under the whole length of the Aqueduct like one continuous stone. It attains a degree of hardness which gives it the appearance of the conglome- rate bearing the name of Pudding-stone, and is an article of the greatest importance in forming foundations for walls of 21 82 great weight ; superseding in many instances, where the soil is soft, the use of piles or other timber foundation. Though we have evidence that concrete was used by the Ancient Romans in the foundations of some of their struc- tures and even in the formation of their roads — such as the Appian-Way, and though we find it used in the foundations of the feudal castles of the Norman Barons of England, still it has not been introduced into the general practice of archi- ture until quite a modern date, and even at the present time is not widely appreciated in this country as a material of so much importance in foundations. The side walls are laid up in a character of workmanship styled "rough-hammered work" the stone required to be of sound and durable quality and laid in a manner to render the work water-tight. Though attention is given in some degree to insure a proper bond to the wall, yet the point more particularly attended to, is to make it compact and impervious to water. The bonding of the wall is not by any means disregarded, in all situations where it is required, yet the position of the work generally, where it is in excava- tion below the natural surface of the ground, renders such precaution of less importance than that of making it com- pact. The mortar used in these side walls is formed by mixing clean sharp sand with hydraulic lime, using the pro- portions of three parts of the sand to one of the lime ; and these are thoroughly mixed and incorporated before they are wet ; when this mixture is wet and thoroughly worked, it is used immediately and always kept properly tempered so as to render it plastic, and to prevent any disposition to become hardened before it is in the wall. After the side 83 walls are finished and the concrete between them has re- ceived its proper form, a coating of plastering, about three eighths of an inch in thickness, is put on over the surface of the concrete and on the face of the walls before the interior facing of brick is commenced. The proportions of this plastering are two parts of sand to one of the hydraulic lime. The bricks used in this work are generally of quite a different character from those used in ordinary house-build- ing ; being harder burnt and of a superior quality of ma- terial. They are required to be burnt to such a degree of hardness that they present a cherry red, or brownish color, and give a clear ringing sound when struck ; and when broken, must present a compact and uniform texture. All bricks brought upon the work which are soft and of a pale color, such as are usually denominated salmon brick, are rejected. Those which are used, possess nearly the hard- ness and durability of ordinary building stone, and are calculated to resist the action of the water, to which they will be exposed. The advantage of using brick is, that a smooth channel offering little resistance to the flow of water can be formed with less expense than with stone, and greater security can be obtained against any leakage ; for besides the coat of plastering which covers the face of the walls and the top of the concrete, there is also a mortar joint between this plaster- ing and the brick work. The bricks being of good form and easily handled, can be more expeditiously and closely laid than the face of a wall of stone, and afford a smooth and uniform face to the wall with less expense. They are re- quired to be bedded full and flush with mortar, so that on 84 lifting one from its position in the work, no imperfections be discovered, but the impress of the brick be found distinct throughout. The proportions of the mortar for the brick work, are two parts of sand to one of hydraulic lime. The inverted arch of brick, as well as the brick facing on the sides, is four inches thick, and the roofing arch of brick is eight inches thick. After the masonry is finished the excavation which was done to receive it, is filled up around it, and over the top of the roofing arch generally to the height of 3 to 4 feet, and in some instances of deep excavation, up to the natural sur- face. If the natural surface be not