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Mr. Fairbaim — Experiments on the Effects [Feb. 4, 



known that tbe powers of resistance to strain in wrought iron are widely 

 different, according as we apply a force of tension or compression ; it is even 

 possible so to disproportion the top and bottom areas of a wrought-iron 

 girder calculated to support six times the rolling load, as to cause it to yield 

 with little more than half the ultimate strain or 10 tons on the square inch. 

 For example, in wrought-iron girders with solid tops it requires the sectional 

 area in the top to be nearly double that of the bottom to equalize the two 

 forces of tension and compression ; and unless these proportions are strictly 

 adhered to in the construction, the 5-ton strain per square inch is a fallacy 

 which may lead to dangerous errors. Again, it was ascertained from direct 

 experiment that double the quantity of material in the top of a wrought- 

 iron girder was not the most effective form for resisting compression. On the 

 contrary, it was found that little more than half the sectional area of the top, 

 when converted into rectangular cells, was equivalent in its powers of resist- 

 ance to double the area when formed of a solid top plate. This discovery 

 was of great value in the construction of tubes and girders of wide span, as 

 the weight of the structure itself (which increases as the cubes, and the 

 strength only as the squares) forms an important part of the load to which 

 it is subjected. On this question it is evident that the requirements of a 

 strain not exceeding 5 tons per square inch cannot be applied in both cases, 

 and the rule is therefore ambiguous as regards its application to different 

 forms of structure. In that rule, moreover, there is nothing said about the 

 dead weight of the bridge ; and we are not informed whether the breaking- 

 weight is to be so many times the applied weight plus the multiple of the 

 load, or, in other words, whether it includes or is exclusive of the weight 

 of the bridge itself. 



These data are wanting in the railway instructions ; and until some fixed 

 rinciple of construction is determined upon, accompanied by a standard 

 measure of strength, it is in vain to look for any satisfactory results in the 

 erection of road and railway bridges composed entirely of wrought iron. 



The author was led to inquire into this subject with more than ordinary 

 care, not only on account of the imperfect state of our knowledge, but from 

 the want of definite instructions. In the following experimental researches 

 he has endeavoured to ascertain the extent to which a bridge or girder of 

 wrought iron may be strained without injury to its ultimate powers of resist- 

 ance, or the exact amount of load to which a bridge may be subjected with- 

 out endangering its safety — in other words, to determine the fractional strain 

 of its estimated powers of resistance. 



To arrive at correct results and to imitate as nearly as possible the strain 

 to which bridges are subjected by the passage of heavy trains, the apparatus 

 specially prepared for the experiments was designed to lower the load quickly 

 upon the beam in the first instance, and next to produce a considerable 

 amount of vibration, as the large lever with its load and shackle was left 

 suspended upon it, and the apparatus was sufficiently elastic for that 

 purpose. 



