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THEORY AND PRACTICE OF ENGINEERING.

Book II.

effect; they are more in accordance with the ancients; by forming them in such a manner thatthe ribs become portions of cones, they will fit each other and make a very strong covering.

Fig. 603. TILES IN IMITATION OF THE ANTIQUE.

Numerous other specimens might be given of the ornamental tile : among them the fol-lowing show the great variety of pattern that may be produced; and it is to be regretted

TILES ALTERNATELY CONVEX AND CONCAVE.

that the covering of our buildings should continue to exhibit the monotonous plain andpantile, when so many picturesque designs might be substituted for them.

To tiles as a covering for a roof there are some objections, and generally slate is now pre-ferred; but their manufacture is capable of great improvement, and when their surfacesare glazed, so as to render them impervious to wet, they are upon an equality with slate.Tiles, in the ordinary state, when exposed to moisture or to rain, will imbibe about aseventh of its weight of water, while slate, if of a good quality, will not take up more thana two-hundredth part: if tiles remain saturated with water for any length of time, theycertainly injure the timber on which they are laid, and will probabiy occasion the roomsconstructed in the roof to be affected by damp.

Pavements are frequently formed of glazed tiles ; those in imitation of the ancienttesserae have a good effect; in the middle ages our churches were paved with a squaretile, each of which had a different pattern, or formed figures and devices resembling thoseof a carpet.