Arriving at the bridge of _Saint Erth_, the traveller will perceive that a considerable portion of the breadth of the peninsula is here penetrated by an arm of the sea, and that the land which succeeds it in a direction towards the south is so low, that a canal might easily be cut along the hills which terminate at Marazion, and a communication be thus opened between the English and Irish Channels; or that an iron rail-way for the conveyance of coals, sand, &c. might be constructed at a comparatively small expense. At _Saint Erth_, were formerly situated the "_Rolling Mills_" for reducing blocks, or bars of Copper, into flat sheets, as described in the first edition of this "Guide;" since, however, the Copper-works at Hayle have been abandoned, these mills have been used for rolling and hammering iron. In the neighbourhood of _Saint Erth_ is _Tredrea_, the Cornish residence of Davies Gilbert, Esq. M. P. On our return to Penzance an opportunity occurs of witnessing the operation of _smelting_ Tin ore.[81] It consists in first heating the ore, with about an eighth part of _Culm_,[82] in a reverbatory furnace for six hours, during which period the sulphur and arsenic are volatilized, and the ore is reduced to its metallic state; the furnace is then _tapped_, and the liquid metal _run out_; a second melting, however, is necessary before it is sufficiently pure to be cast into _blocks_,[83] and assayed at the Coinage. After this last melting, and before the Tin is poured into the moulds, a piece of green apple-tree wood is thrown into the liquid metal, and kept under its surface; the effect of which is to throw up the _scoria_ with rapidity; it would seem to act merely in producing a violent ebullition by the sudden disengagement of steam. One hundred parts of the oxide of Tin ("_Black Tin_") at an average will yield about 65 parts of metal, or _White Tin_, as it is technically termed.