were overcome, some of the more striking facts elicited during the enterprise, and the gradual manner in which the conception grew in its author's mind, and was finally worked out by reasoning and experiment into a practical form.

The Menai Strait, which separates the island of Anglesey from the mainland of Wales, is about eleven miles and a half long, with * width of water-way varying from about 1000 feet to three-quar- ters of a mile. A tortuous course, extensive sandbanks at either end, and numerous rocks or groups of rocks, render the naviga- tion difficult. This difficulty is further increased by a tidal pecu- liarity, that causes violent and baffling currents ; for the main tidal wave, as it advances Northward up the Irish Channel, branches of into the Menai Straits over the sandbanks of Carnar- von Bay, and arrives in Beanneiris Bay at the other end of the Straits before the main tidal wave has completed the circuit of the island.

"As soon, however, as the main tidal wave enters Beaumaris Bay, it re- pels the current that ins set in from Carnarvon, and the tide flows into the Straits in opposite directions. This meeting of the waters gradually retiree before the Beaumaris wave, and arrives at the Britannia Bridge about twenty minutes before high-water there ; so that the tide continues to flow, or the water to rise, twenty minutes-after the current has changed its direction. •

"The shelter of the Straits is, however, so important, and the saving of distance is so considerable in avoiding the journey of sixty miles round the unsheltered and dangerous coast of the island, that the bulk of the ooasting vessels, some of them of large tonnage, avail themselves of its advantages, as do also a great number of vessels employed in the carriage of slates from • The Britannia and Conway Tubular Bridges; with General Inquiries on Beams and on the Properties of Materials used in Constriction. By Edwin Clark, Resident Engineer. Published with the sanction and under the supervision of Robert Stephenson. In two volumes, with Plates in folio. Published (for the Author) by Weak,

the Penrhyn, Llanberris, and other .slate-quarries anion the Carnarvon hills."

It will be necessary to bear these peculierities of the chanciel in mind ; as it was owing to them that the tubular principle was in- vestigated, the present structures erected, and an extensive modi- fication if not a new feature introduoed into engineering and archi- tectural construction.

When Telford, in 1826, threw his,celebmted suspension-bridge across the Menai Straits, he selected the narrowest part for the site.. On the original survey of the line for the railway by the late Mr. Ste- phenson, in 1840, it was proposed to use A portion of this suspension- bridge for the passage of the trains ; dividing them, and employing horse-power for the transit. The Woods and Forests nominally consented to a partial use of the, bridge, but with a clause which rendered their consent nugatory., Objections were also made by other parties to the proposed line as being longer than needful by three-quarters of a mile, or as interfering with the privacy of the Bishop of Bangor. A deviation was accordingly ordered by the. directors, and it was finally determined to cross at the Britannia:, Rock. This rock emerges as nearly as possible in the centre of the Straits ; it is 350 feet long, 120 broad, and rises 11 feet above low-, water level. The idea of a suspension-bridge was abandoned by Mr. Stephenson, because he intended to cross by locomotives, and in that form of structure there is, he says in his evidence before the Committee of the House of Commons' "a difficulty in keeping the platform steady : when the train went on to the Stockton and 71Dar- Iington line, the rails rose up three feet in front of the engine; they were unable to use it."

