ARMOUR-PIERCING PROJECTILES, 
497 
of these experiments, which is already completed, consisted in firing 
increasing charges until a pressure of about 16*4 tons per square inch 
(2500 atmospheres) was arrived at, with a projectile weighing 91*998 lbs. 
and with a chamber of the service size. 
The second series will be carried out in a similar manner, out with 
a powder chamber having a greater diameter. 
In all these experiments the pressures are found by means of two 
Rodman gauges placed in the breech wedge, and the muzzle velocities 
are determined with two chronographs. 
By the time these preliminary experiments are finished, the work of 
preparing the inner tube at the Oboukhoff foundry will be sufficiently 
advanced to render it possible soon to enter on the principal object of 
these trials. 
As these trials are not of less interest to the War Department than 
to the Marine Department, the artillery section of the Marine Com¬ 
mittee has considered it necessary to communicate the results of the 
first trials to the chief authorities of the artillery, so that the artillery 
committee can, if it thinks necessary, give directions as to the design 
of the tube with which the bore of the 6-in. gun is about to be 
lengthened, and as to the general direction of the trials. 
The artillery section of the Marine Committee endeavoured at first 
to obtain, in spite of some difference in the inclination of the rifling, 
all the data of construction which had been communicated to them 
concerning the inner tube of the 6-in. Armstrong gun, of which the 
bore was 23 calibres in length, and had 16 grooves with an increasing 
twist. The inclination of these grooves at the breech is such that they 
make one complete turn in 91 calibres; this inclination goes on 
increasing until it makes one complete turn in 38 calibres, then at a 
distance of 7*874 ins. from the muzzle the rifling has a constant 
inclination of 1 in 38 calibres. As the construction of similar rifling 
would have given much trouble at the Oboukhoff foundry, and would 
have greatly retarded the completion of the gun, and as, besides, the 
results of a competitive trial in Austria between two siege guns of 
6 -in. (15 cm ) calibre (one having uniform rifling of 1 in 45 calibres, and 
the other being rifled with an increasing twist) showed that although 
the accuracy of the first gun was a little inferior to that of the second, 
it was very good in both cases, they thought that they could without 
difficulty provide this same uniform rifling for the 6-in. experimental 
gun. In the opinion of the artillery section of the committee, the 6-in. 
gun with a uniform rifling would have given satisfactory results when 
firing projectiles of the Russian service form, but as the trials afterwards 
carried out by Krupp with a 6-in. gun showed the power obtained when 
an increasing twist is used, having an inclination of 1 in 25 calibres at the 
muzzle and with a shell 4 calibres long, it did not seem sufficient to 
arrange the programme for the experiments, exclusively according to 
the trials made in England, and it became necessary to give up the 
uniform rifling proposed for the 6-in. gun. 
Considering that in the guns of pattern of 1877 (excepting, however, 
the field guns) the development of the groove is parabolic, the artillery 
section considered that a similar development might be given to the 
