28 THE ART AND PRACTICE OF LANDSCAPE GARDENING. 



There are two most general forms of terrace walls. One is 

 parallel with the terrace walk and the line of the house, for which 

 it seems to constitute a base ; this may or may not be surmounted 

 by a balustrade. From this terrace there may be steps conducting 

 to a lower level, perhaps to the pleasure grounds, but possibly to 

 the second terrace with its formal gardens, whence flights of steps 

 give access to the naturally-treated landscape gardens. To this 

 second terrace there is generally a balustrade, which bears the 

 expression of a boundary. In either case the treatment is archi- 

 tectural. It is well to remember that an access other than by steps 

 from the terrace to the gardens should be devised for the possible 

 use of invalids. 



As to dimensions, the following may be generally stated : vertical 

 walls should have a thickness in cut stone of '35 of the height ; 

 in brick, -4 of the height ; in dry rubble, -5' of the height. A wall 

 with a " battered " or sloped front will need less material, and 

 have strength equal to a vertical wall, both having an equal thickness 

 of base. Care must be exercised, especially in falling ground, that 

 the foundations are carried deep enough to prevent sliding. There 

 must be drainage from the back of walls, either by pipes built 

 through the structure at proper intervals of distance, or by the 

 insertion of a porous backing from which pipes to carry off 

 water communicate. A backing of sods, laid by hand, will prevent 

 thrust, and admit of a lessened thickness in the wall. The back 

 of the wall should be left rough in construction. The average 

 slope of ground is i ^ ft. horizontal to i ft. vertical. The weight 

 of dressed granite is i56lbs. per cubic foot; sandstone i37lbs., 

 lime concrete 119 Ibs., cement concrete i37lbs., brickwork ii2lbs., 

 sand 100 Ibs., and of clay 119 Ibs. 



