180 



FIXED STRUCTURES FOP GROWING 



it entirely with water, as in fig. 149. In some cases shallow cisterns 



are cast on the pipes, but their 



Fig. 148. 



power is insufficient, and in general 

 zinc cisterns may be considered the 

 best. Cisterns so placed on pipes 

 heated to 200 will contain water 

 at 145 to 150; but this will not be 



Fig. 149 



Zinc cistern for double pipes. 



Zinc cistern for a single pipe. 



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 145, evaporate 

 about three-quarters of a gallon per square foot of 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 the flues, we 

 should no longer hear so much of the dry disagreeable atmosphere 

 produced by this, mode of heating. It is almost unnecessary to observe 

 that the cisterns will be most effective where the flues are most effec- 

 tive ; or that, as the covers of flues have not interruptions like the 

 joints of pipes, the cisterns may be made of any length. Slate cisterns 

 placed above the pipes may be advantageously used for increasing the 

 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. Much may be done towards 

 keeping a wholesome degree of moisture in houses by covering the 

 shelves, beds, and in fact every surface, with an inch of gravel or 

 broken spar, which should be kept well moistened. 



Ventilation and Aeration. Till lately the subject of giving air to 

 plant-houses has been very imperfectly understood ; and, indeed, as it 

 was generally supposed that a very small supply of air was sufficient 

 for the growth of plants, ventilation was principally employed to lower 

 the temperature of a hothouse when the heat was too great, or to let 

 off sulphurous or other noxious gases which might be generated by 



