ROD 



ROL 



way, and usually two inches within 

 the ground, but thrive better if grown 

 on the surface. A very small bed is 

 sutFicient for the supply of the largest 

 family." 



ROCHELLE SALT. Tartrate of 

 soda and potassa ; used as a gentle 

 purgative. 



ROCK CRYSTAL. Transparent 

 quartz. 



ROCK SALT. The coarse native 

 salt, found in immense masses in 

 some parts of the globe. It consists 

 of salt chiefly, but adulterated with 

 chloride ofcalcium.gvpsum. and marl. 



ROD. The same as a pole, 16^ feet. 

 Four of these make Ounter's chain. 



RODENTS, RODENTL\. Gnaw- 

 ing quadrupeds, with two long chisel 



[ teeth in the front of either jaw. Rats, 

 rabbits, and squirrels are of this 

 class. 



ROE STONE. Oolite, rocks the 

 structure of whicb is made of small 

 rounded particles. 



ROIXERS. Rounded logs, or cyl- 

 inders of iron or stone turning on an 

 axis, and capable of being drawn by 

 hand or horses. They are of great 

 service in levelling the surface of 

 fields and breaking lumps. On loose 

 soils they compress the earth and 



I render it more fertile, and are much 

 used to imbed small seeds and run 

 over grass. The common roller is 

 no more than a heavy log, but nu- 

 merous varieties have been proposed. 

 The improved kinds (see Fig. 1) are 



constructed of iron, in two or more 

 pieces, so as to run over two lands, 

 the horse walking in the ditch, or 

 central furrow. They also carry a 



i6 



wooden frame for rocks, so as to be 



weighted according to circumstances. 



Instead of the frame for weights, 



or in addition to it, a box (a) is some- 



K kk2 



666 



