162 Mr. Mallet on the Physical Conditions 



These phenomena are plainly brought into evidence in mortars ; but they 

 exist, in less degree, in cannon, though, from the length of chase of the latter, 

 it is extremely difficult to make any perfect examination of the state or qua- 

 lity of the metal at the bottom of the bore. 



46. Figs. 2, 3, 4, and 5, Plate m., represent portions of castings, in section, of 

 various external forms, in which sudden changes of volume frequently produce 

 internally actual cavities, technically known as " draws" amongst workmen. Fig. 

 2 is an excentric shell. Fig. 3, partial section of a girder, with a thick base 

 rib, and thin vertical web. Fig. 4, part of a heavy flat casting, with a thin flange 

 projecting from it. Fig. 5, part of the upper (or mouth) portion of the cylinder 

 of an hydraulic press (one which occurred in the author's practice), but quite 

 similar to the conditions so well described by Mr. Edwin Clarke, of the first 

 defective cylinders cast for raising the Conway and Britannia tubes. 



47. These internal cavities are usually found more or less perfectly lined with 

 adherent crystals of cast-iron, and with plates or crystals of exuded graphite. 

 The figures given sufficiently illustrate their general character. The main or 

 longest directions of these irregular cavities always tend to follow the " planes 

 of weakness," or to place themselves, at right angles to these. 



Their magnitude depends upon — 



1°. The coefiicient of cubic contraction of the particular " make" of cast- 

 iron. 

 2°. The high temperature at which the fluid metal has been " poured," 

 i. e., the length of range of temperature through which it has 

 cooled, and the inefficiency with which the mass has been subse- 

 quently "fed" while solidifying. 

 3°. The mutual relations as to volume of the adjacent and successively 

 solidifying parts of the whole mass. 

 No British makes of cast-iron are so subject to violent " draws" as the best 

 and toughest of the South Wales irons, such as the Blaenavon, &c. 



6. — Effects of Bulk and Fluid Pressure. 



48. It is a remarkable fact, though one not yet fully explained, that a small 

 bar, cast on-to, or projecting from, a casting in iron of very large scantling, when 

 afterwards broken ofi"and tested, will not sustain, by a good deal, as great a trans- 



