378 PLI>'Y'S NATURAL HISTOHY. [Book XXXYL 



couple of fingers in thickness ; an inclination being carefully 

 observed, of an inch and a half to ever)* ten feet. This done, 

 the surface is well rubbed down with a polishing stone. The 

 general opinion is, that oak should never be used for tho 

 planking, it being so very liable to warp; and it is considered 

 a good plan to cover the boards \vith a layer of fern or chaff, 

 that they may be the better able to resist the action of tho 

 lime. It is necessary, too, before putting down the planking, 

 to underset it with a bod of round pebbles. "Wheat-ear 54 tes- 

 selated pavements are laid down in a similar manner. 



CITAP. 63.- GUJIICANIC PAVEMENTS. 



We must not omit here one other kind of pavement, that 

 known as the "Gnecanic." The ground is well rammed down, 

 and a bed of rough work, or else broken pottery, is then laid 

 upon it. Upon the top of this, 'a layer of charcoal is placed, 

 well trodden down with a mixture of sand, lime, and ashes; care 

 being taken, by line and rule, to give it a uniform thickness 

 of half a foot. The surface then presents the ordinary ap- 

 pearance of the ground; but if it is well rubbed wiu* the 

 polishing-stone, it will have all the appearance of a black 

 pavement. 



CHAT. 64. AT WHAT PERIOD MOSAIC PAVEMENTS WERE FIRST 



INVENTED. AT WHAT PERIOD ARCI1KD HOOPS WERE FIRST 

 DLCORATKD WIT II GLASS. 



Mosaic 75 pavements were first introduced in the time of 

 Sylla ; at all events, there is still in existence a pavement, 

 formed of small segments, which he ordered to be laid down 

 in the Temple of Fortune, at Pncneste. Since his time, these 

 mosaics have left the ground for the arched roofs of houses, 

 and they are now made of glass. This, however, is but a 

 recent invention ; for there can be no doubt that, whenAgrippa 

 ordered the earthenware walls of the hot baths, in the 

 Therm which he was building at Home, to be painted in 

 encaustic, and had the other parts coated with pargetting, he 



3 " Qucrcus." 



74 "Spicata U-stacea." These pavements were probably so called be- 

 cause the bricks were laid at angles to each other (of about forty-five 

 degree*), like the grains in an ear of wheat; or like the spines projecting 

 from cither bide of the back-bone of a fish. 5 Lithostrota." 



