EEMARKS ON ARTIFICIAL CLIMATE. 89 



with sufficient accuracy the dew point according to Daniell. 

 The air at the limits of congelation holds about xlo^t'i P'''"* of i^^ 

 weight of aqueous vapour, and every increase of temperature 

 equal to 20^ of Fahrenheit will double its capacity for abstract- 

 ing moisture. As the difference between the night and day 

 temperatures will frequently exceed this, the necessity of provid- 

 ing against an injurious degree of dryness is apparent. In our 

 summer months, during very hot weather, judicious shading may 

 be a less evil than the excessive drj^ness which it is chiefly in- 

 tended to prevent. 



Ventilation, in hot-houses, is required to serve a double pur- 

 pose : to renew the atmosphere, or exchange a portion of the 

 external for the internal air, and thus to give a gentle motion or 

 current amongst the plants ; if this be done judiciously, it may be 

 practised at all times or seasons with but little waste of fuel. 

 Motion, and consequently change, exist in every natural climate, 

 and cannot be entirely withheld from plants in hot-houses with- 

 out a corresponding loss of vigour and compact growth. The 

 other purpose of ventilation is to prevent an injurious increase 

 of temperature during hot weather, or sudden bursts of sun- 

 shine. 



Although the principles of ventilation are simple enough to be 

 understood by all gardeners, I may mention that there sliould 

 always be one set of ventilators, or sliding sashes, at tlie top or 

 upper part of the house, and another near the floor where the 

 newly admitted air may, by passing over a part of the heating 

 surface, have its temperature graduallj^ raised to that of the 

 house. By giving more air by the lower than by the upper ven- 

 tilators, it will be gradually disciiarged at the top of the house 

 with but little cold draft among the plants. If the upper venti- 

 lators only are opened, we siiall experience counter currents and 

 drafts of cold air. The lowest may be sliding ventilators in the 

 brickwork, and they should be comparatively numerous in order 

 that small quantities may be admitted by each, thus preventing 

 sudden gusts of cold air at one place. Our stoves being span- 

 roofed, with pediment-like ends, in the very apex or upper angle 

 is a sliding sash, wliich proves to be an excellent ventilator at all 

 times, but particularly during the winter months, allowing the 

 air to escape in quantities as small as need ever be required in 

 practice. Various contrivances will suggest themselves, accord- 

 ing to existing circumstances, for warming and moistening the 

 air before it is admitted among tlie plants. This is chiefly of 

 importance in plant-stoves and early forcing-houses during severe 

 weather. 



It is apparent that all our contrivances for warming and regu- 

 lating temperature and humidity in confined atmospheres will 



