Having thus described in detail all the different portions of the construction of the building, we must proceed to give some account of its actual erection, which will enable us to mention many very ingenious mechanical contrivances which were employed in the course of its progress. The First Operations on the Ground. From the great extent of the area required for the building, it was not to be expected that any site would be found of the necessary size, perfectly level. On the ground occupied by the building there is a difference of level between the two extreme ends of about eight feet. In consequence of this fall of the natural surface from west to east, and in order to avoid having a considerable flight of steps at one end of the building to compensate for it, it was determined to arrange the floor with an inclination following nearly that of the ground, such fall being at the rate of one inch in twenty-four feet. All the lines of the building which would be called horizontal in fact follow this line of the floor, and those which are supposed to be upright are placed at right angles to the floor, and therefore slightly inclined from the perpendicular towards the east. The deviation, however, is so exceedingly small as to be perfectly imperceptible even to those who are aware of the fact; and no one who was not previously informed of it would be able to detect it. It has been mentioned that Messrs. Fox and Henderson's tender for the building was verbally accepted on the 16th of July, 1850, and on the 30th of that month they obtained possession of the site from the Commissioners of Woods and Forests. The first proceeding was to inclose the whole area (including a considerable space at each end more than would be covered by the building) with a hoarding about eight feet high, put together in a very simple manner, so that the boards were afterwards available for the flooring. The supports for the hoarding consisted of pieces of timber fixed in the ground in pairs, at intervals of the length of the boards, leaving a narrow space between them, into which the boards were dropped, and thus held in their place without any nails. Temporary offices were then erected in a convenient portion of the site, and were covered with a roofing which was a specimen of that to be used in the building itself. Considerable ranges of carpenters' sheds were also put up, and even stables for twenty or thirty horses, which were required in the progress of the works. Setting out the Ground. The first thing to be done towards the building itself was to set out accurately all the points where the columns would stand, as well as the general outline of the building. It will be readily understood that this was an exceedingly important part of the work, as upon its accuracy depended the fitting together of the various parts that had afterwards to be put in place.