THE ROYAL ARTILLERY INSTITUTION. 
209 
great forgings, and to account in a highly instructive manner for the trans¬ 
verse fissures developed in cooling, the result of contractile strains :— 
“ These powerful contractile strains within the mass in cooling, exercise con¬ 
siderable influence upon the arrangement of the crystalline axes and planes of 
separation of the iron. Bearing in mind the general law, that the principal axes 
of the crystals of a cooling, and therefore a contracting mass, are found to arrange 
themselves in the direction of minimum internal pressures, it will be obvious that 
the grasp of the external rigid ring upon the internal nucleus will tend to place the 
crystallising axes of the former in tangential directions, and those of the nucleus— 
if at a temperature sufficiently low for its crystallisation to take place—in radial 
directions, during the first periods of cooling. When the contractile forces of the 
external ring have ceased to be tensile circumferentially, and have become tan¬ 
gentially compressive (in virtue of the radial pull of the contracting nucleus), and 
when, at the same time, radial compression of the nucleus by the exterior has 
given way to the contractile tension of the former, pulling away from the latter, as 
also from itself, then the tendency will be to arrange the external crystalline axes 
radially, and the internal ones tangentially. The change of sign or direction of 
the respective tensile and compressive forces, tends to alter the directions of all the 
crystalline axes during the cooling process. This is the cause of the varied direc¬ 
tions in which the integrant crystals are found in the vast mass of such forgings, 
when broken into, or otherwise examined. 
“ The remedy for this unfortunate play of molecular forces, which was adopted 
with respect to the large forgings at Liverpool, and was at last in a great degree 
successful, was to build up and work them hollow from the commencement. 
When a cylinder has a large concentric cylindrical hole along its axis, it cools at 
the same time, though not equally, on both the internal and the external surfaces; 
and thus the extremes of internal strains are avoided, and the hollow centre yields 
more readily to the forcible compressive grasp of the exterior.” 
7. The value of this quotation will be an apology for its length. Mr* 
Mallet's results are given in six tables, from which I extract a few of the 
particulars as bearing directly on the structural strength of the mortars, 
and interesting for comparison with similar data since made public, for the 
irons in use in the Koyal Arsenal at the present day. See for example the 
elaborate tables printed in “ Extracts of Proceedings of the Department of 
the D.G.O.,” Yol. VII. p. 234, from trials made by Mr. Kirkaldy in 1869, 
with iron and steel, from 9-inch gun No. 281. 
“The coefficients T e and T r were designed by Poncelet, to express the ‘work 
done’ by an extending or compressing force upon any elastic prismatic body at 
the point where its elasticity becomes permanently impaired and its form distorted, 
and the further point where rupture occurs.” (“ Minutes, &c.,” p. 298). 
where i ~ extension in terms of length, assumed to be uniform 
throughout its range. 
P == force in lbs. applied per unit of section. 
7)- is arrived at in the same way. 
28 
