_Mortar and Concrete._—Roman builders early learnt how to make good mortar and concrete, being careful to use clean coarse gravel and finely divided lime. They also found that an addition of crushed tiles and pottery was an improvement, and for their good work used so much of this that the mortar became quite red. “Roman mortar was generally composed of lime, pounded tiles, sand and gravel, more or less coarse, and even small pebbles. At Richborough the mortar used in the interior of the walls is composed of lime and sand and pebbles or sea-beach, but the facing stones throughout are cemented with a much finer mortar in which powdered tile is introduced” (T. Wright). One of the advantages of coarsely-crushed tiles is that it absorbs and holds water so that the mortar made with it dries very slowly and thus hardens perfectly. In _Archæologia_ (lx.) an analysis is given of “mortar made with crushed tiles as grit in place of, or in conjunction with, sand.” In Rochester Museum a dishful of the crushed tile is shown which was taken from a heap found ready for use at the Roman villa at Darenth. I may say here that I have found mortar prepared in this way wonderfully tenacious, and suitable for special purposes like stopping holes in ancient walls. A strong cement made of finely powdered tiles, lime and oil was used by Byzantine and mediæval builders and probably by the Romans also. Villars de Honnecourt (thirteenth century) gives a recipe: “Take lime and pounded pagan tile in equal quantities until its colour predominates; moisten this with oil and with it you can make a tank hold water.” The use of crushed pottery in cement goes back to Minoan days in Crete. In London a long, thick wall of concrete formed between timbering was recently found between Knightrider and Friday Streets; it showed prints of half-round upright posts and horizontal planking; it bent in its course and may have been the boundary of a stream. On the site of the old Post Office a Roman rubbish pit was found, about 50 ft. by 35 ft. in size. “In late Roman times the whole pit had been covered with concrete about a foot thick and a building had been erected on the spot” (_Archæol._ lxvi.). At Newgate the Roman structure was erected on a “raft” of rubble in clay finished with a layer of concrete. Rubble in clay formed the foundation of the City Wall. [Illustration: FIG. 3. ]