possible to make them water-tight when supported by piers above the ground, as they are usually built. But however carefully constructed, these materials are so unyielding to the expansion and contraction they are subjected to, that it is nearly impossible to prevent leakage for any length of time. A large number of brick and cement tanks have come under our notice, and we cannot call to mind a single one of them all that has not been a continual source of vexation and expense to its owner, since its first construction.

The principle objections to tank heating, as usually employed, are an excess of bottom heat and a deficiency of atmospheric heat, with a superabundance of moisture when the vapor from the tank is not properly excluded from the house. Tanks should be covered with some good radiating material, as slate or metal. If slate is employed, the joints should be carefully and effectually cemented. Boards are sometimes used as a covering, but their radiating power is slight, and their decay rapid.

Soil or sand, to the depth of six to ten inches, is usually placed upon the tanks, and used as a plunging bed for pots containing cuttings; or the cuttings are sometimes inserted in the bed itself.

Any arrangement by which vapor from the tanks is admitted to the roots of plants is to be avoided, for however desirable a moist bottom heat may be, it is found from experience that the soil is frequently rendered a mass of puddle, in which no living roots can exist.

A portion of the covering of the tank may be made moveable to allow moisture to escape into the house when required.

By means of the tank, bottom heat for propagating or other purposes, can be very steadily and uniformly maintained, more so than by other modes, and the changes of temperature of the outer air do not materially affect it. But the case is different with regard to the air of the house, which is frequently reduced below the freezing point, in severe weather. If the bottom heat is of the required temperature, any attempt to counteract the coldness of the air of the house by increasing the fire, would produce an injurious excess of bottom heat. It is evident that while the required supply of heat for the bottom is uniform, and that for the top exceedingly irregular, both objects cannot be properly secured except by a separate supply of heat for each. For these reasons we would employ a hot water pipe or pipes, passing around the house, on the same level with the tanks, supplied with a valve to regulate the heat at pleasure, or a brick smoke flue constructed in the usual manner.

Tanks are usually divided in the centre, thus forming channels for the flow and return circulation side by side, equalizing the temperature throughout their whole length. This form is sometimes departed from by carrying the tank around the house, and connecting each end with the boiler, but in this case, except in small houses, a uniform temperature cannot be maintained, as the water will have lost several degrees of heat before it has accomplished its circuit. Another arrangement is to connect the remote end of the tank by an iron pipe for the return circulation, passing under the tank the whole distance to the boiler. This is not as perfect and effective an arrangement of pipes and tanks as that before referred to, as in this case we do not have the heat from the pipe under control.

A writer in a late number of the "Gardeners' Monthly," gives the following description of tanks erected by him to obviate excessive moisture and radiate a portion of their heat into the atmosphere of the house.

"In the winter of 1863-4, I finished two span-roof houses, each 60 feet in length, with water tanks three feet in width, running entirely around on both sides of each house, and heated by a single furnace. The tanks were made with wooden bottoms and sides, and covered with slate carefully cemented. My design was to heat the houses entirely by the tanks, by far the larger portion of the heat being given off from the slate covering, and as a bottom heat for plants. As I understand the various writers upon this subject, this is the approved plan. But I have found considerable difficulty, and have been obliged to modify my plan in various respects:

In the first place, wooden tanks, with the top covered with sand, will not give off heat sufficiently to keep up growth in houses of this size during extremely cold weather. By protecting the houses with shutters, this difficulty may be obviated. Crowding the fire, and raising the water in the tanks to a high temperature, is a more objectionable remedy. In this way the bottom heat is too strong. But my most serious difficulty has arisen from excessive humidity. I put three inches of sand over the whole slate surface of the tanks, using a part for cuttings, and the rest, (say 100 running feet of the three feet wide table), for standing pot plants upon the surface of the sand. The plants dried rapidly, and required watering every morning. The result was, that in watering the plants, and of course the sand on which they stood, to some extent, it was like pouring water upon a flue, or upon hot pipes: a constant steam was given off; all the moisture in the sand was rapidly converted into steam; so, also the water in the pots was quickly expelled. In order to heat the house sufficiently, the bottom heat became too strong, and the plants were in too direct contact with it. In cold days the house was in a perfect fog. It was ruinous to the plants. The remedy was simple: more heat must be allowed to escape from the tank into the house, without coming in contact with the sand-bed, and the moist earth of the plants. Another slate floor was laid, an inch above the tank slate, on which to put the sand and stand the plants. This hot air chamber opens into the house on the back and front side of the tank. Thus the whole radiating surface of the top of the tank may be directed into the house, or may be confined as bottom heat, as may be found necessary. By this plan, excessive humidity may be entirely obviated, and the heat completely controlled, as wanted."

Hot Water Pipes.—It is generally conceded, among practical men, that the circulation of hot water in iron pipes is the best known method of heating plant houses. The property which heated water possesses of retaining for a considerable length of time its heat and transmitting it to the pipes at long distances from the boiler, renders it a most effective agency for such purposes: A perfect control of the moisture of the atmosphere, by means of evaporating pans attached to the pipes; entire freedom from deleterious gases, sometimes escaping from flues, and the substantial character and enduring qualities of the apparatus, are important considerations in favor of this method of heating which are not to be overlooked or underrated.

It is true that a house of a given size cannot as soon be brought to the required temperature after the fire is first lighted, as by other modes of heating, but when once in full operation greater regularity is maintained, and if the fire should by any neglect go out, heat is still radiated, often for several hours, before the pipes become entirely cold.

For heating ornamental houses of glass, pipes are also to be recommended on account of the little room they occupy and the neatness of their appearance compared with the unsightly flues or tank. If properly put up, the pipes never leak at the joints, as is the case frequently with tanks, and scarcely need any repairs for years. The first cost of apparatus for heating by hot water pipes exceeds that of the other methods which we have named, but when we take into account its great