THE EOYAL AETILLEEY INSTITUTION. 
423 
A steel bar, cast for the purpose, is then placed with one end resting on a 
plane table, and the other end supported at a height from the table equal to 
MC, and by means of the various measurements the curve is traced on it, 
and afterwards planed or slotted out. 
If the pinion on the sliding rack and the bore of the gun are of the 
same diameter, the curves of the grooves and of the copying bar are the same; 
but if, as is generally the case (for, excepting the copying part, the same 
machine answers for all calibres and systems), the pinion and bore are 
different sizes, the curvature of the bar will differ proportionately from that 
of the grooves. Thus, if the pinion is half the size of the bore, the bar 
must have half the curvature of the grooves. 
The cut is made when the boring bar is coming out of the gun, a stout 
rim of bristles on the head brushing out the chips before it. The cutter 
can be let in or out as required by means of a spindle which passes through 
the rifling bar, and it is kept up to its work by a weighted lever connected 
with the spindle. 
All the grooves in the gun are first cut out roughly in succession, and 
then finely. The distance between the grooves is regulated by a disc fixed 
to the breech of the gun, and having its periphery equally divided by as 
many notches as there are to be grooves. The gun is fixed each time by a 
pall, and when a new groove has to be cut, is turned round to the next notch. 
The groove is of the “ Woolwich” shape, 1"*5 wide and 0"T8 deep, 
with concave edges. It is the same width for all natures, but it is a little 
deeper for the 10 and 12-inch guns. The number of course varies with 
the calibre, 7-inch guns having three, 8-inch four, 9-inch six, 10-inch seven, 
and 12-inch nine. 
As a rule, about two calibres in length is left plain or unrifled for a 
powder chamber. The exact proportion is not fixed. The unrifled part, 
however, should be no shorter than actually necessary to prevent a detri¬ 
mental air space between the smallest charge used and the base of the 
projectile, as the grooving tends to weaken the barrel very much, and the 
seat of the charge should be the strongest part of the gun. 
(9) Previous to the 23rd January, 1868, rifled M.L. guns were left alto¬ 
gether unvented until after proof, at which they were fired by means of electric 
wires passed in at the muzzle. Since that date, all guns are drilled and 
tapped before proof, and fired through a removable steel vent, which is 
unscrewed after proof and replaced by the permanent vent; the object of 
this is to prevent the proper vent being strained by the large proof charge. 
Proof. 
Before a gun is proved, gutta percha impressions are taken of the whole 
length of the bore in the four quarters. The gun is then proved with two 
rounds—the projectile being equal in weight to the service one, but flat¬ 
headed for 7-inch guns and upwards, in order that it may penetrate as little as 
possible into the butt, and the charge being 1 Jth the weight of the battering 
or highest charge used in service. The gun is fired in the open by means 
of an Abel's electric tube, connected with a magneto-electric battery in a 
bomb-proof shed. 
"With the early rifled guns much larger charges were used at proof, and 
then service charges and a double-weighted shot were used for some time; 
