A brief description of this useful engine may not be out of place here. It consists of an upright mast (E), steadied when the crane is in use by two sloping stays (F F). These stays are fixed into horizontal timbers (G) on the ground, connected with the foundation-plate (H) on which the mast turns. At the foot of the mast is fixed a combination of wheels and working handles for raising the weight, technically called a crab. A beam (A) working at the bottom in a socket (B, Fig. 3) fixed to the foot of the mast, but hanging out from it in a sloping direction, is called the DERRICK, and forms the principal peculiarity of the crane, as it can be raised more to the upright line, or lowered to slope more outwards, as may be desired, by means of the chain (C). The advantage of this is obvious; for a weight may thus be raised from or deposited at any point within a circle of a certain radius, depending on the length of the derrick; whereas, in an ordinary crane, the weight can only be placed at points upon the circumference of that circle. The whole engine revolves on a pivot (H, Fig. 2) at the foot of the mast. Cranes of this description are made varying in power from one to forty tons, and with derricks ranging from twenty to sixty feet radius. Raising and Fixing the Columns and Girders. Many of the persons who visited the building during the progress of its erection were heard to inquire "where was the scaffolding;" and others even imagined that the skeleton framework they saw was, in fact, only the scaffolding for the building, and not parts of its actual construction. This leads us to point out one of the most interesting peculiarities of the structure; namely, that it formed, as it were, the scaffolding for its own erection. In order to raise the columns upon the base-pieces, two poles were placed upright, connected by a horizontal piece, forming what is called shear-legs; the whole being steadied in its position by ropes from the summit fixed to the ground in various directions. A rope with pulleys fixed to the horizontal piece served to hoist the column, and sustain it in a vertical position until the bolts were passed through the projecting rings at the bottom of the column and the corresponding ones at the top of the base-piece, and screwed up. When two columns had been thus fixed, a connecting-piece was attached to each end of a girder, and the whole raised by the same apparatus, and fixed on the top of the columns; bolts being passed through the holes in the projections of the connecting-pieces, corresponding with those on the top of the columns. The shear-legs were then moved on twenty-four feet to perform the same duties to another pair of columns; and two sides of a 24-feet bay were thus formed. To complete the square, two more girders were raised in a similar manner, and fixed between the connecting-pieces over the columns. The square bay then became a firm structure, requiring no further support; and by repeating these