540 ADDITIONS. 



the difference of these two forces by experiments. Several persons 

 pursued researches on this subject; especially Mr. Barlow, of the Roy a! 

 Military Academy, 4 who investigated the subject with great labor and 

 skill, so far as wood is concerned. But the difference between the re- 

 sistance to tension and to compression requires more special study in 

 the case of iron ; and has been especially attended to in recent times, 

 in consequence of the vast increase in the number of iron structures, 

 and in particular, railways. It appears that wrought iron yields to 

 compressive somewhat more easily than to tensile force, while cast iron 

 yields far more easily to tensile than to compressive strains. In all 

 cases the power of a beam to resist fracture resides mainly in the upper 

 and the under side, for there the tenacity of the material acts at the 

 greatest leverage round the hinge of fracture. Hence the practice was 

 introduced of making iron beams with a broad flange at the upper and 

 another flange at the under side, connected by a vertical plate or ivcb, 

 of which the office was to keep the two flanges asunder. Mr. Hodg- 

 kinson made many valuable experiments on a large scale, to determine 

 the forms and properties of such beams. 



But though engineers were, by such experiments and reasonings, en- 

 abled to calculate the strength of a given iron beam, and the dimen- 

 sions of a beam which should bear a given load, it would hardly have 

 occurred to the boldest speculator, a few years ago, to predict that 

 there might be constructed beams nearly 500 feet long, resting merely 

 on their two extremities, of which it could be known beforehand, that 

 they would sustain, without bending or yielding in any perceptible de- 

 gree, the weight of a railroad train, and the jar of its unchecked motion. 

 Yet of such beams, constructed beforehand with the most perfect con- 

 fidence, crowned with the most complete success, is composed the great 

 tubular bridge which that consummate engineer, Mr. Robert Stephen- 

 son, has thrown across the Menai Strait, joining Wales with the Island 

 of Anglesey. The upper and under surfaces of this quadrangular tube 

 are the flanges of the beam, and the two sides are the webs which con- 



O ' 



nect them. In planning this wonderful structure, the point which re- 

 quired especial care was to make the upper surface strong enough to 

 resist the compressive force which it has to sustain ; and this was done 

 by constructing the upper part of the beam of a series of cells, made 

 of iron plate. The application of the arch, of the dome, and of groin- 

 ed vaulting, to the widest space over which they have ever been thrown, 



An Essay on the Strength and Shape of Timber. 3d edition, 



