PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 893 



closed at one end, and enough closed at both ends to make the 

 thing float. (Dr. E. exhibited a model illustrating his idea.) It 

 will float in water at 40°, but sinks in water at 35° or 45°, especially 

 if the tubes are filled with water at 40°. Should ordinary river 

 ice, in this condition, move, it crumbles to pieces, and then cannot 

 sink; but if it is in still water it readily sinks, especially if the 

 surface has Ijeen flowed over by muddy water from spring freshets. 



The remarks of Dr. Rowell brought to the floor several gentle- 

 men who advocated the generally accepted theory regarding the 

 glacier orio-in of icebero;s. 



Mr. Alfred P. Boiler, civil engineer, was then introduced to the 

 audience, and read the following interesting paper: 



PROPOSED BRIDGE OVER THE EAST RIVER. 



Perhaps the most difhcult operation to be met with in the pro- 

 vince of civil engineering, is when the engineer is called upon 

 to erect bridges under peculiar requirements across great valleys, 

 estuaries, or broad and rapid water ways ; and I may safely say 

 that no task demands greater skill and judgment, both in the 

 character of the bridge to be adopted, or in its after construction. 

 This difficulty has been most thoroughly appreciated by the resi- 

 dents of both New York and Brooklyn, especially during the past 

 winter. The absolute cessation of intercourse between those two 

 cities at difierent times, caused by ice blockades, has demanded 

 that some method should be adopted that forever and at all times 

 would allow of a permanent and invariable communication. Num- 

 berless have been the plans proposed to aftbrd relief, from a tun- 

 nel to all kinds and varieties of bridges, most all of which will 

 tumble with their own weight, and fall still-born to the ground. 

 I will not trespass upon your time reviewing all these schemes, 

 proposing to discuss this evening a bridge plan, which I hope will 

 meet with your approbation. The site suggested is the crossing 

 at Rutger's street, where the river is the narrowest, the New York 

 approach starting from Chatham square. In this discussion I shall 

 only treat of the l)ridge proper, leaving the matter of approaches 

 out of the question, simply liecause they are dependent upon so 

 many local considerations, as well as necessary to any bridge that 

 should be adopted. The main question is : What is the best sys- 

 tem of bridging that can be adopted ? To answer this question, 

 Ave must first settle upon the requirements of the case, and then 

 pass in rapid review the known systems, and see in what Vvay they 



