MECHANICS' INSTITUTE NEW HALL. 



[1853 



with great severity, was one cause why the inundation at Montreal 

 was less than usual, — was unaccompanied either on the formation or 

 departure of the ice with any "shoves" — and that the surface of the 

 river, opposite Montreal, presented the evenness of a mill pond instead 

 of the ragged quarry aspect of broken ice usually seen. 



The St. Lambert Approach to the bridge, ij; conjunction with the 

 work of the Champlaiu Railroad Company, will have the effect of re- 

 taining in its place the ice formed between Moffatt's Island and the 

 south shore, and thus prevent the descent of a bordage of equal width 

 as high up, at least, as a poiut abreast of Nun's Island. The retained 

 bordage above Moffatt's Island, with that resting on Nun's Island and 

 the south-western abutment of the bridge, will increase in width so 

 as gradually to narrow the passage between the Nun's Island and the 

 eastern shore, and will thus aid in arresting the descending field of 

 the upper bordage and close the Laprairie basin at the earliest date. 

 A few ice breakers judiciously distributed over the shoals, while they 

 would break the shock of fields descending against the bridge, would 

 aid in retaining the bordage and thus expedite the freezing over of 

 this basin. 



The solid approaches will be cheaper and more substantial than any 

 other portion of the bridge of equal iGngth; and in fact no substitute 

 which will bring the rails down to the level of Point St. Charles can 

 be devised tor them, except that of extending the piers and bays to 

 the shore and carrying the masonry up to the level of the rails. A 

 system of masomy arches giviDg free passage to the water, would be 

 exposed to the risk of being blocked up and overthrown by the 

 " shoves" of the ice. 



To carry out the arrangement of descending from the central arch 

 to each shore on the top of the tubes; it is evident (since the depth of 

 these are 30 feet under the rails) that as the shore is approached the 

 lower side of the lubts would be brought within the reach of the 

 winter flood. Before this point is reached, therefore, the arrangement 

 and character of the structure must be changed, and as it would des- 

 troy the effect of the bridge again to elevate the tubes aud run through 

 them — the solid causeway is necessary. It is true that by abandon- 

 ing the proposed arrangement of running on top of the tubes, raising 

 the masonry of all the piers to the level of the rails, and continuing 

 the piers and tubes to the shores — the solid approaches can be dis- 

 pensed with; but I considtr that there are objections to such an ar- 

 rangement exclusive of economical considerations and the loss of the 

 effect of the solid approach in retaining the bordage. If the spans are 

 such that tubes whether of iron or, wood are required, — passengers 

 would be confined in a tunnel two miles in length with all its disa- 

 greeable connections, and if the spans are so narrow as to admit of an 

 iron bridge open at the top — the side trusses would yet be necessarily 

 so high that it would become a long trough which unless open at the 

 bottom would fill with snow, while it would effectually dtprivo the 

 passengers in summer of that view from the windows of the train 

 which will constitute one of the great attractions of the bridge. Oa 

 the other hand by the arrangement proposed, the appearance of the 

 bridge with passing trains is improved — the snow is avoided — the mo- 

 notomy of the outline is broken by the single elevated tube in the 

 centre, and the channel is thereby clearly displayed to the navigation. 

 The pleasure and comfort of the passengeis enhanced — economy aud 

 safety to the structure are secured — and, if built of wood, the risk of 

 fire is greatly diminished. 



The Piers. The most important question in connection with the 

 structure is that of the piers. The superstructure and approaches are 

 simple matters, and so would the piers be were it not for the ice 

 phenomena. Many persons (astounded by the commotion when a 

 "shove" takes place) entertain the belief that piers cannot be made 

 to stand in the river below the Lachine rapids, or at least below Nun's 

 Island; but the simple connivance described by Mr. Logan shows how 

 easy it is to elude the effects of the ice however difficult it may be to 

 oppose them. That the ice is not, as is often remarked, "irre&istible," 

 may be proved from the fact that the islands, rocks, wooden wharves 

 and stone quays have not been removed by it. Probably there is no 

 point where the ice strikes with greater force than against the long 

 wharf at the Bonsecour Market — but this cribwork has resisted the 

 shock, and forced into the air a broken heap of fragments. The power 

 required to crush a cubic inch or foot of ice is very much less than that 

 required io crush stone, iron or wood. If therefore there is mass 

 enough or support enough, as is annually proved by the stone quays 

 of Montreal, the ice is broken into fragments or ground into powder; 

 but the simpler, more economical and effective method is that univer- 

 sally employed where ice is to be encountered of turning the ice back 

 upon itself and leaving tho first arrivals to take the shock of all that 

 following after. By sloping the up-stream face of a pier oriec-breaKer 

 so that the ice will ride up upon it, the stability of the pier is in- 

 creased by the additional weight piled upon it and a heavy rampart 

 of ice receive all future assaults. 



