Dee. 15, 1870 | 

the limit of resistance, for hard and crystalline substances 
have often flaws which no inspection can discover, and 
which only reveal themselves at the moment of destruction, 
and also such guns, when they burst, always do so explo- 
sively, without previous warning. This is not the case 
with tough material, which can yield through a very consi- 
derable extent before breaking. It may be objected that 
this could be overcome by increasing the thickness of cast-, 
iron guns. But it is found that, after a limit very soon 
reached, increase of thickness does not produce increase 
of strength. The following law has been ascertained, that 
“no possible thickness can enable a cylinder to resist a 
pressure from within greater per square inch than the 
tensile strength of a square inch bar of the same material.” 
That is—to take an example—if a cylinder of cast iron, 
whose breaking strain is ten tons per square inch be 
subjected to that amount of interior pressure, it will from 
the very first occasion begin to give way. At first, the 
inner surface would be ruptured, and the expanding gas 
or other source of pressure, taking advantage of the 
fissures, would quickly extend and complete the work of 
destruction. The inner portions of such a cylinder have 
much more work to do than those farther out, the same 
amount of force acting over a greater extent of surface 
or lamina. It may therefore be roughly estimated that 
the strains on the successive portionsor rings vary inversely 
as the square of their radii. Dr. Hart, Fellow of Trinity 
College, Dublin, taking into account the compressibility of 
-2 
the metal, has given the following formula, t= 4 
a 2 
Rk? + p? 2 ; a a 
—_1 © , where s is the strain on the inner surface, o that 
K? +? 
on a ring of which p is radius, and & and 7 are external 
and internal radii of the cylinder. Tocompare the strains 
on the insideand outside, let S be the latter, and, asp = 2, 
the formula becomes Spt hPa 
Ss e+ 7 
case of a gun of ten inches calibre, with a thickness of 
side of five inches, y = 5, R = Io, and Sis tos as 50 to 
125, so that more than twice as great a strain is borne by 
the inner surface as by the outer. 
This brings us face to face with one of the great 
objections to cast-iron guns. In all castings the outer 
part, which cools first, is stronger than the interior. The 
metal contracts in cooling, and as the heat first leaves the 
exterior it first becomes solid, and the inner particles 
successively unite to it in layer after layer of crystals ; 
so that the centre contains metal but imperfectly coherent. 
In a casting of two or three feet diameter, a central 
portion of six, eight, or even more inches in diameter, is 
found consisting of a spongy mass of scarcely coherent 
crystals of iron, sometimes with cavities visible to the 
eye when a section is made. This is exemplified in the 
annexed drawing (Fig. 1) of a 13-inch sea-mortar shown 
in section with the head of metal with which it is cast 
remaining attached, the parts to be cut off and bored out 
being marked by a black line. The shaded portions 
represent the weak and porous parts of the metal, which 
extend down through the centre below the bottom of 
the powder chamber, where it leaves a soft spot, easily 
hammered and burnt away by the shock and blaze of the 
discharge. Itis plain that in any piece of artillery thus 
formed, the strength of the sides must gradually decrease 
from the exterior to the interior, which is precisely the 
reverse of what is required. To remedy this the Rodman 
cast-iron guns, used inthe United States, are cast round 
a core or closed tube, which is inserted in the mould and 
represents the bore of the proposed gun. Into this tube 
a stream of cold water is kept continually pouring, so 
that the molten metal first solidifies round it; and, further, 
to secure this a fire is kept up round the mould for some 
time after the castng has begun. Here we have the 
conditions required in a cast-iron gun, viz. the best 
Applying this to the 
NATURE 


129 

and strongest metal in the interior round the bore. 
Fig. 2 represents a 15-inch Rodman cast in this manner. 
It will be observed that in its shape, which resembles a 
soda-water bottle, all angles are avoided and a rounded 
form carefully preserved. By this another source of weak- 
ness is avoided, through compliance with an important 
law of nature. When any substance solidifies under 
the influence of heat leaving the mass, “the principal 
axes of the crystals will always be found arranged in lines 
perpendicular to the bounding planes of the mass, that is 
to say, in the lines of direction in which the wave of heat 
has passed outwards from the mass in the act of consoli- 
dation.” (The Construction of Artillery, by R. Mallet, 
M.I.C.E., &c., &c. 1856.) This direction is that of least 
pressure within the mass, being that of the motion of the 
heat waves ; and the law above stated is part of the far 
more general principle that in nature.the line of least 
resistance is the one invariably chosen. From this law it 
follows that wherever there is an angle or sudden change 
in the form of a casting, through that angle there runs a 
plane of weakness, arising from irregular crystallization, 
as the crystals arrange themselves perpendicularly to the 
surfaces. Every abrupt change in the form of the exterior 
of a casting, every salient or re-entering angle, no matter 
how small, upon the exterior of a gun or mortar, is accom- 
panied by one or more planes of weakness in the mass. 
This was strikingly exemplified in the cylinders of the 
hydraulic press made for raising the tubes of the Britannia 
Bridge. The first was made with a flat bottom, conse- 
quently it had planes of weakness as shown by the lines 
VV in Fig. 3, and under the enormous pressure to which 
it was subjected it gave way, the bottom curving out in 
the direction of those lines. The second cylinder was 
made with a rounded bottom, as in Fig. 4, and success- 
fully resisted the pressure to which it was exposed. Cast- 
iron guns give evidence in bursting of planes of weakness 
in accordance with this law. The usual lines of fracture 
are shown in Fig. 5. Any visitor to Woolwich Arsenal 
can, by inspecting the cemetery where the guns burst in 
proof or for experimental purposes are preserved, verify 
this law of nature from many examples. By the form of 
the Rodman gun this source of weakness is avoided. It 
is a cast-iron gun, made on thoroughly scientific principles, 
in which the material is used to the utmost advantage, and 
therefore it may justly be compared with the Fraser gun, 
in which the same thing is done with wrought-iron, to 
measure the value of two materials. A casting is always 
cheap compared with a forging ; in this point the Rod- 
man has the advantage. But at its best, cast-iron has only 
one-third the strength of wrought-iron. | Consequently 
the Rodman gun cannot be safely rifled. It fires 
heavy round shot, but with a range and accuracy greatly 
inferior to that of an elongated rifled projectile. Its 
initial velocity is high, but it is not long kept up. Atclose 
quarters its racking effect upon armour plate would be 
very severe, but its penetrative power is low. In the ex- 
tensive experiments made at Shoeburyness in 1868 the 
ro-inch Fraser gun of 18 tons weight penetrated fifteen 
inches of iron (in three 5-inch armour plates), upon 
which the Rodman 15-inch gun of 20 tons only made a 
shallow indent. Further, being cast-iron, they are liable 
to burst explosively without any previous indication, and 
from the metal being in a state of great tension, being 
cooled from the inside, they have been known to break 
up in store. 
The facts stated in the comparison of the best and 
most scientifically-constructed cast-iron with wrought, 
seem to be very decisive against the former. Two other 
materials have to be noticed. Ore is bronze, or gun- 
metal. It has some admirable qualities for making a 
gun. A bronze gun is hardly ever known to burst under 
ordinary circumstances. So great is its tenacity that 
with continued firing such a gun has been known to swell 
and change form without bursting. 
