Lecture on Steel Bridges.

A good-sized audience, composed chiefly of students, assembled in Sander's Theatre last evening to hear Mr. Clarke's lecture on steel bridges. In a few words, President Eliot introduced the speaker, but omitted, as the latter facetiously remarked, to mention the fact that he was a graduate of Harvard. Mr. Clarke began by stating the importance of modern bridge-building and the rapid progress which has been made in the branch within the last fifteen years. One of the greatest undertakings of the age is the spanning of the Hudson at Poughkeepsie, by a massive bridge, 3094 feet, and with the approaches, one and one third miles in length. The object of this great work is to aid in the cheaper transportation of coal to the New England States, so that the manufactories will be developed to a large extent. The navigation of the Hudson is chiefly by tows, so that it is very important that the bridge will not interfere with the mode of transportation. The piers are 500 feet apart in order to allow these great tows to pass through easily. As the Hudson at Poughkeepsie is 70 feet deep and has a large mud deposit, it is necessary to lower a large cassion with double sides. This box is 100 feet long, 60 feet wide, and 60 feet high, and weighted with gravel. Through holes in the top the mud is dredged out by a large machine, which lifts ten tons every five minutes. After the mud is dredged out the space is filled up by concrete, which hardens under the water. Upon this bottom stone piers 30 feet high are built, and above these steel piers 100 feet high. This concrete masonry is the strongest made, and will sustain a pressure of 100 tons.

The company which Mr. Clarke represents is now building a bridge at Hawesbury, New South Wales, which is similar in construction to the one at Poughkeepsie. One of the greatest advances which has been made in the work is the substitution of iron and mild steel for brick. The spans of the Poughkeepsie bridge are built by means of a massive staging supported on large piles. This staging contains over one million feet of lumber and is within thirty feet as high as Trinity Church steeple, New York. Mr. Clarke then illustrated his remarks by a series of stereopticon views, which showed the manner in which the spans were constructed. At the conclusion of the lecture, Mr. Clarke was greeted with loud applause.