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. [Sidenote: Cofferdam.]