ENGINEEKING. 



253 



feet long, and rise 30 feet 

 above high water, afford- 

 ing support for steel tow- 

 ers 16 feet by 60 at the 

 base, 16 feet by 30 at the 

 top, and 100 feet high to 

 the lower part of the 

 superstructure. The 

 tower construction is 

 shown in the illustration, 

 consisting, in effect, of 

 two pyramidal struct- 

 ures securely braced in 

 every direction. The 

 cantilevers are three in 

 number, of 548 feet each, 

 and two connection spans 

 of 525 feet each. It will 

 be noticed that the two 

 end spans and the cen- 

 tral span give a greater 

 clear height above the 

 water namely, 160 feet 

 while the connected 

 spans give 130 feet, a 

 plan which facilitated 

 the placing of the canti- 

 levers without staging, 

 and therefore with less 

 obstructiontocommerce. 

 The wind-pressures are 

 estimated on the basis of 

 30 pounds per square 

 foot of surface, including 

 towers, spans, and area 

 of trains. The pressure 

 on the caisson bases is 

 about 3 tons per square 

 foot, and the material 

 upon which they rest is 

 hard gravel. 



From end to end, in- 

 cluding approaches, the 

 structure is about 1^ 

 mile long, and it is an 

 excellent example of the 

 latest ideas in bridge 

 construction. 



The Tay Bridge. Some 

 of the ingenious devices 

 for laying the founda- 

 tions of this grand bridge 

 were described and illus- 

 trated in the article on 

 engineering for 1885. 

 The year 1887 saw the 

 completion of the whole 

 structure, and trains were 

 passed over it early in 

 June. Throughout the 

 whole course of con- 

 struction the ruins of the 

 old bridge, which fell in 

 1879, were utilized, and 

 the engineers were thus 

 enabled to dispense with 



BRIDGE AT POUGHKEEPSIE. 



