142 
IRON. 
Iron is used for its strength, but how differently various 
qualities of iron perform this function, everyone is not aware ; 
thus some iron is of such a brittle nature that it will break 
with a slight shock, while other iron though no stronger 
under a pull or thrust applied gradually, will bear a heavy 
weight applied suddenly, and this principally constitutes the 
difference in the quality of iron. It is of the greatest 
importance, that our structures should not only be strong 
under regular weights or work, but that they should 
remain iirm if this weight or work be applied suddenly. 
The iron made from the ores found in the Leeds and 
Bradford districts, is the best in the world that can be pro- 
duced in large quantities. The pig-iron is converted into 
wrought-iron, and used for the manufacture of crank axles for 
locomotives, wheels, wheel t3^res, and boilers ; crank axles 
and screw shafts and boilers for steamers, and indeed for 
any purpose, where the cost is not so much an object as 
safety, and where the result of breakage would be 
disastrous. 
In some places, iron is made from magnetic ore directly 
into wrought-iron of very good quality, by melting it in 
small hearths with charcoal, but in this way it is only made 
in ver}^ small quantities, quite inadequate to supply the large 
demand for iron of the best quality. 
In the vicinity of Leeds and Bradford, however, it is 
made largely, but the supply of ore must in a short time 
become worked out, as, although iron nodules are diffused all 
through the coal measures of the district, it is in such small 
quantities as not to be worth working. 
The Ironstone worked is in three principal layers of 
nodules, which lie about 2 feet above the Black-bed coal, their 
aggregate thickness would only make a seam of about 4 or 6 
inches ; its value may thus be judged, for in the Cleveland 
