2 
which is now under trial at Woolwich. So far as these 
guns have been tricd they have given very exceptionally 
good results, both in France and England, and they 
promise to excel all others in strength, facility of con- 
struction, and economy as regards cost. Let us then 
attempt to explain in a popular manner the principles 
and methods of this system of construction. 
A gun is a machine the object of which is to send 
heavy bodies to a great distance at a very high velocity. 
The motive power acts on the body for a very short time, 
a fraction of a second only, it must therefore be of great | 
intensity, and consequently the machine must have very 
great strength. Formerly all guns were made of cast-iron 
or bronze ; after this wrought iron and steel came into 
use or a combination of the two, Krupp and Whitworth 
adopted steel, Armstrong and Woolwich a combination 
of wrought-iron and steel, Palliser again, a combination 
of cast and wrought-iron. 
In making a vessel to resist great internal pressure, it 
was natural to conclude that by increasing the thickness 
of the vessel, its resisting strength could be proportionately 
increased, but as was first pointed out by the late Prof. 
Barlow, it was found that the limit in this direction was 
very soon reached, and that no vessel, whatever the 
thickness, could resist an internal pressure greater than 
the tensile strength of the material of which it was made. 
If the cylinder be composed of a material whose tensile 
strength is 10 tons per square inch, and if the internal 
pressure be Io tons per square inch, and if the cylinder 
be conceived as to be divided into a great number of 
30 TONS PER SQ.INCH. 
és 
20 
<a Ss 67 SB we Tie Td narimenEs 
successive indefinitely thin layers, then, whatever be its 
thickness, the first of these layers will be strained to 10 
tons, its maximum strength, the next Jayer will be strained 
less, and the strains will go on decreasing according to a 
fixed law as we proceed outwards. Now these outer 
layers cannot exert any more force, except it be trans- 
mitted from the innermost one, and consequently any 
further assistance can only be got from them by increasing 
the strain of the innermost layer, which, being already 
strained to its maximum strength, must necessarily give 
way. 
In order to meet this radical defect in all homogeneous 
cylinders the principle of initial tension was adopted. 
This was done by building up the cylinder of several 
concentric rings, or hoops, each of which was put on the 
one below it with an initial strain, thus compressing all 
NATURE 
those below. If now, by this method, the innermost hoop 
or tube be put into a state of compression of, say, 5 tons | 
per square inch, it is evident that the first thing the 
internal pressure has to do, is to remove this compression | 
to zero. 
sure. It has then to overcome the tensile strength of the 
material, or 10 tons per square inch, which requires an 
additional pressure of 10 tons per square inch. Thus the 
This will absorb 5 tons per square inch of pres- | 
resisting force of the cylinder has been increased from 10 | 
| use of wire. 
to 15 tons per square inch. 
Now the greater the number of the hoops in a given 
thickness of cylinder, the greater is the additional strength 
[Wov. 2, 1882 
proper initial strain, and if the hoops were infinite in 
number and therefore infinitely small in thickness, we © 
could obtain the maximum strength for the thickness of . 
cylinder, and each ring would, at the moment of rupture, 
be strained to its maximum tensile force. In such a 
cylinder the strength would increase in the exact ratio of 
the increase of thickness, and when it burst every layer 
would give way at the same time, but as there is no limit 
to the possible increase of thickness, there is also no limit 
to the possible increase of the internal pressure. Of 
course this theoretical construction is practically impos- 
sible, but we can approach to it very closely by making 
the hoops very numerous and very thin. The limit of the 
number of hoops is however very soon reached in the 
system of hoop construction. 
Sir Wm. Armstrong’s roo-ton gun is built up of a steel 
tube and three wrought-iron hoops on it. The Woolwich 
81-ton gun has a steel tube and two wrought-iron hoops. 
Sir Wm. Armstrong’s gun is therefore a better gun than 
ISp TONS PER SQ. INCH. 
By 
str TONS FER 5Q.INCH. 
20 rT TONS PER SQ.INCH. 
iby 
anal 
: i 
10, i i 
: H 
: 
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5 Xi 
is : 10 1S 120 INCIIES, Bo 
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fF UONT FER SQ.Inss. 
the Woolwich, assuming in both cases that the initia} 
tensions are correctly adjusted, but if in either case the 
number of hoops had been doubled, the total thickness 
remaining the same, both guns would have been greatly 
increased in strength. The practical difficulties of in- 
creasing the number of rings are, however, very great, 
and the expense would be enormous. The proper initial 
tension, or sirinkage as it is called, depending on ex- 
treme accuracy of workmanship, would be extremely 
difficult of attainment, and Sir Wm. Armstrong has 
probably gone nearly as far as is practically possible in 
this direction. 
“The regulation of the initial tension in guns of the 
hoop construction is so important that it is necessary to 
go somewhat more into detail, in order that our readers 
may thoroughly understand its importance, and be in a 
position to appreciate the advantages attendant on the 
We therefore introduce to their notice a series of 
t | diagrams showing the distribution of the strains through- 
imparted, provided that each hoop is put on with the | out the thickness of a gun. The first is the case of a - 
