It may further be noticed, regarding this useful implement, as strongly marking the prejudices of habit, that Mr Mylne, the proprietor of Mylnefield Quarry, who, with enlightened views, furnished his works with machinery of the very best description, among other articles, provided a number of these jack-machines for his quarriers, but, for a long time, they could not be induced to make use of them. One of the men, however, happening, of his own accord, to apply the jack in turning a heavy block, its utility soon became apparent; and Mr Allan the manager, who had previously taken considerable pains to get the jack introduced, was at length not a little pleased to find it, after having been laid up in store as useless, in much request throughout that extensive quarry.
A Lewis Bat, of some form, for lifting large stones, is believed to have been known to the ancients. But that now in common use is generally understood to have been at least improved by the French engineers, who, in honour of their Sovereign, gave it the name of Lewis. This useful implement is so universally known in practice, for its great utility in building with heavy materials, that it is hardly necessary to do more than simply allude to it. It consists of five pieces of iron, three of which, forming a dovetail, like an inverted wedge or the keystone of an arch, are inserted into a corresponding hole cut in the stone. The fourth is the bolt connecting the shackle-piece, by which the weight is suspended, as will be better understood by referring to the sketch or diagram illustrative of it in Plate XI. Of this implement, it became necessary to furnish several dozens, as well from the variety in the weight, as from the figure of the stones, many of them requiring two Lewises to produce a proper balance. But the number was more particularly encreased, from the different sets required for the workyard, the stone-lighters, and for the Bell Rock, where it was necessary to provide against loss, to which this service was so peculiarly liable.
As the whole of the stones of each course or tier of this building were connected or let into one another, by a system of dovetails, diverging from the centre to the circumference, after the manner of the Edystone Light-house, as will be seen from Plate XIII., each particular stone required to be cut with accuracy, to fit its precise place in the building; and as even the form into which the blocks of granite were made, often depended upon the adventitious produce of the quarries, it became a very considerable operation to prepare the necessary moulds or patterns for the respective courses. When, therefore, the thickness was ascertained that a lot of these stones would admit being dressed, a plan of the particular course was first drawn upon paper by the Clerk of Works; a certain compartment of the course was then protracted of the full size, upon a platform of polished pavement, measuring 70 feet in length, and 25 feet in breadth, and occupying part of the ground floor of the workmen’s barrack. From this enlarged draught, Mr James Slight, the principal mould-maker, took his dimensions in making the moulds of the full size of the ground-plan of each stone, on which were marked the necessary directions for the stone-cutter, both as to the thickness of the course, and the position of the connecting joggle-holes, trenails and wedges.
