THE ROYAL ARTILLERY INSTITUTION. 339 
weaken the fibre. Hence the tension on no coil should exceed -roVtfth 
of its diameter. 
The compression varies inversely as the density and rigidity of the mass, 
the first layer of coils will therefore undergo more compression than the 
second, and the second more than the third, and so on. Accordingly, in the 
Armstrong or original construction, a greater proportion of shrinkage was 
given to the inner layers than the outer, because so much of it was 
absorbed by compression. The shrinkage, however, never exceeded * *002 
per inch of diameter. 
The position of the coil must be also considered. The shrinkage over 
the seat of the charge is greatest of all, as that part of the gun must be the 
strongest and firmest, whilst the shrinkage over the muzzle is the least, for 
an opposite reason. 
Add to all the foregoing conditions the expediency of shrinking on the 
several parts so that each shall do its proportion of work on the discharge 
of the piece (according to a law not exactly known), and it will be seen that 
the problem is a very difficult one to solve in practice. Indeed, Sir William 
Armstrong, who introduced the system, has admitted that he did not carry 
out his plan with theoretical precision, but that the coils were simply shrunk 
together sufficiently to secure the stability of the fabric and that a small 
variation was immaterial.* His primary object therefore was to secure 
cohesion throughout the mass, and in doing this within the limits of 
shrinkage before stated, he built up his gun to a certain extent in accordance 
with theory. 
It is at all events certain that by a combination of his two principles, 
one of Sir William Armstrong^ guns is considerably stronger than a gun 
of the same shape and weight would be if made out of a solid forging like 
the Horsfall gun. 
The Woolwich guns built on his system and lined with toughened steel, 
are sound and strong, but from the fine iron used and the great number of 
exquisitely finished coils and a forged breech*piece, their manufacture was very 
costly; and as it was probable that several heavy guns would be required, the 
War Office pointed out the desirability of procuring some cheaper plan. 
Accordingly the attention of the Eoyal Gun Eactories was devoted to the 
question, and their efforts have been crowned with success. Eirst, a cheaper 
iron sufficiently strong for the exterior of the gun was obtained, and 
secondly, the plan, which was proposed by Mr Eraser, the principal executive 
Officer of the Department, was found to be less expensive than the original 
one. 
Mr Erasers plan is an important modification of Sir W. Armstrong^ 
from which it differs principally by building up a gun with a few long 
double or triple coils, instead of several short single ones and a forged 
breech-piece. There is less material, less labour, and less fine working, and 
consequently less expense required for the “ Eraser” or present service 
construction. 
10 feet long and 1 inch square, the extension was only i 0 b o oo of the length, for every ton, per 
square inch of section up to the elastic limit, after which it rapidly increases up to the breaking 
strain. 
* Report on Ordnance, 1863, p* 162. “ Construction of Artillery,” Inst; Civil Engineers* 1860; 
