1420

THEORY AND PRACTICE OF ENGINEERING.

Book II.

considerable hardness, and if occasionally bonded through with flat tiles, as at IlichboroughCastle, Kent, are difficult of demolition : such walls consist of three thicknesses, two formingthe outer faces, and one the rubble core in the middle.

When the opus incertum is made with large polygonal facing stones, and piers of tile orbrick are occasionally introduced, the effect is greatly improved, and the strength of thewall considerably increased ; they are frequently met with in Italy , built with great care,and filled in with rubble composed of excellent mortar. This kind of masonry bears astrong resemblance to the cyclopean walls that are scattered over the country inhabited bythe early Greeks; but the latter construction was executed with large blocks, and generallywithout mortar, its strength depending upon the locking of the several stones together, andthe weight superimposed to keep them in their places.

Fig. 2393,

OPUS INCERTUM.

The Opus reticulatum is considered by Vitruvius the most beautiful species of construction ;the term applied to it originated in its net-like arrangement, and we have an examplein a part of the west front of Rochester Cathedral, supposed to have been built before

Fig. 2394.

OPUS RETICULATUM WITH STONE.

the Norman Conquest : it is, however, liable to split, from the bed of the stones beingunstable, and its deficiency in respect to bond; there are several varieties of this kind ofwork remaining in Italy, and some examples are attributed to the time of the Etruscans .