436 ARCHITECHNICS. 



or sulphate of lime, may also be used for building ; but it is much 

 softer than marble, and found in less abundance. Of silicious stones, 

 granite is the most valuable for building; being sufficiently hard and 

 strong, and very durable. Basalt, and hornblende, are much harder, 

 and of rarer occurrence ; but sienitic granite, and sienite, containing 

 hornblende, are very useful and durable, though hard to cut. Gneiss, 

 and especially mica slate, are highly useful for flagstones ; on 

 account of their splitting in thin layers. Sandstone, also called 

 freestone, is more easily wrought, but less durable ; and argillite, or 

 clay slate, is chiefly used for covering roofs, and as a material for 

 writing. Steatite, or soapstone, is valuable for resisting the effects 

 of fire. To these and other similar materials, some reference has 

 already been made, under the branches of Mineralogy and Geology. 

 The quarrying of stones, is analogous to mining : but quarries are 

 generally open to the sky ; and the stones are loosened by drilling 

 holes, and either splitting the rock with wedges, or blasting with 

 gunpowder. 



Clay, a hydrous silicate of alumina, is extensively used in brick- 

 making, as well as in pottery. Bricks, are made of well kneaded 

 clay, struck in moulds of proper shape and size, then thoroughly 

 dried in the sun, and afterwards burnt, to give them the requisite 

 hardness. Lime, is obtained by calcining marble, limestone, chalk, 

 or shells, to drive off the carbonic acid. Pure lime slakes freely, 

 and swells greatly by the absorption of water ; hence it is also called 

 fat lime ; but mortar made of it does not harden under water. Hy- 

 draulic lime, contains alumina, silex, iron, or some other impurity ; 

 which makes it hard to slake, but causes it to harden under water, 

 and thus to form what is called hydraulic mortar. Common sand 

 may be used with hydraulic lime, to form this mortar ; or it may be 

 formed of common lime with burnt clay, or bricks finely pulverized, 

 which constitute what may be called a hydraulic base. Common 

 mortar, contains from two to four parts of sand, to one of lime. 



Masonry, is the art of building with stones or bricks, and mortar ; 

 or it is the structure itself thus built. When laid without mortar, it 

 is called dry, or open masonry ; to distinguish it from mortar 

 masonry. The stones, or bricks, should always be laid with their 

 upper and lower surfaces horizontal, except in arches. When the 

 stones are laid without any regular order, they form rubble masonry : 

 but when in horizontal courses, they form coursed masonry; and 

 when the vertical joints are also regular, they constitute ashlar 

 masonry. A header, is a stone laid crosswise in the wall ; and a 

 stretcher, is one laid lengthwise ; the bottom of a stone being called 

 the bed, and the top, the build. Masonry is strongest, when the stones 

 break joints: the vertical joint, in one course, coming over or under 

 the middle of a stone, above or below. The art of Stone cutting, 

 rests on principles of Geometry, which we have no room here to 

 apply. 



3. Of the Vegetable Materials, used in the arts, the most im- 

 portant are, the different kinds of wood, obtained from the trunks of 

 trees ; and vegetable fibres, used for making cloth and cordage. Of 

 animal materials, we shall have no room here to treat. The most 