The first plan for crossing the Menai Straits was to erect a gi- gantic pier on the Britannia Rock across its entire width, and two- other piers (instead.of the usual abutments) rising from low-water mark of sprang-tide on either side of the Strait. These three piers were to be 55 feet shave spring-tide, and to be connected by two cast-iron arches of 360 feet span, the centres of which were to be 50 feet above the piers and 105 feet above high-watent As eon-, tering to support the arches while in the course of erection would interfere with the navigation and would at the Menai Straits be objectionable on account Of the expense, Mr. Stephenson de- termined upon a modification of a plan of Sir Isam.rd BrimeL. "The erection of the arch was to be proceeded with by placing equal and corresponding voussoirs on opposite sides of the pier, at the same time tying them together by horizontal tie-bolts"; a distinct idea of which principle -the reader can form, by passing a skewer through a couple of apples, resting the skewer on the top of a book standing upright, and pressing the apples close to the volume. No sooner, however, was this plan of the bridges made known, than it was opposed by all the interests connected with the navigation of the Straits. Various hinderances and evils were predicted; but they were substantially resolved into the diminution of head-way and water-way from the low springing of the arches and the occupation of the Britannia Rock by the central pier. Mr. Stephenson and Mr. Clark make light of these objections ; though there does appear to be weight in the following among other facts from Captain Vides report to the Admiralty. "The diminution of head-way is occasioned by the form of the bridge, its piers being only 55 feet above high-water; and as a sloop of 37 tons has a mast of 73 feet in height, a numerous class of -vessels will meet with inter= ruption ; and it has been explained why the lofty sails cannot be dispensed with. Again, we are informed, that frequently vessels to the number of fifteen, or more, may be seen beating down together under the influence of a favourable tide ; and in the eddies and baffling winds with -which they have often to contend, it becomes a matter of extreme difficulty to take exactly that part of the arch through which they may wifely' pass. "At this time of tide, [the turn,1 under present circumstances, vessels could tack with their jib-booms over the spot where it is proposed to place the South pier of the bridge, and also over the Ihitannia Bock, where the great central pier is designed; and to weather the Western end of the Bri, tannia it is necessary they should stand over close to the South pier; which, lithe present plan of the bridge be carried out, they oeluld not-do, both on account of the pier itself and the arch which rests upon it."

Right or wrong, however, the navigation interests triumphed. The entire width of the rock was not to be occupied, (the actual size of the tower is about 60 feet by 50,) and a clear head-way of 100 feet along the whole span was peremptorily insisted on by the Admiralty. Thus, everything had to be begun again, and Mr. Stephenson was thrown back upon original principles, at least as far as re- gards extent and structure. In 1841 he had made a design for a wrought-iron bridge of 110 feet span, for carrying a common road over the river Lea, in which the conditions only 'admitted of a platform 18 or 20 inches in thickness. This bridge was not erected in conformity with the design, but Mr. Stephenson now recurred to it ; rejecting various suggestions that were made to him, ortuther fusing them all into his own plan: The object sought was a straight stiff road-way, one end resting en the Welsh, the other on .the Anglesey shore, and the middle, or middle ends, supported on a pier built on the Britannia Rock ; a petty example of which is

seen daily in a plank-bridge over a brook. In practice, however, such a road-way could not be attained • a solid body of that length would break by its own weight, every addition to its strength in-

creasing, so to speak, its weakness ; it would vibrate more or less ; and if people wouldhave trusted themselves on such an open plat-

form, Par t would not have waged the bill—though they are as safe probably without as with the seeming protection, since none of the walls of bridge or viaduct anywhere could resist the weight of a train if it got off the rails. Sides of some kind, therefore,

* The nearest -approach to this design is Southwark Bridge, whose centre arch is 240 feet span. and the rise of its arch M -the centre is 24 feet: but the difficulties WI erection at the Menia Straits were of course much greater. -as well as a top, are absolutely necessary, for they give stiffness Vad strength to a lighter bottom. Arches notbelhg permitted, the suspension-bridge being rejected, and a solid beam being fin- a tube whether in the form of a pipe or an oblong box became imperative. The combination of the self-supporting prin- ciple with suspension by chains was indeed pressed upon Mr. Stephenson, and he started with the idea of using them as part of the process of erection ; but they were finally discarded altogether.

About the practicability of the plan Mr. Stephenson seems ne- ver to have entertained a doubt after the idea had once been formed in his mind. If sometimes he was appalled at the responsibility, "reflection satisfied" him "that the principles on which the idea was founded were nothing more than an extension of those daily in use in the profession of the engineer." The directors of the com- pany were equally or even more confident than himself, and they seem to have given him almost carte blanche : but the world was not so confiding. The Committee of the House of Commons was incredulity itself. When the first general experiments, though really successful, became known, the scientific world were as sceptical.

