244 
Iron . 
and so much less will the diameter of the gun be found 
when cold, to what it would have measured at the climax 
of its expansion. 
3d, Contraction immediately takes place of the metal 
ceasing to expand : to its effects are chargeable the reduc- 
tion of the increased diameter of the gun, and which seems 
merely in consequence of the escape of the caloric. 
The action or effect of these separate laws will intimate- 
ly depend upon the quality and fluidity of the metal : 
with the same quality of iron different effects will be pro- 
duced according to the division of the fluid, and with the 
same degree of division in the fluid the extent of the ope- 
ration of these laws will be different with the different quali- 
ties of crude iron. Soft cast iron very hot will shrink 
and contract less than iron equally hot but of a harder 
quality, or, which is the same thing, than iron containing 
less carbon. 
In casting cylinders, pipes, and other hollow machinery, 
the effects of expansion and contraction are manifested 
without any great degree of shrinkage appearing. 
The diameter of the mould in all these castings is gene- 
rally made from l-8th to 3-16ths per foot in diameter lar- 
ger than the casting is wished to be ; while the space or 
vacuity left betwixt the exterior and interior of the mould, 
called the thickness, is made less than the strength in me. 
tal is wished to be. 
When the cylinder is cold, however, the diameter, if 
properly allowed, will be found correct by the operation 
of contraction in cooling • while the thickness in metal will 
be found increased, though still correct, by an expansion 
or separation of the exterior and interior parts of the mould* 
This last is by the moulder called straining ; and if great 
care is not taken to compress the sand firmly round about 
the mould, the thickness is sometimes increased so much 
as to render the manufacture unsaleable* Should this 
