Iron, 
135 
gazine of compressed air generates a considerable portion 
of heat, which greedily seizes the damps, which are una^ 
voidable in underground excavations, and conveys them 
to the furnace. The blast is, however, steady and uni- 
form ; and when the inside of the building is completely 
j secured against the passage of air, it is productive of con- 
siderable effects in the furnace. In the summer months, 
however, the air becomes so far debased as to affect the 
quality of the iron, and change it from grey to white. 
Every change in the temperature of the atmosphere during 
this period, is indicated by various changes in the furnace. 
The largest air- vault hitherto in use was excavated out: 
of solid rock at the Devon iron works : the fissures of the 
rock admitted considerable quantities of water ; and the 
same degree of damp would always prevent the possibility 
of making the side-walls and roof air-tight by means of 
pitch and paper, &c 0 
Besides the various natures of blast, as to the strength 
and equality of the current afforded by different modes of 
constructing the blowing- machines, a variety in the qua- 
lity of the air obtained is also an invariable consequence ° 
this is sufficiently knotvn by the effects which it produces 
| in the blast-furnace, and ought to be subject to scrupu- 
j ions examination. 
In this, as in other countries, larger products of cast 
: iron are obtained in the winter months than during the 
summer and autumn seasons : the quality of the metal is 
also much more carbonated, and with a less proportion of 
j fuel. In many parts of Sweden, where the summer heats 
are intense, the manufacturer is obliged to blow out or stop 
his furnace for two or three months : not onlv is he unable 
• / 
to make carbonated metal, but is frequently incapable of 
keeping the furnace in such trim as to make a produce of 
any quality whatever. In Britain, during the months of 
.Tune, I uly, and An gash more especially in dry seasons, 
