50 CULTURE OF THE GRAPE. 



surface. The walk in the centre of the house is then to 

 be cut down two feet nine inches below the surface, 

 and is to be two and a half feet wide when boarded 

 or cemented. The next work in the order of construc- 

 tion is the building of a water-tank for the purpose of 

 generating bottom-heat, and also heating the house. 

 Wooden tanks have been often recommended, and they 

 are now generally in use; but they have always been 

 found to be more or less troublesome, and by no means a 

 perfect success in any case : they are apt to swell and 

 shrink with the changes in temperature. Wood being 

 porous, they give off more or less moisture, if they do net 

 do worse, and come to a positive leak. If the top of the 

 tank is covered with wood, it is a poor conductor, and 

 does not sufficiently heat the propagating-bed, unless the 

 water is raised to a high temperature, when too much 

 steam is generated. If the wooden tank is covered with 

 slate, it is difficult to join the two materials with cement, 

 so as to endure the extremes of winter and summer, and 

 keep the work impervious to steam. It is not denied that 

 wooden tanks have done some good service, but they are 

 imperfect; and, as there is a better material and a cheaper, 

 they should never be used. Tanks have been made by a 

 thick coat of cement upon the level surface of the ground, 



