The tenacity of the mortar which was used, forms an important element in the strength of the whole fabric. That which is in use now is generally spoiled, from a variety of circumstances. The prevailing practice is, first of all, to slack the lime by pouring a quantity of water upon it when lying in a heap; in most cases this does not sufficiently pulverize it: it is then mixed with any earth bearing the least resemblance to sand, and the two are worked together very imperfectly with a shovel. The mortar thus made often stands and hardens, so as to require to to be once and again mixed with water, and worked up before it is used. It thus becomes quite impoverished; and, after all, for the convenience of the mason, it is employed in so dry a state that the stone soon takes all the moisture from it, and it becomes little better than powder. The gigantic railway operations of recent times have driven men out of the beaten track, and compelled them afresh to discover the Roman method of preparing mortar. On the authority of engineers well acquainted with the Roman Wall, I am enabled to state, that the mortar of that structure is precisely similar to the grout and concrete[45] of the railway mason of the present day. Specimens of the ancient and modern grout are before me, and there cannot be a doubt as to the identity of their preparation. The following is the mode in which the railway engineer prepares his mortar. The lime, in the state in which it comes from the kiln, is first ground to powder, and is then mixed with sand and gravel, and chippings of stone. The purposes for which the mortar is required indicate the coarseness and quantity of the intermingling gravel. When wanted as concrete, to form, independently of other materials, the foundation of some heavy structure, stony fragments of larger size are mingled with the lime than when the mortar is to be used to cement chiselled stones, or even than when wanted to constitute with rubble the interior of a wall. The mixture of pounded lime and gravel, when made, is not mingled with water, until the moment of its application to the work for which it is required, but it is then intimately united with an abundant quantity of it. When used as concrete, the mass will, in three hours, have solidity sufficient to bear the weight of a man, and in about three days it will have acquired a rock-like firmness. Such, doubtless, is the way[46] in which the mortar of the Roman Wall was prepared, and it would have this very important advantage over that generally used at present, that, in a very short time, the work would acquire a massiveness and strength, sufficient to resist the attacks of an enemy. The mortar of the Saxon and Norman periods is of the same character.