But it is to be expected that the violence of the ice shocks will bo 

 diminished rather than increased by erection of a bridge. At present 

 when the dam slips and the ice begins to move it is earned on with 

 increasing momemtum until it strikes the shore But if sustained at 

 intervals of loO yards or less across the stream by piers, the initial 

 velocity would be checked and the ice would rise and fall in situ with 

 the variations of the water level. 



The plan I have proposed contemplates the planting of very large 

 " cribs" or wooden "shoes," covering an area of about one-fourth of an 

 acre each, aud leaving a clear passage between them of about 210 feet 

 — a width which will allow ordinary rafts to float broadside between 

 them. These islands of timber and stone will have a rectangular well 

 left open in the middle of their width toward their lower ends, out of 

 which will rise the solid masonry towards supporting the weight of 

 the superstructure, and resting on the rocky bed of the river- The 

 enclosure of solid crib work all round the masonry yet detatched from 

 it, will receive the shock, pressure, and " grinding of the ice, aud yield 

 to a certain extentby its elasticity without communicating the shock 

 to the masomy piers. These cribs, if damaged, can be^repaired with 

 facility; and from their cohesive powers will resist the action of ice 

 better than ordinary masonry. During construction they serve as 

 coffer dams, aud being formed of the cheapest materials — their value 

 as service ground or platforms for the use of machinery, the mooring 

 of scows, &c, during the erection of the works will be at once appre- 

 ciated. Their application to the sides of the piers is with particular 

 reference to preventing the ice from reaching the spring of the arches 

 which will be the lowest and most exposed part of the superstructure 

 if wood be used. 



The class of superstructure proposed for these wide spans, if of 

 wood, would be a strong rectangular open built hollow beam, assisted 

 by a deep open built arch. The two systems of arc and truss, howe- 

 ver objectionable in iron bridges, have been proved to be susceptible 

 of advantageous combination in the numerous and excellent bridges 

 built on what is known as the " Burr" or Peunsylvanian principle — 

 decidedly the best class of wooden bridges in existence. The elasticity 

 of timber permits both systems to come into play without injury to 

 either wheu a strain is upon them, (which is not the case with iron) 

 while the two great elasticity of the wooden arch is counteracted by 

 the rigidity of the truss to wnich it is attached, 



Experiment at Menai proved the superiority of the rectangular form 

 for hollow beams in iron. It is somewhat singular, that the best form 

 of wooden bridge in America for wide spans was, long previous to 

 the Menai experiment, a type in wood of the celebrated tube. The 

 strength of both bridges is collected near the four angles; the sides top 

 and bottom, in the iron wonder, serve chiefly to maintain the relative 

 position of the vital parts. The strength of the wooden tube must be 

 wholly in the top and bottom chords — the inferior capacity of wood 

 for the connection of its pails being in some measure compensated for 

 by the practibility of employing the auxiliary arch. 



The wooden railway bridges of America are progressive improve- 

 ments upon the ordinary road bridges of Pennsylvania and New Eng- 

 land, in which there was apparently an excess of strength:— the arc 

 carrying the load and the truss (with plates instead of chords for the 

 top being a mere frame work to preserve its shape. In adapting 

 these structures to the passage of railway trains every part has'been 

 from time to time increased in weight and size as experience dictated, 

 but it is questionable whether as a class they are not generally too light, 

 and wanting in that inertia which attempts at stiffness cannot compen- 

 sate for, and which is requisite to absorb a portion of the momentum 

 communicated to the structure by the sudden impact of locomotives 

 weighing twenty-five to thirty tons, and moving at a speed of thirty 

 miles the hour. These wooden bridges with arcs included, are not 

 more than one-third or one-half the weight of tubular iron ones for 

 the same span. 



I have proposed a class of superstructure more weig"hty than usual, 

 and while recognising the objections to the extra weight to be sus- 

 tained, I conceived it practicable to build a truss of the long span pro- 

 posed which shall sustain at least its own weight, and to apply an 

 auxilliary arc to that truss which can at least resist the effect of the 

 load. 



While instances are numerous of the failure of wooden bridges not 

 supported by arches, by their in time sinking below the horizontal 

 line, lam not aware of any well built "Burr," bridge having failed 

 from this cause, although many have spans of 200 feet. 



Mechanics' Institute N\.w Hal!. 



We understand that contracts have been entered into by the Com- 

 mittee of the Institute, for the erection of their New Hall, accoiding to 

 plans furnished gratuitously by F. W. Cumberland, Esq., and which 

 we have no doutt will be highly creditable to that gentleman, and also 



