THE EXHIBITION OP 1862. 
161 
second, does not, at 2° of elevation, attain two-thirds of the 
range it would have in vacuo, the Armstrong, with an initial 
velocity of about 1,050 feet per second, exceeds it by about 
120 yards. 
From the above-mentioned obliquity of the elongated pro- 
jectile to the curve of its flight, and the consequent unequal 
action of the resistance of the air above and below, arises a 
deviation in its flight, which is termed derivation ; but this is 
constant for similar ranges, and can be allowed for. 
We now come to a practical difficulty in the manufacture of 
large guns. This difficulty was found even in making round-shot 
guns of above eight inches in diameter of bore, strong enough to 
withstand the charge required to impart high velocities to sohd 
shot ; and with elongated shot this difficulty is greatly increased, 
because the weight of the projectile is increased in proportion 
to the size of the bore. For as was pointed out, we believe, 
first by Dr. Hutton, it is of no use to increase the thickness 
of metal in the gun beyond a certain point. Let us take the 
cross-section of a gun, that is to say, an annulus, and inquire, 
what is the precise action of the exploded powder on this? 
We know that iron in resisting a tensile strain is extended 
(a small portion of the extension, from want of perfect elasti- 
city, being permanent), and the gun, or the portion of it 
which we are considering, — the annulus, is therefore enlarged, 
this enlargement being, to a great extent, only momentary. 
But though it is enlarged, the area of the annulus, which, 
being the cross section, represents the amount of metal in the 
gun at that point, must remain the same ; therefore the width 
must diminish ; or, which is the same thing, the circumference 
of the inner circle must be increased in a greater ratio than the 
outer; i. e., the inside of the gun must be more stretched than 
the outside. Now, if we consider the annulus divided into its 
component concentric rings, the greater the distance of each 
successive ring from the centre, the less will it be stretched, 
the less will be the strain on it, and the less good will it 
do, — till we arrive at a point when the inside of the bore or 
inside ring will be ruptured, before any strain is taken by the 
outside ; so that, beyond a ceertain point, increasing the thick- 
ness of the metal is useless. 
In order to make the gun sufficiently strong, various means 
have been suggested (Captain Blakeley, some of whose guns 
were in the Exhibition, having been, we believe, one of the 
first and most successful in practically meeting the difficulty), 
such as shrinking or driving on rings of metal on an inner 
core, or by winding wire on it with an increasing strain, 
so that what is called an “ initial tension ” is given to the 
outside of the gun ; and the successive rings of the annulus 
