2 REPORT—1862. 
used and copper loop heated, a loose contact produced a current from copper 
to iron across the joint, giving a deflection of 100 divisions. A tight contact 
gave nothing decided. When the iron loop was heated (the copper cold) the 
loose contact produced a current from iron to copper across the joint, giving 
a deflection of 90 divisions. A tight contact in this case gave a weak current 
in the opposite direction. When the joint was heated in the middle, as the 
temperature gradually rose, a maximum deflection of 3 divisions was first 
reached, showing a current from copper to iron across the joint; and as the 
heat increased still further this current was reversed, and finally, at a white 
heat, gave a maximum deflection of 3 divisions with a current from iron to 
copper. 
On the Mechanical Properties of Iron Projectiles at High Velocities. 
By W. Farrsarrn, F.R.S. 
A VALUABLE series of experiments were made at Manchester upon portions of 
plates fired at by the Iron Plate Committee at Shoeburyness. These experi- 
ments comprised the determination of the resistance to punching, to a tensile 
strain, to impact, and to pressure. 
They show that the tenacity varied from 11 to 29 tons per square inch in 
the iron plates, and from 26 to 333 tons in the homogeneous iron plates. The 
average strength of the iron plates between 14 and 3 inches thick varied 
from 234 to 243 tons per square inch, and this, or about 21 tons, may proba- 
bly be insisted upon as a measure of strength in future contracts for iron 
lates. 
a The elongation of the plates under a tensile strain may be taken as a mea- 
sure of the ductility of the material ; it varied in the thicker iron plates from 
0-91 to 0-27 per unit of length, and averaged 0:27 inch in the homogeneous 
metal plates. The maximum observed was 0:35. 
The most important results in connexion with the question of the resist- 
ance are, however, those obtained by combining the tensile breaking weight 
with the ultimate elongation, as first indicated by Mr. Mallet in a paper read 
before the Institution of Civil Engineers. By finding in this manner the 
product of the tenacity and ductility, numbers are obtained which, though not 
identical with those expressing the resistance of the plates in the experiments 
with guns at Shoeburyness, are yet in close correspondence with them, The 
average value for Mr. Mallet’s coefficient in the thicker iron plates was about 
6500 lbs., and in the steel or homogeneous plates 8300 lbs. But the resist- 
ance of the iron plates increases with the thickness, whilst that of the homo- 
geneous metal diminishes. The correspondence of these numbers is indicated 
in the Report addressed to the War Office and the Admiralty ; but a more 
extended series of experiments are yet wanting to determine the true value 
of the coefficient as a guide to be insisted upon in the manufacture of iron 
plates. 9000 foot-pounds is the maximum for iron given by the *results 
already obtained ; but an extended series of experiments might develope new 
features of resistance and new improvements in the manufacture. 
The experiments on punching afford an explanation of the greatly increased 
perforating power of the flat-headed shot overthat of the round-headed 
projectiles. They also lead to a formula for the ordinary cast-iron service 
shot, which appears to give with approximate accuracy the law of the resist- 
