Iron . 
necessary precaution be slightly performed, and the thick- 
ness considerably increased, the usual expansion which 
takes place when the metal passes into the solid state be- 
comes so extensive as to effect a permanent increase of the 
diameter of the casting, and destroy its use*. The united 
effects of these two causes force the sand to assume an ele- 
vated posture all round the mould, and occasion violent 
rents and fissures, which become immediately filled with 
pale blue flame, accompanied by a crackling noise like 
the snapping of electric matter. 
Shrinkage in these castings, particularly if large, would 
affect the solidity of the vessel by taking place to a consi- 
derable extent upon the upper surface, immediately where 
the runner discharges the metal into the mould. This is 
in a great measure counteracted by feeding these gates or 
runners , after the mould is filled, with several ladles full of 
fluid iron, and keeping the communication open to the 
edge of the casting by moving t small iron rods up and 
down in the gate. The metal is thus allowed to percolate 
into the chasm, if any is formed, and prevents any bad 
consequences likely to ensue from the general shrinkage 
of the mass* 
It is impossible to convey an exact idea of the extent or 
quantity of shrinkage that takes place in castings, or pro- 
portion it to the height or dimensions of the original mass. 
The subject of contraction is more within the reach of 
measurement, and in many cases may be ascertained with 
great precision. 
* The additional thickness always takes place to the exterior of 
the mould. The pressure can more easily act with effect against the 
concave than the convex side of the mould. The moulder is 
fully aware of this in the act of cooling, particularly if the metal has 
been very hot and of a sharp quality. After he conceives the iron 
fairly consolidated throughout, he cuts two openings at least in the 
core or interior part of the mould, and penetrates to the red-hot 
surface. This gives scope to the contraction of the vessel* and 
preserves the casting frequently from being destroyed. 
