ENGINEERING. 289 



it ventilates, and its sudden expansion cools the tunnel or the mine where it is 

 used. 



The first, or one of the first tunnels in this country in which the rock was 

 drilled by compressed air, was Nesquehoning, by Mr. J, Button Steele. Since 

 then many have been made by the same means, one of the most memorable of 

 which is the Musconetcong tunnel, a mile long, made under the direction of Mr. 

 Robert H. Sayre. This difficult work gave occasion for the valuable treatise on 

 tunnels by Mr. Drinker, who was in immediate engineering charge of it. The 

 Hoosac tunnel, 24,000 feet long, after a lopg continued struggle, was completed 

 several years ago, and is now in use. 



Among the tunnels now being constructed is one half a mile long under the 

 plateau of West Point, and another 4,000 feet long through the hard trap rock of 

 Bergen Ridge, at Weehawken ; both on the line of the road now in construction 

 on the west shore of the Hudson. Nearly all the debris from the latter is raised 

 through shafts. 



The project is now under serious consideration of making a tunnel some 

 twenty-one miles long under the Straits of Dover. A few years ago such a pro. 

 ject would have received only a laugh of incredulity. 



The admiration of the world has not yet abated for the boldest of arched 

 bridges yet built, that over the Mississippi at St. Louis, with its steel arches of 

 500 feet span, its piers of heavy masonry sunk to solid rock more than a hundred 

 and thirty feet below the high water surface of the river, through shifting sands, 

 and during the most fearful floods. 



The Brooklyn bridge, 1,595 feet, or nearly a third of a mile long, over an 

 arm of the sea more crowded with commerce than any other in America, and 

 high enough to allow a line of battle ship to sail under it — is drawing to comple- 

 tion, and will be (though perhaps only for a few years, 'till something more stu- 

 pendous comes), one of the wonders of the world. 



Probably the boldest plan for a bridge ever proposed, is that now in contem- 

 plation over the Forth at Edinburgh, but of which it is yet premature to speak. 

 Many very long spans and important bridges are now in progress in this 

 country, such as the one over the Missouri by Mr. Morrison, but time does not 

 permit even a glance at them. 



We are now so familiar with the success of suspension bridges for railroads, 

 that we can hardly realize the almost universal disbelief in that success before 

 they were tried. The late John A. RoebUng told me before his bridge was fin- 

 ished, that Robert Stephenson had said to him, " If your bridge succeeds, mine 

 is a magnificent blunder." And yet, unexpectedly to the best engineers in the 

 world, the suspension bridge over the Niagara answers the purpose quite as well 

 as the tubular bridge over the St. Lawrence. 



The mention of the St. Lawrence reminds us of the great and interesting 

 improvement of that river now going on under the direction of Mr. Kennedy. 

 The original low water channel between Quebec and Montreal, had, in places, a 

 depth of only eleven feet. Now they are increasing the low water depth to twen- 



