1422

THEORY AND PRACTICE OF ENGINEERING.

Book [I

perfectly smooth bymeans of a sand or grit-stone ; at others it iseither drove, broached, orstriped; the former me-thod is the least costly, isusually adopted, and ge-nerally known in London as random-tooling or boast-ing.

The tomb of CeciliaMetella at Rome iscased on the outside withstone in courses, laid veryregular, filled in behindwith rubble work, whichseems to have been doneat the same time with thecourses, as there are indi-

'■mmm

Fig. 2399.

cations where the levels have been left and again commenced.

Wherever ashlar facing is adopted, the stones of each course should be placed on theirnatural bed, otherwise they will be liable to flush off at their joints; and where piers arerequired, unless they are of large dimensions, this kind of work must be abandoned; forthere cannot be the requisite strength without the courses passing entirely through: inseveral churches constructed iu the tenth and following centuries, we find the polygonaland cylindrical shafts of columnsfaced with ashlar, and hearted orfilled in with rubble or chalk, runtogether with a durable mortar;where the latter has lost its te-nacity, the columns have in severalinstances split asunder.

Pseudisodomum had the coursesunequal in height: in other respectsit resembled the isodomum. Thereare many remains of Roman wallswhere the stones of each course areof the same form as well as dimen-sion throughout, ordinarily 3 feetlong, 18 inches wide, and 9 inchesin thickness; in some instances onecourse is laid flat, and the othersare placed on their edges, so thatthe first reaches to the middle ofthe wall, which is filled in withrubble.

Diatonous was a term given by

Fig 2400

PSEUDISODOMON.