WALLS. 23 



must be inconvenient from their bulk, or the large space 

 which they occupy ; and hence they have never come 

 into general use. Where, however, the natural slope 

 of the ground is too great for carrying on the ordinary 

 operations of gardening, sloping terraces may advan- 

 tageously be converted into a kind of inclined wall, to 

 be faced with slate or some other material that does 

 not readily absorb moisture. 



Bricks afford the best and the most kindly material 

 for garden-walls. Being rough and porous, they absorb 

 radiant caloric, and, being bad conductors, they accu- 

 mulate heat ; when thus rendered warmer than the am- 

 bient air, they rapidly part with the extra heat, and 

 maintain the temperature amid the branches nailed to 

 the wall ; they do not retain moisture, and, by their 

 numerous interstices, they furnish every facility for nail- 

 ing in the twigs of the fruit-trees. Where freestone 

 (that is, sandstone capable of being easily dressed) is 

 abundant, the exterior wall is often formed of coursed 

 masonry, and the interior is faced with bricks. The 

 foundation should, if possible, be formed of stone. 

 Whimstone (that is, either the greenstone or the basalt 

 of mineralogists) forms an excellent material for fruit- 

 walls. It is susceptible of a neat hammer-dressing ; it 

 does not readily imbibe moisture, and therefore is not 

 much cooled by evaporation ; and being of a very dark 

 color, it absorbs more solar heat during sunshine than a 

 lighter surface, while at night the radiation from both is 

 nearly the same. Diiferent parts of the principal fruit- 

 wall of the Horticultural Society's Garden at Edinburgh 

 are built of brick, of freestone, and of greenstone ; and 

 the plants trained against the greenstone portion have 



