This part of the work was executed with great precision by Mr. W. G. Brounger. He commenced by determining the four extreme angles of the building, and the centre lines of the main avenues. These formed fixed points from which were determined the whole of the centres for the columns. Our readers will recollect that the dimension of twenty-four feet occurs horizontally throughout the building, either in multiples or sub-multiples. In order to measure off the different distances, rods of American pine were made, into which, near the ends, pieces of metal were fixed, having corresponding notches at the exact distance of twenty-four feet apart. By these means the lengths were measured off with great accuracy, as the wood used is not liable to alteration in the length of its fibre; and by means of the metal notches the rods were sure to be placed correctly together. It was necessary to make these sockets or notches of metal, from the great amount of work the rods had to perform. In determining the length of the rods, the standard of the Astronomical Society was used; and this was referred to in all important measurements for the castings and other parts of the building, to insure their precise eventual agreement in length. This will hardly be considered to have been unnecessary when it is remembered that, from the great length of the building, a very minute error in any of the parts would have been so multiplied as sensibly to throw out the ends. To those who are unacquainted with the fact, it may be well to mention that the standard of length referred to is obtained from a pendulum, which oscillates seconds, in the latitude of London, in a vacuum, at the level of the sea, at a certain fixed temperature. The length of this pendulum is then divided into a certain registered number of feet and inches. The rods above described were carried along the centre lines of the columns, and the position of each column was marked by a small stake driven into the ground; and in order still more accurately to fix the centre, a long nail was driven into the head of the stake. In this manner the position of every column throughout the building was determined. The level at which the floor was to be fixed was the next point determined by the ordinary method of levelling, and stakes, with a [T section] piece at the top, called boning-sticks, were fixed in different parts of the building; by the aid of which the tops of the base-pieces for the columns were all afterwards fixed in one plane of the required slope. Fixing the Base Plates.