306 Mr Robertson on the Mechanical Arts of Persia. 



ing a small proportion of lime. While building, the workmen do 

 not use a trowel, but lay the mortar with the hand. The bond is 

 simple, as the bricks are square, and donot admit of much variety 

 of arrangement. The mortar-joints are usually from one to two 

 inches thick and very irregular, unless in arches or door- ways, 

 when a good deal of neatness is often exhibited. As timber is 

 very scarce, brick-arches and domes are common. The semi- 

 cylindric arched roof is built in this way ; after the side walls 

 andgablesof the space intended to be coveredhavebeen erected, 

 the curve of the arch is marked out upon one of the gables, 

 and this is plastered over with the common clay-mortar ; a 

 layer of brick is then stuck upon the mortar'; and as the bricks 

 are thin and light, they remain firm till the ring is completed, 

 and then small chips are pinned into the joints, at the opening 

 ends. When one layer is finished, it is plastered over with 

 mortar, and a second layer is stuck upon it in the same man- 

 ner. In this way an arch of any required length, and of con- 

 siderable span, is quickly constructed, without centering. If 

 the bricks were made sufficiently thin and light, this mode of 

 building would answer well in this country for arching tunnels 

 and drains, and for mining purposes. 



Large spaces are often covered over by a brick-dome, or by 

 a series of domes supported on pillars. The pillars being built, 

 thin arches are thrown with the assistance of a slight cen- 

 tering from pillar to pillar, thus dividing the space to be 

 covered, into square compartments. The domes are then com- 

 pleted in the way described for the common arch, without 

 centering, the workmen placing layers of brick on the four 

 sides, alternately. These layers get shorter and shorter as the 

 work proceeds towards the centre, and the workmen, judging 

 by his eye alone, gives the whole in trades such a curve as to 

 form a neat dome when completed. When the domes are 

 very large, stucco is used as mortar, and the bricks, instead 

 of being placed on edge, have their faces downwards, and their 

 edges joined together by the cement. 



For light walls, hollow building is common. The first 

 course is of one brick on bed ; in the second, two rows of bricks 

 are placed on edge, forming the two faces of the wall, and 

 an upright brick is placed across at every joint ; the third 

 course is brick on bed again, and so on. This kind of building 



