38 
NATURE 
[Wov. 9, 1882 
tion. For instance, in a recent discussion at the Institu- 
tion of Civil Engineers, Dr. Siemens asserted that not a 
single unit of heat would be set up in the body of the gun 
by compressive action, and maintained that the whole 
heat produced was due to the heated products of com- 
bustion of the powder. But an experiment recorded by 
Hirn in his Treatise on Thermodynamics seems to sup- 
port the view we have above set forth. He found that if 
an elastic bar of india-rubber was extended by tension it 
grew sensibly warmer, if then it was allowed to contract 
by the gradual decrease of the extending force, it cooled 
again to its original temperature ; but if on the contrary 
it was let go suddenly, it did not cool, but remained at its 
higher temperature. In the one case the mechanical 
€nergy was given out in work done in the extending force, 
whilst in the other no external work was done. This is 
exactly what happens in the gun. 
There is moreover another cause which operates in 
heating the body of agun. The explosion of powder is 
an impact. Now in the impact of two elastic bodies one 
portion of the vs viva is expended in overcoming the 
elastic force of the material ; another portion is converted 
into heat, and this portion remains in the body after the 
elastic force has restored it to its original form, and can 
only be got rid of by convection. 
Thus there are two causes operating in heating a gun 
exclusive of the very small effect due to the heated pro- 
ducts of combustion. Let us now examine what would 
be the result of this heating upon the various constructions 
of guns. 
Take first the homogeneous gun, of which the state of 
strain is represented by diagram A, page 12. The strain 
at the inner surface of the gun during explosion is about 
27 tons, whilst at the outer circumference it is only 3 tons 
per square inch. Now when the internal pressure is re- 
moved, the energy stored up in this strained mass is con- 
verted into heat, and we may suppose the amount of heating 
to be directly as the amount of energy so converted and 
inversely as the quantity of material heated. This being 
so, it follows that the inner layer of the gun would be 
heated nine times as much as the extreme outer layer by 
reason of conversion of energy, but the mass heated in 
each layer being in proportion to its length, and the 
lengths being as 43 to 19, or as I to 4°3 nearly, the rise 
of temperature would be as 4°3 X 9 to 1, ze. thirty-nine 
times greater in the innermost than in the outermost 
layer, and it is easy to see how this inequality of tempera- 
ture must cause great internal strain by expansion, and 
thus weaken the gun. 
Let us now consider the case of the g-inch gun, the 
strains of which are shown by diagrams B, and By. As 
regards the steel tube, the result of the explosion is to 
change the inner surface from a state of compression of 
II tons to a state of tension of 12 tors per square inch, 
and the outer layer from about 7 tons compression to 
about 2} tons tension. Whilst this is going on the tube 
is giving out work in aid of the powder guns until it 
arrives at the neutral state, after which it is absorbing 
work ; the whole tube is therefore cooling. Now let us 
take the outer hoop. The effect of the explosion here is 
to increase the initial tension of 6 tons to 17 tons at the 
inner, and from 2 tons to 4} tons at the outer surface. 
Now when the internal pressure is removed the energy 
given out is expended, first in the compression of the 
tube, and this part of the energy gives rise to no change 
of temperature, but the whole of the rest of the energy 
represented by 11 tons at the inner and 2} tons at the 
outer surface is converted into heat, and taking into con- 
sideration the masses the relative rise of temperature will 
1 : 
beas 1! is to ai, or as 113 to 1 nearly. Thus it ap- 
7 193 
pears that whilst from this cause the tube is cooled, the 
hoop is heated and expanded, which is equivalent to 
reducing the initial shrinkage of the hoop. 
But we have still to deal with the heat set up by the 
percussive force of the explosion. This we may assume 
to be some direct function of the induced strain. It will 
therefore, as regards the tube, be a maximum at the 
inner and will be zero at the outer surface, whilst it will 
be greater at the inner surface of the hoop as compared 
with the outer in the proportion of 11 to 2} (assuming it 
to vary directly with strain). 
Lastly, as regards the heat imparted from the powder 
gases. It may be shown that in the very short time of 
the operation this is confined to a very thin layer of the 
inner surface of the tube. The final result then is, thar 
the inner surface of the tube is heated, whilst the outer 
surface is probably actually cooled, at the same time the 
inner surface of the hoop is considerably heated, and the 
outer surface also heated, though to a much less degree. 
The effect of the changes must therefore be to weaken 
the gun, though in a very different manner from the case 
of the homogeneous gun. 
We come now to the wire gun, diagram E. Here the 
work done by the powder gases is represented by the 
arm BHOMNB, less the area BCN, that is, by the area 
CHOMNC. When the internal pressure is removed, the 
whole of this is converted into heat, but a portion of this 
between C and N would be neutralised by the cooling 
effect of the wires whilst converted into mechanical energy 
in passing from the compressive to the neutral state, and 
consequertly the heating of the gun, though not abso- 
lutely uniform throughout, would be very nearly so. The 
heating from the percussive action would also be nearly 
uniform, being rather greater towards the inner surface. 
Now it can be shown that if a gun properly constructel 
either with hoops or wire be uniformly heated, the strains 
are not affected, and it therefore follows that in the wire 
gun the effect of heating is very slightly to alter the condi- 
tions and strength of the gun, and the wire gun, there- 
fore, is in this respect far superior to the hooped systems. 
We have now pointed out the difference in the mode of 
construction with hoop and with wire, we have compare | 
the two systems and shown that for strength, facility, 
and economy of construction, the wire system has greatly 
the advantage; we have refuted the objections which 
have been taken to it, and the task which we undertook 
is completed. Doubtless it will occur to our readers to 
ask how it is that a system which promises so fair, and 
which was brought prominently forward upwards of a 
quarter of a century ago, has never till quite recently 
been tried by the gun-makers. How is it that millions 
upon millions have been spent at Woolwich on hoop guns 
and that this system has been persistently neglected ? 
We know that not only was it brought before the 
Ordnance Select Committee, twenty-seven years ago, and 
that not as a mere idea, but accompanied with experi- 
mental facts, which, as the late Mr. Bidder, then (1860) 
President of the Institute of Civil Engineers, stated 
publicly, established such a Arima facie case as should 
have received the attention of Government, but we know 
further that at various times since it has been fruitlessly 
urged that trials of the system should be made. 
We presume that those who had the decision of such 
matters were so satisfied with what they were doing, and 
had so much confidence in their own system.that they 
never gave their serious attention to what they thought to 
be the dream of a theorist. The inexorable logic of facts 
seems, however, at last to have come into play, and we 
believe that the recently-constituted Crdnance Committee 
is at present seriously engaged in the reconsideration of 
the-whole subject of gun construction, and that wire guns 
will be admitted to be within the region of practical gun- 
making. 
We trust it may be so, and that the system may be 
fairly tried, but in order that the trial may be fair, it is 
essential that it be conducted with due regard to those 
principles which it has been our object to explain—that 
