168
HISTORY OF ENGINEERING.
Book I.
was not needed for the baths, but was intended to supply the arena of the Coliseum , whenconverted into a naumachia. The walls are solidly built above and below, and all thearches and vaults well turned; the outer wall is buttressed up, and the spaces betweenformed into hemispherical recesses. The stucco which has lined the walls is encrustedwith a tartareous deposit, similar to what we find in the channels of the aqueducts, and thePiscina mirabilc at Baia?, and is so hard that it bears a polish equal to marble. The
Fig. 186.
various communications between these halls are set out with great regularity, and standingwithin either, looking diagonally, a fine effect is obtained.
Piscina were intended for the same purpose as our cisterns, and by Frontinus theyare designated Limaria when they were used for allowing water to deposit its impurities ;such a piscina remains on the Latian
Way, built in the manner already j ~ \
alluded to: the water flowing in and ' u i~i >
out of apertures made at right angleswith each other. When a reservoirwas covered by an arch, it was termedcontectis piscinis; such received thewaters of most of the aqueducts, andkept them from any influence that thesun’s power might produce ; when leftexposed, vegetable matter would formon the surface, and render the waterunwholesome. Frontinus describes avariety of arrangements to keep waterpure, and in a piscina near the Latian
Way arc three divisions, which formed Fig. 187. limaria.
a perfect system of filtration ; from the
galleries above it could be drawn out in any state of purity, and every precaution was
'•my.ni
CONTECTIS riSCINIi
ruuui.
I
innnr