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HISTORY OF ENGINEERING.

Book I.

air is admitted, and, after warming, is suffered to pass through apertures in the pavementprepared for the purpose. In the Roman hypocaustum the fire was laid at the mouth orat the foot of the prefurnium, and the air passed over it, in the same way that it does in amalt kiln, into a chamber under the floor of the apartment to be warmed. The floor, beinglarger than the top of the kiln, was supported by small pillars, and the heat, insteadof passing through it, was made to circulate around the walls and lose itself in its cir-cumambulation. These species of hypocausta were common in the country houses of theRomans. Under the rooms of many of their villas are found a hollow space, from 2to 3 feet high ; the floor being supported upon flat tiles which rest upon a series of smallbrick pillars built with fire-clay, or with a cement not containing any calcareous matterlikely to be acted upon by a strong heat. Along the sides are still found the square pipesof burnt clay which carried off the rarified air from this subterranean chamber into theapartments around or above the triclinium or room immediately over the space first warmed.

A narrow passage usually led to the mouth of the furnace; this being vaulted, andhaving an aperture at the end to admit air, acted as a blower to the furnace or blazingwood placed at the mouth of the hypocaustum.

De la Valle observes that the Persians warm their apartments by stoves made under ground ;these are a species of cauldron, in size and : shape like a small barrel, which are placed inan iron vessel filled with burning coals. The barrel or stove is covered with a wooden topover which is laid a carpet; the embers are excited to burn more strongly by means of asmall tube, which acts as a blowpipe: here the air admitted into the barrel becomeswarmed, as it docs in the oven of a French poele, and may be diffused throughout theapartment.

By the introduction of warm air, thus admirably circulating throughout a Roman house,there was no need of a chimney shaft like those we require to carry off the smoke from acoal fire; and Vitruvius makes no mention of such contrivance. He says that cornices and mcarved mouldings should be omitted in rooms where fires are lighted, because they aresubject to be injured by the smoke. It is true they had open fires, as they have in Italy atpresent, made in chafing-dishes or brasiers, which were placed in the middle of the apart-ment. These are very handsome, and are supported upon a tripod, or made to resemble awalled town with towers in miniature.

The wood used was the white and common willow, which produced little smoke andfew sparks. They first peeled off the rind, and then laid it some time in water, afterwhich they exposed it to the sun and air to dry. Sometimes this wood was soaked in oillees, or had oil poured upon it, or was partly burnt in the open air, before it was introducedinto an apartment, when it was called ligna cocta, and under such a term it was sold atRome . The warehouses called tabernse coctiliari® were very numerous, and many personswere employed to prepare wood for fuel, or to rid it of its smoky qualities.

Harbours and Buildings in Water .—When a harbour was to be formed, the Romans madechoice of such situations as would ensure safety to their vessels in stormy weather, and preferredthose bays which were protected by long promontories jutting considerably into the sea, andforming curves and angles. Around these, porticoes, arsenals, and market-places were esta-blished, and the mouth of the harbour was secured by iron chains, suspended by means ofmachinery contained in two towers at the entrance. When nature had not so provided, andthe shore was flat, they constructed piers by throwing into the sea heaps of stones, whichthey projected to a vast distance in the following manner. An earth was procured betweenCum® and the promontory of Minerva, which, when mixed in one proportion of lime to twoof the earth, had the property of hardening under water. When the situation of the pierwas determined upon, dams were formed in the sea by driving oak piles, secured firmlytogether with chain pieces. Between two ranges of piles thus driven, the earth was takenout, and the bed properly levelled; mortar, composed as above, was then thrown into thespace between the piles, together with a proportional quantity of stones, until it was entirelyfilled. If the sea was too deep or violent for this method to be pursued, they constructedon the margin of the sea a foundation of the greatest possible strength, with courses ofstone laid horizontally throughout rather less than half its length; the remainder, whichwas towards the sea, was made to incline. On the side towards the water, and on the flanksof the inclined plane, walls were built, projecting over the face eighteen inches, afterwhich the whole was filled with sand to the level of the horizontal part. On this sandthey commenced building a wall, which was raised as high as possible without incurringdanger ; this was left for a couple of months exposed to the action of the air, that the wholemight harden. The walls inclosing the sand were then removed, and the sea, washingagainst the sand, speedily carried it away ; the whole mass then sliding on the inclined plane,was shot into the water, and this sort of operation was continued until a sufficient numberof artificial blocks were constructed to form the mole or pier required. Sidonius Apolli-nans, in his panegyric of Anthemius the consul, says : “ The new land advances into thesea. and its mass contracts the old ocean; Dicarhean dust when thrown into the waterbecomes solid, and the hard mass sustains fields, thrown into the foreign waves.” This kind