Two kinds of mineral are smelted for iron in Britain. The first is the hiematites of Ulverstone and the neighbourhood of Whitehaven, which, as it contains much iron (60 per cent.), is carried by sea to smelting furnaces in different parts of Britain. The second is the argillaceous ironstone, which constitutes some of the strata that accompany pit coal. This is more generally used than the hiematites. And it is in the vicinity of the masses of stratified minerals, which yield coal and ironstone, that the principal iron-works in Britain are set down. These strata are found in various parts of the island, and are portions of that class of depositions called by geo logists the coal-formation. A stratum of coal or of ironstone of considerable extent, is termed, by the coal and iron masters, a coal-field, or an ironstone field.
Pit-coal cannot be employed entire in the blast furnace; the bituminous part would be conglutinated by the heat, and the furnace would be choked, and the materials would no longer descend gradually as they ought to do. The coal is, therefore, burnt to drive off its bitumen, and it is then .in a state of cin der and called coak. It requires a larger mass of coak than of charcoal to smelt ironstone. Hence the coak blast-furnaces are large, and the machines employed to blow them are more powerful than the wooden spring bellows invented in Germany, in 1620, and which continue to be employed in the charcoal iron furnaces in Germany and France. Bel lows connected by leather, and worked by water, were used to blow the blast-furnaces at Carron, at the commencement of that establishment in 1760.
Sometime after, these bellows gave place to blowing machines, composed of pistons working in iron cy linders, constructed by the celebrated 'Smeaton, and described in his Reports. The blowing machines of the blast-furnaces in Britain are now always com posed of pistons moving in iron cylinders. The im provements in the steam-engine have rendered prac ticable the working of blast-furnaces in situations where there is no tall of water ; and, on the other hand, the manufacture of the various parts of nume rous steam-engines, has called forth the abilities and ingenuity of the iron founder.
In consequence of the advanced state of the Eng lish cast-iron manufacture,, several foreign nations have been desirous of introducing the English me thod, and have procured English workmen for that purpose. In this way, iron-works, on the English plan, were erected in Russia, about 1780, in Silesia m 1790, and in France, at Creusot, near Mont cenis, there were three English coak blast-furnaces, begun about 17.90.
The strength of guns depends on the strength of the metal of which the gun is composed, and on the quantity of metal and the manner in which it is dis posed. This subject is considered in the article', GUNNERY of the Encyclopedia. The nature of the subject does not admit of determining with precision the best weight and length that can be given to a gun fitted for exploding a ball of a given weight. Guns of the same calibre are conse quently made of different dimensions, even in the same country. The four tables that follow contain the weight, length, and other dimensions 9f British and of French guns :