46 ANNUAL OF SCIENTIFIC DISCOVERY. 



of iron, on the Pratt truss principle. Every piece of wrouglit iron 

 in the ties, links, bolts, etc., was tested in a hydraulic press up to 

 23,600 pounds to the square inch, and struck with a hammer, 

 while under tension, before being used in the bridge. Theoreti- 

 cally, the strength before the effect of the load becomes apparent 

 in stretching is 28,000 pounds to the square inch ; while the ulti- 

 mate strength is 60,000 pounds to the square inch. The bridge is 

 so proportioned that a train of 2 locomotives and the heaviest 

 freight cars strain the iron only about 7,500 pounds to the inch." 

 The tests made were these : — 



Three of the heaviest locomotives were coupled and placed at 

 rest centrally upon the span 250 feet long, and the deflection or 

 yielding of l3ridge very accurately observed by means of instru- 

 ments. The total weight of the load was 300,000 pounds, and the 

 maximum deflection at the centre of the span was 2.4223 inches, 

 being one-sixteenth of an inch less than the deflection previously 

 calculated. 



The same load was then placed upon a span 157 feet long, and 

 a deflection produced of 1.375 inches, which varied but little from 

 the result of previous calculations. 



The 3 locomotives, still coupled, were then run over the 157 

 feet span several times, at rates of speed varying from 10 to 16 

 miles per hour. The deflection produced was 1.406 inches, being 

 an increase of only 3.1 inches over the deflection while at rest. 

 Probably no severer strain than the above will ever be applied to 

 the bridge in actual use. In each case, on the removal of the 

 load, the bridge at once resumed its previous form. 



The strain applied to-day was 5,100 pounds to the square inch 

 of wrought iron, and 5,800 pounds per square inch of cast iron. 



On the 157 feet span, the strain applied was 9,000 pounds to the 

 square inch on the wrought iron, and 10,200 pounds to the square 

 inch on cast, being about one-quarter more than the strain pro- 

 duced by the pa'=.sage of the heaviest freight trains. All the 

 "w^rought iron had been tested before being used bj'^ a strain of 

 23,000 pounds per square inch. Specimens of the wrought iron 

 which were subjected to the ultimate strain broke only at from 

 60,000 to 80,000 pounds per square inch. The total cost of the 

 structure was $1,500,000. — Scientific American. 



THE LIFE OF IRON BRIDGES. 



It may be assumed that a wrought-iron girder-bridge, subjected 

 at intervals to a dynamical load not exceeding the fourth part of 

 its powers of ultimate resistance, will be safe for traffic for a pe- 

 riod of 328 years. This assumption is based upon the proviso 

 that the successive alternations of strain and repose should not be 

 repeated more than 100 times during the same day. With the 

 exception of some country lines and rural branch railways, the 

 number of trains of every description passing over bridges in 24 

 hours considerably surpasses the limited number 100. Taking 

 the traffic during the night to be only one-third of that during the 



