Barnard's propagating-tank, Fig. 39, is a practicable device for attachment to a common stove. A similar apparatus may be attached to the pipes of a greenhouse. The tank consists of a long wooden box made of matched boards, and put together with paint between the joints to make the box water-tight. The box should be about three feet wide and ten inches deep, and may be from ten to thirty feet long, according to the space required. In the middle of the box is a partition, extending nearly the whole length of the box, and on the inside, on each side, is a ledge or piece of moulding to support slates to be laid over the entire surface of the box. The slates are supported by the ledges and by the central partition, and should be fastened down with cement to prevent the propagating sand from falling into the tank. One slate is left out near the end, next the fire, to enable the operator to see the water and to keep it at the right level. On the slates sand is spread, in which the cuttings may be struck, the sand nearly filling the box. At one end of the box is placed a common cylinder stove, with pipe to the chimney. Inside the stove is a lead or iron pipe (iron is the best) bent in a spiral. This coil, which is directly in the fire, is connected by iron pipes with the tank, one pipe leading to one side of the partition and the other to the opposite side, as shown in the drawing. If water is placed in the tank it will fill the pipes and form a continuous circulating system through the pipes and up one side of the box past the end of the partition, and down the other side. A fire in the stove causes the water to circulate through the tank and impart to the bed a genial warmth. [Illustration: Fig. 40. Zinc Propagating-tank.] There are various tanks designed to rest upon the pipes in a greenhouse. The principle of their construction is essentially the same as of those described in previous pages--bottom heat, a tray of water, and a bed of soil. Earthenware tanks are commonly employed, but a recent English device, Fig. 40, is made of zinc. It is about seven inches deep, and holds an inch or two of water in the bottom. A tray five inches deep sets into the tank. The water is supplied through a funnel at the base.