26 ANNUAL OF SCIENTIFIC DISCOVERY. 



central portion of bars two inches square and three inches square is 

 composed of comparatively large crystals, and bars of three inches 

 square in section, planed down on all sides alike to three fourths of an 

 inch square, are found to be very weak to resist both transverse and 

 crushing pressure. Hence it appears desirable, in seeking for a unit 

 for the strength of iron of which a large casting is to be made, that 

 the bar used should equal in thickness the thickest part of the pro- 

 posed casting." 



The commissioners then go into a detailed notice of the relative merits 

 of the different kinds of iron bridges in use on railways. The sim- 

 plest bridge, and that which admits of the greatest, amount of headway 

 at a given elevation, is, undoubtedly, the straight girder-bridge. The 

 length of a simple cast-iron girder appears to be limited only by the 

 'power of making sound castings and the difficulty of moving large 

 masses. In the employment of wrought-iron combined with cast-iron, 

 in the manner of trussing, the greatest skill and caution are necessary 

 to render such combinations safe. The general opinion of engineers 

 appears to be, that the cast-iron arch is the best form for an iron bridge, 

 when circumstances permit its use. Lattice bridges are of doubtful 

 merit. For low bridges the bow-string girder is recommended. The 

 hollow girder form of the Britannia and other bridges appears to pos- 

 sess many advantages, but has not yet been sufficiently tried to warrant 

 the expression of a decided opinion. 



In conclusion, the commissioners sum up the results to which they 

 have arrived as follows : " That it appears advisable for engineers, 

 in contracting for castings, to stipulate for iron to bear a certain 

 weight, instead of endeavouring to procure a specified mixture. That, 

 to calculate the strength of a particular iron for large castings, the 

 bars used as a unit should be equal in thickness to the thickest part 

 of the proposed casting. That, as it has been shown that, to resist 

 the effects of reiterated flexure, iron should scarcely be allowed to suf- 

 fer a deflection equal to one third of its ultimate deflection, and since 

 the deflection produced by a given load is increased by the effects of 

 percussion, it is advisable that the greatest load in railway bridges 

 should in no case exceed one sixth of the weight which would break 

 the beam when laid on at rest in the centre. That, as it has appeared 

 that the effect of velocity communicated to a load is to increase the 

 deflection that it would produce if set at rest upon the bridge ; also, 

 that the dynamical increase in bridges of less than forty feet in length 

 is of sufficient importance to demand attention, and may, even for 

 lengths of twenty feet, become more than one half of the statical de- 

 flection at high velocities, but can be diminished by increasing the stiff- 

 ness of the bridge ; it is advisable that, for short bridges especially, 

 the increased deflection should be calculated from the greatest load 

 and highest velocity to which the bridge may be liable ; and that a 

 weight which would statically produce the same deflection should, in 

 estimating the strength of the structure, be considered as the greatest 

 load to which the bridge is subject. Lastly, the power of a beam to 

 resist impact varies with the mass of the beam, the striking body 

 being the same, and by increasing the inertia of the beam without 



