104

HISTORY OF ENGINEERING.

Book I.

grinding it with size; the common sorts, used by plasterers, was charcaol obtained fromburning pine branches, pounded in a mortar with size.

Blue was thus formed:—sand was ground with sublimed sulphur, until it acquired thefineness of flour ; to this coarse filings of Cyprian copper were added, and the whole, bythe addition of water, made into a paste, rolled into balls, and afterwards dried ; they werethen put into an earthen vessel, and placed in a furnace, when a blue colour appeared.

A purple was obtained by plunging a lump of yellow earth, heated red hot, into vinegar.

White lead, verdigris, and red lead were in common use : purple was obtained frommarine shells, which afforded the scarlet dye ; the shells were collected, and broken intosmall pieces with iron bars, •when the purple colour, which oozed out like tears, wascollected into mortars, and ground. Madder root was employed to tinge chalk, and greenwas formed by mixing blue with the herb weld.

The houses at Pompeii are usually constructed of a great variety of inferior material,and on the strength of the mortar depended their stability; the walls were coated withplaster, formed precisely after the method described by Vitruvius . After the rough coat, asecond, composed of sand and lime, called arenatum, and then the marmoratum, composedol sand and pounded marble, which was put on very thin, and rubbed and polisheduntil a surface was obtained equal to marble. Whilst this coat was in a humid state,the colours were laid on, which, according to our author, incorporated themselves with theincrustation, and were not liable to fade: three coats of arenatum, and as many of marmo-ratum, were used in the best works, which received a polish capable of reflecting objects.

Strength of building. When lintels or beams are loaded, they are apt to sag in themiddle, and cause fracture in the work above; but when posts are introduced, properlywedged up, this is prevented: by the insertion of two inclined pieces of timber, it may alsobe acomplished. (Vitruvius , lib. vi. cap. 11.)

The weight of the wall may be discharged by arches formed of wedges, concentricallyarranged ; these, turned over the beams or lintels, relieve the weight, and prevent themfrom sagging. In all buildings where piers and arches are used, the outer piers are to bemade wider than the others, that they may resist the thrust of the arches.

Where walls were constructed to resist the pressure of a bank of earth, their thicknesswas proportioned to the weight they had to resist, and buttresses were added, which wereplaced as far distant from each other as the height of the wall, and made of the samewidth ; they projected at bottom as much as the wall was thick, and gradually diminishedto the top. On the inside of the wall, towards the mass of earth, it was indented like theteeth of a saw, which teeth were made to project from the wall as much as its height; thepressure of the earth was thus distributed over a larger surface.

The Romans sometimes formed walls entirely of rubble, or blocage, as it is termed byFrench writers; depending entirely upon the goodness of the mortar for their strength,

Fig. 121

MINERVA MBDICA.