"Everybody had some doubts and fears to be overcome ; dismal warnings came in on all hands, suggesting every imaginable apprehension ; and Mr. Stephenson appeared at tunes disheartened when he withdrew, as was his

.y custom, to give instructions on the subject, and to deliberate on the weighty difficulties that had to be encountered in his undertaking. Very few are aware of the painful anxiety that falls to the lot of the engineer in circumstances of such deep responsibility : he can be satisfied with no

un- certainty or doubt—and what other foundations were possible ?"

Destruction by its own weight "was prophesied by some of the most eminent mathematicians and greatest mechanics." Thus wrote a late eminent mathematician, who entered minutely into the whole plan-

" In whatever form the tube is made, it mud be so constructed that in every part its strength may be in proportion to the stress ; and all the parts must be firmly compacted, that the whole tube may vibrate nearly as a mu- sical string. "The tube must be braced by strong iron cables, so that the wind may not blow it away, and that a sudden jerk of the carriages may not break it. " But an iron tube 450 feet long, to carry a tmm of carriages 150 tons in weight, at the rate of forty miles an hour, le a chimera. "It Is said that the strength and form of the tube have been satisfactorily determined ; that the stone piers are to be commenced immediately ; and that the whole work will be completed in two years. Almost wherever the statement was read there was a startling apprehension that a great and ex- pensive work, involving the reputation of many scientific men, and periling the public safety, was to be undertaken on very slight and imperfect data."

"With few exceptions, scientific men generally either remained neutral, or ominously shook their heads and hoped for the best." One of our most eminent philosophers expressed himself thus upon the matter; and perhaps the bridge may not yet be beyond his cautious fears, as indicated in the Italics— "I have no doubt that the strength of the tubular bridge, when new, sup- posing ordinary care to be used in most parts, and considerable attention given to accuracy in those parts which are to resist crushing will be more than abundantly sufficient. I think also the permanent strength of the bridge may be quite sufficient ; but I am not so decided upon this. These opinions would be scarcely affected by a moderate alteration in the thick- ness of the plates. But these considerations apply only to the consi- deration of weights placed quietly upon the bridge ; and I should not think myself justified in expressing any opinion which could be supposed to apply as to the capability of the bridge to resist the sudden introduction of a weight at railway speed. On this subject experimental information is want- ing to me. All that I can tell is, that a stiff structure would be likely to suffer more from it than a flexible one. My opinion also depends entirely upon the assumed accuracy of those parts which are to resist compression. If in the tubes intended for this purpose there are no transversal stops or frames, their strength for resisting thrust will be very greatly a matter of accident. My notion would have been to rely upon tube strutture for stiffness, and upon something else—as chain suspension—for absolute permanent strength."

Even Mr. Edwin Clark himself, when he went down to superin- tend the commencement of the works, could not refrain from send:- lug to his anxious chief a little of Job's comfort. He writes—" It as a fearful span when looking at it on the spot."

But to return to the narrative, from this digression on the anxieties of an engineer. In the experiments instituted to test the practicability of the conception, three objects were to be kept in view,—strength, form, and proportion. The first series of experi ments was made on circular tubes, the next on oval, the third on rectangular. The tube having been formed, was supported at either end just as the bridge would be; a perforation was made in the bottom, midway between the supports, and a suspension-link sustained by a cross-bar of wood inside the tube upheld a frame, work charged with weights, continually increased till the tube gave way in some part. The experiments upon the tubes were thirty-four in number; twelve of which were made on circular tubes, seven on oval, and fifteen on rectangular. The result was to establish the fact that rectangular tubes were the strongest, and that the top is the greatest source of strength. "In the whole. of these," says Mr. Stephenson'a report to the directors, I "this remarkable and unexpected fact was brought to light, viz. that in ouch tubes the power of wrought iron to resist compression was much less than its power to resist tension, being exactly the reverse of that which holds with cast-iron : for example, in east-iron beams for sustaining weight, the proper form is to dispose of the greater portion of the material at the bottom side of the beam, whereas with wrought-iron, these experiments demonstrate, beyond any doubt, that the greater portion of the material should be distri- buted on the upper side of the beam. We have errived, therefore, at a fact having. a most important bearing upon the construction of the tube, viz. that rigidity and strength are best obtained by throwing the greatest thick- ness of material into the upper side."

