216 FIXED STRUCTURES FOR GROWING 



will not be the case unless they are properly fitted, and luted on the pipes 

 with wet sand ; for the smallest interstice is found to make a great difference 



in the heat transmitted. Mr. Rogers 

 finds that cisterns fixed in this manner 

 with water, at a temperature of from 

 120 to 146, evaporate about three 

 quarters of a gallon per square foot of 



Fig. 153. Zinc cistern for double pipes. Fig. 154. Zinc cittern for a single pipe. 



surface in twenty-four hours. The proportion which he employs in an 

 orchidaceous stove is about one square foot of evaporating surface to ten 

 square feet of glass, and in stoves and forcing-houses, he is of opinion 

 (Proceedings of the Horticultural Society, 1840, p. 149) that there ought 

 to be one square foot of water for every fifteen square feet of glass. If 

 houses heated by flues had this proportion of cistern placed over them, 

 we should no longer hear so much of the dry disagreeable atmosphere pro- 

 duced by this mode of heating. It is almost unnecessary to observe that 

 the cisterns will be most effective where the flues are most effective ; or that, 

 as the covers of flues have not interruptions like the joints of pipes, the cis- 

 terns may be made of any length. Slate cisterns placed above the pipes 

 may be advantageously used for increasing l;he moisture, serving at the same 

 time as a reservoir of heat, and of water for watering the plants, and also 

 for growing aquatics ; but as the water in such cisterns will seldom exceed 

 the temperature of 80 to 85, a much larger surface is required than in the 

 case of zinc cisterns accurately fitted to the curvature of the pipes. On 

 smoke-flues the water in such cisterns will rise to a much higher temperature 

 than on pipes, because the slate bottoms will come in close contact with the 

 entire surface of the covers of the flue. 



510. Steaming, that is, the discharging into the atmosphere of a house, 

 in large quantities, the steam of water heated to the boiling-point, has been 

 adopted as a means of producing atmospheric moisture ; but it is objection- 

 able on account of the high temperature of the steam, excepting in large 

 houses where the volume of air affords room for the steam to part with heat, 

 so as to be converted into vapour before it reaches the plants. Steaming 

 may also be useful in combination with fumigation, or the diffusion in the 

 atmosphere of matters noxious to insects. Mr. Rogers proposes the fol- 

 lowing method of using steam in such a manner as not to prove injurious 

 to plants. " A shallow cistern, about six inches deep, and carrying at 

 least four square feet of area, with a false bottom of wire or pierced zinc 

 about one inch from the real bottom, being provided, the steam-pipe from, 

 the boiler should be introduced so as to discharge itself between the real and 

 false bottom ; the cistern should now be filled with water nearly to its 

 brim, and the steam laid on. The water will soon be raised to a pretty 

 considerable temperature, and yield an abundant supply of innocuous 

 vapour." The use of the false bottom is to prevent the water from boiling 

 up and flowing over before it is converted into steam. 



