338 
MINUTES OF PROCEEDINGS OF 
breech^piece was made from a solid forging with the fibre in the required 
direction. 
To explain the second principle it will be first necessary to define the 
terms “ shrinkage/' “ tension/' and “ compression/' as here employed. 
When two coils are prepared for shrinking, that is, when their surfaces 
which are to be in contact are brought to their proper dimensions and to 
the necessary degree of smoothness, the excess of the exterior diameter AB 
of the inner one above the interior diameter CD of the outer one> both coils 
being cold, is termed the “ shrinkage." 
In order to shrink the two together, the outer coil must be expanded by 
heat* until it is large enough to drop over the inner one where it is allowed 
to cool and contract. Now the power of heated metal to resist contraction 
is less than that of cold metal to resist compression to a certain point and 
to a certain point only, hence the inner coil becomes compressed in the 
process, and the amount its diameter AB is decreased thereby is called the 
“ compression," whilst the outer one remains partially extended, and the 
amount by which its diameter is thus increased beyond its original and 
natural length CD is its “ tension." 
The shrinkage is equal to compression plus the tension, and the amount 
must be regulated by the known tension and compression under certain 
strains and circumstances. 
The tension on a coil when shrunk on should in no case exceed that due 
to the elastic limit of the iron. The elastic limit of bar iron is about 
12 tons per square inch and causes an extension of about toVo^ 1 of its 
length,f any weight beyond this would stretch the iron permanently and 
* 500 F. is quite sufficient; this will allow a working margin of expansion beyond the amount 
calculated, so that the iron need never be raised to the high temperature at which scales form. 
f The amount of elongation varies not only on the quality of the metal, but with the size and 
shape of the specimen tested. The specimens tested in the Royal Gun Factories’ machine are 
two inches long and stretch from •003 // to •004" at the limit of elasticity, but from the experiments 
quoted by Mr Kirkaldy (Experiments on Wrought-iron and Steel, p. 66) made from a bar of iron 
