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
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.
HE next proceeding was to excavate the holes for the concrete, on which the base-pieces were to stand. To do this, the stakes marking the centres of the columns had to be removed, and it was therefore necessary to adopt some method of finding those centres again with precision. For this purpose a large carpenter's square, as it is called, was made. This instrument forms a right-angled triangle, and in this instance was used in the following manner:—The centre of its longest side, or hypothenuse, was marked by a line, which, if continued, would pass through the right angle of the triangle, and at an equal distance along each of the other sides of the triangle from the right angle an upright saw-cut or notch was made. The square was then placed horizontally, so that the line marked on the hypothenuse coincided with that of the centres of a row of columns, and so that the right-angled corner of the square touched the nail marking the exact site of a column. Two small stakes were then driven under the notches in the short arms of the square, and nails were driven into them through the notches. It will be seen that by these means the site of the first stake could easily be again ascertained after its removal. The holes for the concrete were then dug of an oval form and of the various sizes and depths required, and the concrete filled in to the proper height. The gravel used for the concrete was raised in a pit at one end of the ground.
Next to the setting out of the positions of the columns, perhaps the operation of fixing the base-pieces was that in which the greatest accuracy was required; for as there were in some parts three storeys of columns to be fixed over them, any inaccuracy as to their level or position would be very much increased at the top of the building. To fix the base-pieces over the centres that had been determined for the columns, another carpenter's square was made use of, like that already described, but having the right-angled corner cut out to the form of the section of a column. This square being placed with the notches in its short sides over the two stakes already described, the upright portion of the base-piece was fitted into the notch at the angle; and as the reader will at once see, if he has followed us in the description of the various processes, its correct position was thus exactly found.