102 
BIRNIE. 
This result was, however, to be expected, inasmuch as the 
theoretical curve of initial tension depends upon the thick¬ 
ness of wall in calibers, and the actual curve evidently obeys 
the same law. 
This experiment leaves no room to doubt that initial ten¬ 
sion strains of as great intensity as are desirable can be pro¬ 
duced in a hollow forging by interior cooling, and if these 
strains should be more than needed they can be reduced 
by annealing. The effect of the subsequent annealing in 
the present case was beneficial, particularly in the muzzle 
section, which now shows the peculiarly interesting case of 
a resistance to interior pressure which closely approximates 
the possible maximum. The ordinates of the stress curve 
in action are all nearly equal and differ but little from the 
limit of 60,000 pounds. 
Without disparagement to the built-up, hooped gun, which 
has proved to be excellent, it may be said that the apparent 
superiority of a gun made of a single forging, with initial 
tension produced by interior cooling, rests not only upon 
claims for reduced cost and increased longitudinal stiffness, 
but also for increased tangential strength in every section 
where the actual thickness of wall is insufficient in practice 
for the division of the built-up gun into as many as four 
layers, since this number of layers is in general required 
in that construction to enable the bore to be worked through 
the double limit of elastic movement. 
Inasmuch as the walls of built-up, hooped guns below 10 
or perhaps 8 inches caliber cannot be conveniently divided 
into four layers, and this division, moreover, can only be 
applied in the thicker portions (i. e., the reinforce), the con¬ 
clusion from the theoretical standpoint, at least, is that an 
equality of tangential strength will exist under the two 
modes of construction for the reinforce of guns of 8 or 10 
inches caliber and upwards; but for guns of smaller caliber 
and for the chase portions of all guns the greater tangential 
strength will pertain to the single forging with initial ten¬ 
sion produced by interior cooling. 
