MECHANICS AND USEFUL ARTS. 33 



enterprise and sagacity: they determined to take the opinion of the most 

 eminent engineer whose advice and counsel they could obtain. 



The Britannia Bridge across the Menai Straits was opened in 1849, and it 

 was not, therefore, unnatural that, in 18-3:2, the directors should look to Mr. 

 Robert Stephensou, as the engineer most competent to advise them. Mr. 

 Stephenson considered the subject of so much interest and importance, that 

 he determined to go out to Canada, personally, for the purpose of dealing 

 with it. He accordingly repaired there, at the end of the summer of 18-33, 

 and, after examining into the facts, made a public declaration of his opinion, 

 that a bridge across the St. Lawrence was practicable. On the 2d of May 

 following, Mr. Stephenson addressed to the Grand Trunk Railway Directors 

 a report, in which he considered the whole question in three branches : first, 

 as to the description of bridge best calculated to prove efficient and perma- 

 nent; second, as to the proper site; and thirdly, as to the necessity for such 

 a structure. Upon the first point, he did not hesitate at once to recommend 

 the adoption of a tubular bridge, as the description of bridge best fitted for 

 a permanent, safe, and substantial structure, in such a situation; on the 

 second point, he was not a little influenced by considerations affecting the 

 flow of the river, and " those almost irresistible forces " consequent upon 

 the breaking up of the ice in spring. 



Mr. Stephenson, on his arrival in Canada, met with numerous alarmists, 

 who could graphically describe to him the effect of the ice, but he met with 

 no one who had in any way measured or calculated the amount of its pres- 

 sure. In considering the question whether a bridge could be constructed to 

 withstand that pressure, it appeared to Mr. Stephenson to be of primary 

 importance to ascertain really and precisely what that pressure was. This 

 was a question of calculation; though, in the absence of any data, the 

 difficulty was how to calculate it. And here, before the reader proceeds fur- 

 ther, he may, perhaps, not without advantage, pause for a moment to ponder 

 on the way to solve the problem, What is the amount of the pressure 

 of ice four or five feet thick, in a running stream of a certain inclination, 

 velocity, and breadth ? 



This problem puzzled Mr. Stephenson himself at first; but it was not long 

 before he hit on an expedient. He first got at the inclination of the river; 

 next at its velocity. He then assumed that the ice upon that river was what 

 they told him it usually was, from four to five feet thick. He then inquired 

 into the condition of the river, and he found that, about nine miles above 

 Montreal, there was. a fall called the Fall of Lachine, which, of course, sepa- 

 rated the body of ice above the fall from the body of ice below it. Taking 

 these data, he calculated what would be the pressure of nine miles of ice, 

 from four to five feet thick, lying on a plane of a given inclination, and 

 pressing against the piers of a bridge across the channel. The result of that 

 calculation in figures it would be unnecessary, even if it were possible, to 

 state; but, whatever were the figures, they enabled Mr. Stephenson at once 

 to realize one all-important fact. He arrived at the conclusion that " the 

 almost irresistible force " of this mass of ice would crush or sweep away any 

 ordinary bridge, and that all the suggestions previously made for encounter- 

 ing the difficulty were only likely to result in disaster if carried into effect. 



For, up to the period of Mr. Stephenson's report, great difference of opin- 

 ion existed in Canada and elsewhere, as to the probable effect of the ice 

 pressure. One party held that no bridge whatever could stand against it; 

 another, whilst admitting the difficulty to be formidable, thought timber 



