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.
Moulds.
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.
These moulds being made with great precision, were carefully marked and numbered with oil paint, according to the positions which the respective stones were to occupy. They were made of well seasoned fir timber, and dressed clean in the form of open frame-work, measuring from three to four inches in breadth, and from one-half to three-fourths of an inch in thickness. At the angles and joints, thin plates of iron were screwed upon these frames, to strengthen and preserve them, while the workmen were making their draught-lines, and in their numerous applications of them in the process of hewing the stones. Each course of the solid part of the building required from three to five moulds, of the form delineated in [Plate X.], which were carefully laid aside in sets, till the particular course to which they belonged should be landed upon the rock, and secured in the building. In a work of this kind, such a precaution was indispensably necessary; for, in case of loss or accident to any of the stones, in landing them at the Rock, it would then only have been necessary to send to the work-yard, referring to the particular number of the mould, from which another stone could speedily have been prepared.
Cofferdam.
In the first designs for the Bell Rock Light-house, the writer had modelled a cofferdam, five feet in height, intended to have been erected of cast-iron, round the site of the building, that the work in its early stages might be continued for a longer period, both during the ebb and flood tides. The experience of last season’s work, however, shewed that the erection of the proposed cofferdam would have been attended with considerable difficulty; and, to have rendered such an apparatus equally useful during ebb-tide as flood-tide, would have required the pumping of water by machinery more complicated and powerful than the situation of the Bell Rock would have admitted.
Pumps.
This idea was therefore laid aside, and two Pumps, of a simple construction, were prepared, for clearing the foundation-pit of water. They measured about twelve feet in length, and were of a square form, both externally and internally, having each a void of ten inches. They were made of fir timber, three inches in thickness, strongly jointed, and put together with white-lead paint, having also a number of cross bars and bolts of iron, to strengthen them for withstanding the atmospheric pressure upon so considerable a surface. These pumps were furnished with a wooden spear or rod, having a cross head or handle at one end, and a leathern valve attached to the other. This valve was of a very simple construction; it collapsed when plunged into the water, and was inflated by the return draught delivering a quantity of water equal to the cubical contents of the void or chamber of the pump.
Winch-machines.
There were four Crabs or Winch-machines prepared for working the different purchases required in the various departments of the work, as, for example, in lifting the stones from the praam-boats, as represented in [Plate XI]. Another of these machines was fixed on the temporary wooden bridge of communication, erected between the Beacon-house and Light-house, as will be seen in [Plate IX.] Other two of these machines are likewise represented for raising the stones from stage to stage upon the building, as will be more particularly described in the letter-press description of this Plate. These machines were made wholly of iron, excepting the bushes for the gudgeons working in, which were of bell-metal. They were calculated to work with what is called double and single purchases, according to the weight of the stones to be lifted. They were very powerful in their operation: the winch or barrel being twelve inches in diameter, gave the single purchase a power of about fifteen to one, and the double purchase about sixty to one. These machines were calculated to work with five tons. The weight of the largest size was altogether about 10 cwt., so that they were not easily shifted by the impulse of the sea, when batted to the rock, as represented in [Plate XI.]