1126

THEORY AND PRACTICE OF ENGINEERING.

Book II

In ancient structures wrought stone was laid without mortar, or the small quantityused was so clear and fine that it only served to fill up the inequality of the bed, anddid not prevent the parts betweenthese inequalities from bearing oneach other. The Romans observedthat walls constructed of soft stone,or inferior brick, were subject to beaffected by the changes of the atmo-sphere and driving rains ; to obviatewhich Vitruvius advises that inapartments on the ground floor at aheight of 3 feet from the pavement,the first coat of stucco be of potsherdsinstead of sand, and that a thinwall be built on the inside, leavinga cavity between for the air tocirculate. Where there was notsufficient space for this second wall,tiles 2 feet square were placed, oneover the other, in front of a flueleft in the wall; G shows the positionof the tiles up the face of the wall,

D its effect when plastered over orstucooed: A is a channel or drain,contrived to receive any moisturethat may run down the flue, or hol-low part of the wall.

For cottages the same arrange-ment might be adopted with greateconomy, and in several parts ofEngland walls are constructed in asomewhat similar manner, by settingthe bricks on edge, both on the innerand outer face, so that in a 9-inchwall, there is a space of 4 inchesbetween them. To produce therequisite strength, each header passesthrough the entire wall, and by care-fully excluding the mortar or dustfrom occupying the vacuum betweenthe two faces of the brick, a dry wallis secured, and a much less quantityof material consumed than is re-quired for a solid 9-inch wall. Stonewalls might be so constructed, andit has been suggested to work with-in thin walls vertical plates of sheetiron, pitched previously to insertion,to keep out moisture.

The Romans formed the floors oftheir rooms very carefully; afterexcavating the earth to the depthof 2 feet, they well rammed the bot-tom, and laid over it a layer of brokenstone or potsherd prepared for thepurpose : on this was spread a com-position of pounded charcoal,lime, sand and ashes, 6 inchesin thickness, which was ren-dered perfectly smooth by firstrubbing it with stone and thenpolishing it. Rakes, rollers,scrapers, and trowels, were

Fig. 2400.

PLAN OF WALL .

Fig. 2410.

ELEVATION OF WALL .

made use of during the processes, and several of these floors remain at Pompeii , which attesttheir strength and durability.

It was the practice of the Romans to construct the upper portion of their walls with tes-taceous materials, forming a projecting cornice to carry off the water. When mouldings