All the materials employed in the Exhibition of 1851, with the exception of the glass on the whole roof, and the framing of the transept-roof, have been used in the construction of the Crystal Palace. The general principle of construction, therefore, is identical in the two buildings. The modifications that have taken place, and the reasons that have led to them, have already been stated. Two difficulties, however, which were unknown in Hyde Park, had to be provided against at Sydenham: viz., the loose nature of the soil, and the sloping character of the ground. Means were taken to overcome these difficulties at the very outset of the work. The disadvantage of soil was repaired by the introduction of masses of concrete and brickwork under each column, in order to secure breadth of base and stability of structure. The slanting ground was seized by Sir Joseph Paxton with his usual sagacity, in order to be converted from an obstacle into a positive advantage. The ground ran rapidly down towards the garden, and Sir Joseph accordingly constructed a lower or basement story towards the garden front, by means of which not only increased space was gained, but a higher elevation secured to the whole building, and the noblest possible view. The lower story is sufficiently large to serve as a department for the exhibition of Machinery in Motion, and a very interesting exhibition of Agricultural Implements, which important branches of science and human industry will thus be contemplated apart from other objects. Behind this space, towards the interior of the building, is a capacious horizontal brick shaft, twenty-four feet wide, extending the whole length of the building, and denominated “Sir Joseph Paxton’s Tunnel” (A). Leading out of this tunnel are the furnaces and boilers connected with the heating apparatus, together with brick recesses for the stowage of coke. The tunnel itself is connected with the railway, and is used as a roadway for bringing into, and taking from, the Palace all objects of art and of industry; an arrangement that leaves the main floor of the building independent of all such operations. Behind the tunnel, and towards the west, the declivity of the ground is met by means of brick piers of the heights necessary to raise the foundation pieces of the columns to the level at which they rest on the summit of the hill. [Illustration]