1869.] Slating and Slate. 453 We have mentioned above, that the felt should be laid on in two thicknesses. This should always be done, when the roof is of an ordinary pitch ; but when it is steep, such as, for instance, a French rocf, a single thickness will suffice, pro- vided a proper overlap is given In addition to the advantage gained by the use of felt, in the place of mortar, which we have already stated, viz.. that it precludes all possibility of any suc- cessful attempt at passing off a make- shift and bad work ; it also possesses another, which is equally important, viz.: that it can be used on roofs, that are laid quite late in the season, at a time Avhen, from the inclemency of the weather, frost, &c, it would be utterly impossible to employ mortar. The old method was, to lay slate in mortar, coating every course as it was laid. This was done with mortar com- posed of lime and sand, and spread very thin, so that it left but little substance This, however, has fallen entirely out of practice ; and has been abandoned, since the introduction of the two other modes, ■which we have described. The slate are usually secured to the laths, or boards, which should be of hem- lock, with iron nails, boiled in linseed oil, in order to protect them from rust. Sometimes copper nails, and, again, gal- vanized iron nails are used. In cases where the framework of the roof is constructed with iron, the slate are secured with copper wire to the iron lath. The use of slate for roofing purposes, is of very remote antiquity. Pliny men- tions, that the Celtae employed a white stone, that was divided by them more easily than wood, and sawn into thin plates for tiles. Some Roman build- ings, the remains of which have been discovered of late years in Great Britain, afford indisputable evidence, that they were actually roofed with slates, fas- tened on with nails of iron, hooked, long and large. Slate is of common occurrence in dis- tricts of the metamorphic and silurian formations; but, wherever found, it is rarely met with of a sufficiently good quality for Working, until it has been dug, down to a depth, beyond the reach of atmospheric influences. Quarries of great extent are worked in North Wales, and in parts of England, Scotland and Ireland ; but the first are those from which the market in America has been almost exclusively supplied, up to with- in a short time past ; and which are, therefore, perhaps, deserving a brief description, which we are enabled to give, from notes taken on a personal in- spection. The principal quarries are situated about five miles from the town of Bangor, in Caernarvonshire, in North Wales ; and are of immense extent ; the beds having been traced several miles in length; and the excavations being of a very imposing nature. They extend into the sides of the hills, in a succes- sion of terraces, or ledges, one over the other, sometimes to the number of twelve; each of them sixty feet high; and on these are tramways for wagons, drawn by horses, for carrying away the material excavated. Every improve- ment and appliance in mechanical skill, that is known in England, have been brought to bear upon the working of these quarries ; and some idea of the vastness of the operations here carried on may be formed, when we state, that there are close upon, if not quite, 3,000 men and boys constantly employed ; not including about 500 more, who are daily at work, unloading and loading the slates at Port Penrhyn, at the entrance of the Menai Straits, whence the product of the quarries is exported to all parts of the world. About 200 tons of slate are daily transported from the quarries to this port. The scene, to be here daily witnessed, is one full of interest, which seemed to us, one of the busiest we had ever had the good fortune to view. The succession of terraces, one over
