162 CULTURE OF FARM CROPS. 



flat receptacles, the surface of which is provided with ribs 

 or flat projections, (see illustration, fig. 20). These "become 

 heated by contact, and the result is that each rib becomes 

 a heating surface. By far the most compact, as probably 

 it is the most efficient, heating surface is the " heat-box," 

 of Mr. Walker of Manchester (see fig. 21). This is made 



Fig. 21. 



up of a series of blocks of cast-iron. Each block contains a 

 number of square cells or tubes ; both sides of the divisions 

 between the cells forming effective heating surfaces. These 

 blocks, usually six in number, are placed within a hollow 

 case, of an ornamental form, standing within a few inches of 

 the ground. Steam or hot water is supplied to the space 

 surrounding the case, and the heat is communicated to the 

 blocks. The air passing up the interior of the cells be- 

 comes mildly heated, its place being supplied and a contin- 

 ous current maintained through any fresh air flue over the 

 mouth of which the set of blocks may be fixed. The blocks 

 and their case for the case itself forms a heating medium 

 which may be denominated the honeycomb or cellular ar- 

 rangement, will present more heating surface to the air than 

 can be obtained within a given space by any other form. 



