THE MODERN MODES OF HEATING PLANT HOUSES. 
67 
It is true, that very much has been said on the merits and de¬ 
merits of each, in the gardening periodicals ; but the public have 
often too much reason to believe that some of the writers are 
guided by interested motives ; others are, on the other hand, very 
much influenced by prejudice; while those who enjoy the reputa¬ 
tion of being capable of deducing an opinion from a knowledge of 
pneumatics, hydrodynamics, and physiological investigation, are 
those who have run into the greatest errors : one, by giving an 
unqualified approval of the working of a new invention, because 
it has some good parts to recommend it; another, as readily 
condemning the whole for one defect. 
I scarcely need to say, that to provide an artificial climate for 
plants—the admission of one natural agent (light) being con¬ 
sidered—a supply of heat, pure air, and atmospheric moisture is 
indispensable ; and an apparatus possessing the adaptation to supply 
these, through its medium, with a due regard to economy, dura¬ 
bility, and simplicity in its construction, rendering each element 
subservient to the purpose for which it is required, and placing 
the whole perfectly under the control of the cultivator, is the 
great desideratum in all plant houses. 
The first introduction of hot-water pipes, though an improve¬ 
ment on the old system of dues, as a means of heating the air in 
a house, made no provision for supplying that air with vapour, 
when required, with anything like precision. It is true that any 
quantity of moisture might be raised in the house , even to satu¬ 
ration, by throwing water on the pipes and the pavement : but» 
by the former expedient, steam is produced, which is found to be 
prejudicial rather than advantageous to most plants. It also 
causes a fluctuation of temperature, as a large quantity of heat is 
absorbed in the formation of it. And to expect a supply of vapour 
b} r keeping a cold pavement wet, is not much more reasonable 
than to regard the glass roof as an evaporating surface, because the 
rate of evaporation is always regulated by the degree of heat in 
connexion with the water ; therefore, it is the stratum of air im¬ 
mediately above the floor which furnishes the heat requisite for 
the transition from the liquid to the gaseous form. And as this 
stratum of air is, from physical causes, the coldest in the house, it 
follows, as a matter of course, that it is not the most favourable to 
facilitate evaporation. The fact, therefore, must be obvious, that 
it is utterly impossible to imitate nature in balancing the tem¬ 
perature and moisture of the air by either expedient above 
