216 Report on the Progress and State of Applied Mechanics. 



son's experiments are capable of being accurately represented by suitably 

 modifying the constant multipliers in a formula originally proposed by 

 Tredgold ; and this formula indicates the fact, that for a pillar whose 

 length is less than twenty-seven or twenty-eight times its diameter, 

 cast iron is thfe stronger material, — while, for a pillar whose length is 

 greater in proportion to its diameter, wrought-iron is the stronger. To 

 Mr. Hodgkinson is due the experimental investigation of the resistance 

 of wrought iron tubes to a direct longitudinal thrust, and of the manner 

 in which all granular substances give way to a du'ect crushing force, by 

 splitting into wedges, cones, and pyramids, of a certain inclination for 

 each material. 



38. The resistance of wrought iron rivets to shearing, has been 

 determined by Mr. Doyne (who commanded the Army Works Corps 

 in the Crimea), and found to be nearly equal to the tenacity of boiler- 

 plate. 



39. The experiments which have recently contributed most to the 

 advancement of our knowledge of the strength of iron, are those of Mr. 

 Fairbairn. By him was invented that cellular construction, which 

 enables wrought iron plates to withstand a thrust, and is essential to 

 the practicability of those tubular girders large enough to transmit a 

 train, the original idea of which was the invention of Mr. Stephenson. 

 Mr. Fairbairn has determined tlie strength of iron at different tempe- 

 ratm-es (showing it not to be impaired at 600° F.), the strength of cast 

 iron, after repeated meltings, — the strength of various kinds of plate 

 iron, "along and across the grain, — of various forms of plate and bar iron 

 beams, — of different forms of boilers, — and of the different parts upon 

 which the strength of a boiler depends, so that the giving them their 

 proper proportions is now a matter of calculation. Many of Mr. Fair- 

 bairn's experiments have been made at the instance of the British 

 Association; and, amongst others, those which have just been com- 

 pleted upon the resistance of thin tubes, such as the flues of boilers, to 

 a pressure from without, tending to make them collapse. 



40. Mr. W. H. Barlow, experimenting on the resistance of cast 

 AND WEOTJGHT IRON BEAMS to a TEANSVEESE LOAD, has recently found 

 that resistance to be greater than that which corresponds to the direct 

 tenacity of the material, in a proportion depending on the figure of the 

 cross-section of the beam, — being greater as that figure becomes more 

 compact, and approaches a solid rectangular shape. When a solid 

 rectangular cast iron beam is broken under a transverse load, Mr. 

 Barlow finds that the tension upon those particles which are most 

 strained, viz., — those at the middle of the lower side of the beam, is 

 about two-and-a-half times as intense as the tension which tears a bar 