The experiments, though highly satisfactory in establishing the correctness of the principle, could not be considered conclusive as regarded the best mode of reducing it to practice. An exact model of the intended bridge was therefore constructed, one-sixth of th

size ; that is, the model was 75 feet long, .2„frt ,,Ivitieufuld 4 feet 6.iiichei deep ; and it was placed in it pos4toriarla.that

. . . of the intended bridge. Six different experiments were made upon it ; the fractures- being repaired as they occurred, (for which wrought-iron affords great facilities,) and the weak points thus developed being of course strengthened. The third experiment, however, was decisive, since the model itself would have borne a loaded 4ain.

" The result of the last experiment illustrated the importance of the p11. hire in the sides, as, with an addition of only 2 hundredweight to the weight of the tube, the top and bottom remained precisely the same as before, while breaking-weight was increased from 43 tons to nearly 56-5 tons, or more than ten times its own weight. This thin model, therefore, was capable of carrying 113 tons eqyally distributed over it; and was of itself sufficient for railway traffic, as the weight of a line of locomotives upon it would only be

75 tons." .

Thus a wrought-iron tubular bridge, made of plates riveted to- gether, was decided on ; and nothing now remained but the con- struction of the bridges, the_floating of them from their place of manufacture on the shore to their rdace in the stone piers or towers, and raising them by means of hydraulic machinery from high-water mark to their position in the Menai Bridge, 100 feet above it. All these processes are described minutely in Mr. Clark's volumes, and illustrated fully by diagrams or plates. The account of the construe- tion is curious, from the magnitude of the work and the machinery requisite to carry it on, as well as from the importance of thoroughly good workmanship in every part. It has not, however, the interest involved in the discovery of a new principle and the gradual expe- riments by which that principle is tested and applied. Neither has the constructive part the breathless feeling attached to the struggle with and control of the ocean, the military precision of the preparations, and the instant obedience to.one master 'will, that distinguish the floating and placing of the. Tubular Bridges. Neither does it so fill the mind with wonder, as 'the raising such immense and unwieldy masses into mid-air by the simple process of forcing water into a cylinder, which raises a ram, which raises whatever is placed upon it. The floating and placing of the tubes was formerly described in the newspapers, with ample minuteness, if not with such precise knowledge as in these vo- lumes. The proper description of the hydraulic press would in- volve plates and much elaborate explanation ; altogether the space requisite to convey a full idea of this mighty work would extend our notice to too great a length. There are incidental pas- sages, however, which are interesting in themselves, and also in- dicate the kind of interest attaching to the more popular sections of Mr. Clark's book : of them we will quote a few.

Magnitude, one of the sources of the sublime, is the great feature of the undertaking. It is the magnitude of the span that caused the doubt of the practicability of the bridge; it is the magnitude of its suspended height that impresses the mind of the spectator or reader,—for the principle is the same whether it were raised ten feet or a hundreci Everything was correspondingly great, dawn to the men and materials. These are some of the statistics.

"Three steam-engines were employed for raising the stones of the towers and abutments, with 26 travelling-cranes over different parts of the work, simultaneously engaged in transporting and setting them ; 2177 cargoes of stone and other materials were discharged for the masonry alone. The stones were worked on the shore on either side of the Straits. From 500 to 600 men were constantly employed in the erection of the towers, besides 300 or 400 men occupied in the quarries, and in bringing the stone to the Straits, • • * * "The whole quantity of timber employed in the scaffoldings for the .ma- sonry was 175,000 cubic feet, and for the construction of the land-tubes 118,230 feet. The platforms on which the large tubes were constructed con- tained also 110,105 cubic feet ; the total quantity of timber employed as scaffolding for the whole bridge being thus 403,33,5 feet. * * * * p

