ON BRIDGES. 139 



is borne Ly the cables themselves. The bridge has now been in con- 

 stant use, and since the eighth of March, 1855, when the first engine 

 passed over it, with perfect success. 



It is the duty of the engineer to construct his works so that, while 

 durability is insured, the object shall be attained at the least cost. 

 It has been frequently said that there is no limit to engineering 

 achievements but the want of money, and much less credit is due to 

 him who accomplishes an important result with a large expenditure 

 than to him who does the same with limited means. This is espe- 

 ciall}'" true of the public works of this country, where the lines of com- 

 munication are very long and frequently pass through sparsely settled 

 regions, which could not by their local traflSc support expensive struc- 

 tures. It will be interesting, therefore, to compare the cost and 

 weight of the two structures which we have described with the view 

 of determining their relative merits on these grounds. 



The Britannia bridge is 1,511 feet in length, the Niagara 800. 

 The weight of one line of the first is 4,680 tons or 2,340 tons for 755 

 feet against 1,000 tons for the 800 feet of the second, or nearly twice 

 as much. Again, the Britannia bridge, completed, cost three millions 

 of dollars for a double track, while the Niagara bridge cost four hun- 

 dred thousand for a single track — rather more than half the length; 

 and taking into consideration that the piers, foundations, &c., would 

 cost almost as much for a single line as for two, the ratio of cost would 

 be about one to two and a half, a vast difference in favor of the sus- 

 pension bridge. 



The proof of the practicability of cheap bridges of great spans, due 

 to Mr. Roebling's talents, forms a new era in the art of engineering. 

 Communications which were hitherto considered impossible may 

 now be made; regions like those of the west coast of the United 

 States, intersected by deep ravines with vertical sides, may be tra- 

 versed by railroads, and rivers may now be spanned by bridges at 

 places which Avere before entirely impassable. 



Mr. Peter W. Barlow, C. E., of England, after a careful personal 

 examination of the Niagara bridge, has not hesitated to propose a 

 plan for spanning the river Mersey, at Liverpool, by a suspension 

 bridge three thousand feet long, at a height of one hundred and fifty 

 feet above the water, with towers four hundred and fifty feet high, at a 

 cost of five millions of dollars. 



. The greatest engineer is not, however, he who builds the largest 

 work at a greatest cost, but he who, by the introduction of new prin- 

 ciples, or by the ingenious application of those already understood, 

 extends the practice so as to enable us to surmount new diificulties 

 with economy and safety. 



