52 ANNUAL OF SCIENTIFIC DISCOVERY. 



frrailo to tho cities ^vill increase, that of the wa^on tracks beino: 

 niueh sh:iq)i'r than the k>nirer and easier one of the raih'oail. 

 The luiilizi* will be 41 or IJ feit in width, with U> feet in the 

 miihlle, lor trains, one way on either side for cattle and vehicles, 

 and on the ontside of these still the passaj2:cs for foot-passengers. 



The luist Jilver Brid<;e. — " The work is assnmiui:: shape. Tlio 

 caisson for the irreat tower on the Brooklyn shore has been con- 

 tracted lor. Operations are to be connneneed at once. The 

 wood-work at the oil-docks and piers will be torn np and everything 

 down to low-water mark will bo removed. The bottom of the 

 river will be exeavatt'd to a de)>th of '22 feet below high tide. 

 The space to be cleared and levelled is 170 feet long by i02 feet, 

 extending out into the river. Divers will be cmi)loyed to remove 

 tlie obstructions at the bottom, and blasting will have to be 

 resorted to. 



"The caisson is like a large scow, or tlat-bottomed boat, turned 

 upside down; nothing more. Then, if one imagines its being 

 sunk to the bottom of the river on a level space prepared for it; 

 that the water is forced out of the boat, or * air-chamber,' as it is 

 called, by means of compressed air; that workmen are sent down 

 into the air-chamber in a shai't, cut througii the top of the caisson 

 (^bottom of the boat), who, with the aid of calcium lights, dig out 

 the material beneath them, which is hoisted up to the world 

 above; that they continue excavating until the proper depth is 

 reached, the caisson sinking, and, of course, on a perfect level as 

 the work progresses, and that the 'air-chamber' is last of all tilled 

 up with cement, a general idea can be formed of the way in which 

 the foundation of the tower will be laid. 



''Experiments which have been maile on the quicksand bed of 

 the East lliver while excavating a dry dock prove its bearing 

 power to be 10 tons per square foot. By Mr. lloebling's plan, it is 

 proposed to rest upon this bed a weight of only 4 tons per square 

 foot. The weight of each tower is to be somewhat over 75 

 tons. 



**To distribute this vast weight so that no part of the pressure 

 on the base shall be over 4 tons per foot, it has been decided tlnvt 

 the area of the foundation shall be 170 feet long by 102 feet broad. 

 This area will be composed of huge timbers resting on the sand, 

 and bearing the masonry-work of the tower upon it. The timber 

 will be 20 feet thick, and this vast mass of 20 feet by 170 by 102 

 will be securely bolted into one solid frame, so that the weight of 

 the tower above can never deflect in the slightest degree at any 

 point. 



" The board unanimously hold that oOO feet high of a masonry 

 structure could be safely and unvieldinglv erected on such a 

 timber fouutlation as proposed by Mr. Koebling, and that tho 

 superstructure thereon, if properly built, would easily bear tho 

 weight of the bridge, ami all the weight that could be put on tho 

 bridge. 



"The bridge company have purchased about 4,000,000 feet, 

 broad measure, of yellow Georgia pine, the greater part of which 

 is now on hand. Before the contract with the builders was made, 