"Great precautions were taken to preserve the scaffolding from fired, to which the number of red-hot rivets constantly thrown about continuallYekj posed the scaffolding for the land-tubes, which on several occasions actually took fire. The means at command for extinguishing it were, however; so complete, that it was on all occasions immediately and easily subdued, or,the consequences would have been most serious, as the tubes themselves *Tula inevitably have been destroyed, having no other support during their con- struction. To avoid such a disaster, two fire-engines were eonstantlf on the spot in working order ; and tanks containing 8000 gallons of water were erected on the scaffolding at either abutment, with cast-iron pipes leading to all parts of the scaffolding. Gangways were also constructed beneath the flooring ; and pliable hose, ready for attachment at any portion of the structure, afforded ready means of deluging any part of the platforms with water. The water was pumped into the tanks from cisterns, which were supplied from land-springs in the neighbourhood, constantly flowing into them in wooden channels for that purpose."

As everything was unprecedented, suspense, one of the most powerful sources of interest, -is frequently at work. It was a trying operation to remove the scaffolding that supported the :tubes dung their construction, and the moment arrived to prove whether they would even bear their own weight.

"On the 16th January, the operation of cutting away the platform was commenced, and occupied ten days of intense 'anxiety. * * * • After driving the wedges, and further removing the platform, as the tube descended and compressed the remainder of the timber, the difficulty of extracting became greater and greater and the timbers were cut out piecemeal. They became at length so compressed that great difficulty was experienced even in this -operation. - A hole drilled with an augur was immediately filled up by the pressure, so that it was impossible to bore to aziy depth: and great labour was required to splinter away the timber in small fragments. "The descent of the tube from its own weight Was watched constantly with the greateetanxiety, as the operation proceeded at lengthnight and day. The great problem was now being solved. Many had predicted a deflection of two or 4qm: feet, and a great number of high authorities had affwthed that the tube could not support its own weight- while others foretold the buckling of the top, distortion of the sides, and crushing of the extremities- Everv Ph&momenon was therefore watched incessantly with intense interest as the tube -descended inch by inch. The crushing Of the timber, 'moreover, at times, as it broke away ender the ataxia, alarmed the men who went 199i" • working beneath, and it:irtul. be continual y jresent, with them

to keep them at their fienson, however, awaited the result with confidence -:thou not tininingled with deep anxiety, as tie results of

the operations,finin day twelay were fnrwarded to him' by tho author. By the 2444 slann4433witho. hulkof the PlitifettuldWas _removed; and the tube had deacon tit at ticntini9i inS ,1-1.1;11.eaffeitiSure On the remaining: ProPs Was t' at:pouf:1 ,":ner fecoA; thelongituainal hats being nearly' ellififid (lif 'beet' . 1"1"- 41 'Ai fears, howevei; Were at an end on the 25th, when the w role of the platform was cleared -away, and the tulidi tockfitit -own- weight troin end' to- enklitith- delleetien of 7i inches, being' on-eighth' lea than hart-been calculated ITO., • i.itlflse first emotions on: reflecting on this magnifieelit solution of the pro- blem which. had Unlit-cod-so-Much cafe and Apprehension ivere incxyressibiy :exciting; and though the novelty has seated, no stranger can stand beneath this stupendons beam; ,tapered oft; as it appears, by its vast perspective, with- out feelings or intense -astonishment."

There still remained the test of what weight the tube would bear • betiillell431 wto trial wa ittended with a curious exanitile'dfialtniespheri0. 't.Thq tetitg eif• theite6e Was -MOW rapidly proceeded with. For this pnr- raila. were :laid throfighout ;the tube alid,-platforin. Ballast-wag*eiis xvere.them loaded viktli iron plates, :and drawn into;the- centre of the tube. The first 20 tonsintreased the defleconi .enpleigclith of an mph, and with .50'toiii the deflection, as read by 1ami)t,TI weight was e• it;ft in"all night, and the deflection it niOrning Was only. 8 inches. This was 'attributed at the time to sonic error inadeliiithel reading; but thia,' and manynther anomalies in the deflection;';were afterivarde fully ariceonited• for by local changes of temperature. • A: gleanivf ,sunshine on the top of; the tube raised, it an one occasion nearly an inch in half an hour with 200 tens at the centre: the top plates being expanded by increase of temperature, While the lower plates remained constant from radiation to the water ire- 'inediately beneath Mein: In'a similar manner the tube was drawn sidewise ta the; extent ;of air ineh..from thesan shining on one side, and returned lin- ...sie,diately as clouds, passed oyer the sun; being in fact a. most delicate theme- meter in constant motion , both laterallyaud,y,ertically.''. The,egeaAnf the wind had cileoLexcited great forebodings in most ini,indstexeopting Mr. Steiduln800'04 but its iniluenco seems to be Towthan ;that of the sulk; „ . ; . an extension of the theory of the beams as was involved in the poii4rfictiou of these bridges; it became imperative to. inquire into every property of such structures, lest any plimnomenon, hitherto unimportant in iirditiori beams; should now 'rapidly rise into importance, and increase in eamoltigh ratio of the magnitude.: the effect of isochionous vibration from wind er other causes; and the impact of trains in rapid motion, Were always foremost among the theoretical apparitions that haunted the early history of the bridges. " Mr. Stephenson attached, however, little importance to these considera- tions ;-:depending on the' great weight .of the structure itself fen; obviating any danger from impact, and On the fitful nature of gusts of wind; as afford- ing no apprehension of continued isochronous motion. During the violent gales of -February last, the heaviest that have ,occurred for' many years, the were but little, affected, although one of them .was resting at each end ofily ref a pile of loose -planks, and at an elevation of 100 feet, and was /fe1her connected; laterally nor longitudinally; with the neighbouring tubes, which': must 'nearly quadruple its lateral strength : its lateral motion amounted, under these nireunistancesi: to. about 11 inches.. The 'blow struck the gale was not simultaneous throughout the length of the tube, but ins- pinged locally and at unequal intervals on all parts: of the length which pre- sented a broadside to the gale: It was impracticable to pose along the top of the tube except by clinging-to the windward edge ; and even in this posi- tie-A-the 'fitful nature of the gusts Was disagreeably perplexing. The gale was diverted from its horizontal course, and, descending obliquely into the Water below, ploughed it up in elands of spray for some distance from the tube. 'The maximum vibration did not occur daring the greatest violence of the wind, but-litthe Momentary lulls, when the tube, partially returning to its 'normal shape from its own elastieity, was again met by the succeeding weihre.v. The tube, 'however, on no occasion attained Any serious oscillation, but appeared to some extent permanently sustained in a state of lateral de- flettiiiii, without time to oscillate in the opposite direction. "The'impaet from the passage of an ordinary train must, of course, be inbeiriparalde in effect with the blow of such a hurricane on a surface of 13,000 square feet in one' spaii."

The expense of ,,the„ preliminary experiments was altogether 63501.; a ",great cost," Mr. Clark terms it, but surely money well laid out. • The total cost of the Conway Bridge was 145,000/. • that of the unfinished Britannia Bridge, as near as it could be ascertained at the time Hr. Clark was writing,: was about 600,000/. . The fol- lowing is a Summar , of the general chronology of the line, and of ibis Britannia Bridge in particular. •

Line surveyeelby the late George Stephenson.

SO: July 4. Chester and Holyhead Railway incorporated—Railway

• works commenced: . • ' 1345, June 30. Rill . passed anotioning the construction of the Britannia Bridge—received the Royal Assent.

,i. • July.. Preliminary ;experiments commenced. .

1iii6.,....Apial.13.• •.Firetworkmemengaged on the Bridge. Sept. 21. First stone' laid' Britannia Tower.

447, June: 13...., yirst vessel with iron arrived at the Straits.

-__ Aug. 10. - First rivet inserted in the Britannia Tube by Mr. Edwin

_,.._Wr` , • ,_, 'Clark. • maw Teli.•,22.,,,iCieniarvon,and Anglesey Towers completed. vtoifltA,jisihrs-ultenpietui.brought'froni Conway to Britannia. , ay 4. First Britannia Tube completed, and platform cut away. s,„ , rjune.- 2Q. , . Pipit tube; floated. I ....,-, j... . —• 22. Last, stone on the Britannia Tower laid by Mr. Stephenson. iov, 9. First tube deposited on its permanent bed.

-• „ ir Dec, 4. Second tube floated: •- • ' Feb. 7. Second tube deposited on its permanent bed.

nj .,,.-March 3. Carnary.on small tube lowered. .

-cd on , — 5. First. en,gioe_ passed through the tubes, and last rivet in- serted by Mr. Robert Stephenson. . --• 18. Single line, opened for public traffic. -iiiffsaunei„ ... Third tube .iteated. . • Eo.i.,,,i1. . Last tube floated." . .-

i

...fd, sk,u/iy.„ .:. 1 hird tube deposited on its permanent bed.

b'=illobjettt. of Mr.lelark's yoluinesis threefold. • 1. To convey as lie.' War. ' an account40 possible of the difficulties attendant upon

't*struction an ArIpetiou of the Tubular Bridges, and to trace ' :Step by whielige.yiNere ,theoretically, experimentally, and Practimily overconassrolit. .,To exhibit the experiments ' in detail, ac- companied by mathematical calculations evolving the principles or rules which those experiments contain, in reference to the strength of materials and the models experimented upon. 3. To treat more widely the principle of the beam, and to deduce its general laws from the facts. elicited during the great work with which the au- thor was so closely oonnected. Mother section of a miscellaneous -kind includes subjects of a subordinate or local character— as the tides of the Menai Straits.

In the first or historical part the author attains his object; giving a clear and businesslike narrative of the whole story. Some- thing more of strength might have been gamed by greater com- pression, and greater force, perhaps, by a more lively or artistieal treatment : but the narrative is truthful, and it rises with the occasion. The subject, however, would have been presented more distinctly to the mind had the narrative been continuously pro- steedectwith, and the calculations on the experiments, which now intervene between one step of the process and its successor, been _given elsewhere. The other sections are of a purely niathematical -ellaxaoteZo and contain a complete exposition, of the principles un- folded during this memorable undertaking ; Mr. Clark having been assisted by other engineers, or deriving some of : his 'matter from their reports. These frequently contain information of a popular kind,—such as this calculation as to when a tubular bride would break.

"If the dimensions of the section remained constant and the length only were increased, the tube would break with its own weight when it became about 850 feet long.

"But if the dimensions of the section were also increased, so that, the tube retained the tame proportions, becoming then a similar tube to what it is at present, it would break with its own weight when it become abmit 1570 feet long.; The depth of the tuber would in this case be 100 feet, the breadth 48 feet, and the weight 62,000 tons.

"Other curious properties of similar tubes may be determined from the

preceding reasoning ; practice these theoretical limits cannot be ap- proached. The impossibility of construction, the rapid increase in the weight of the requisite pillars for any great extension of depth, the neces- sary additions for preserving the general form of a large structure considered merely as an edifice, independent of its action as a beam, and the great crushing strain to which the bearings would be subjected,. are all elements increasing. in a high ratio with the dimensions, and rendering. impossible, on similar principles, any structure of much greater magnitude than the tannia Bridge. This will be evident if we call to muid that in that tube twenty-one per cent of the whole weight of the sides is already consumed in stiffening them; and not only would the weight of a larger structure increase as the cube of its length, but it would also require a much larger percentageof stiffening plates in the sides, for as the pillars increase in height the

weight increases as the cube of the length, while the strength is only as the square."

The octavo volumes are accompanied by a large folio of plates, consisting of working drawings and general -views. The drawings exhibit more clearly and in greater detail many things that the text with its cuts and diagrams can only explain generally. The views present the subject to the eye with a distinctness that words can- not attain; sometimes forming a substitute for actual inspection, sometimes preserving features of the construction that vanished when their work was done.