CHAPTER V. 



VENTILATION. 



Ventilation, as applied to dwelling- 

 houses as well as to hothouses in this 

 country, has, till lately, been both much 

 neglected and little understood. This is 

 the more extraordinary, as it is histori- 

 cally known that, in other countries, it 

 was well understood and much attended 

 to, even centuries ago. In the famous 

 palace of the Alhambra, for example, 

 begun to be built in the reign of Maho- 

 met the Second, we are informed by 

 Power, in his " History of the Moors in 

 Spain," that " in every apartment two 

 currents of air were continually in 

 motion, apertures being formed near the 

 ceiling to discharge the warm and un- 

 wholesome air, which the pure inferior 

 current forced upwards. So well directed 

 were these currents of air, as to come 

 refreshed every instant with that deli- 

 cious coolness, breathed only in this 

 edifice." How few dwelling-houses at 

 the present day have provision made 

 near the ceilings for the escape of impure 

 air ; and yet it is at those parts, and by 

 such means as those described above, 

 that we are to get rid of those pestilen- 

 tial elements with which the air of even 

 our best-regulated houses is so often 

 contaminated. 



All ventilation is founded upon the 

 simple principle that cold air is heavier, 

 and has a tendency to sink downwards, 

 whilst hot air is light, and rises to the 

 top. At first sight it may appear that, 

 for the purpose of ventilating any build- 

 ing, it is only necessary that holes should 

 be supplied at the bottom of the apart- 

 ment for the air to enter, and other holes 

 be placed at the upper part, for the hot 

 air to escape. Practically, however, ven- 

 tilation is far from being so simple an 



affair; and if there is anything more 

 difficult than another for a scientific man 

 to accomplish, it is that of causing cur- 

 rents of air to obey his will, and take 

 that course through any building, and 

 with any velocity, which he may desire. 



" Ventilation is necessary, not to enable 

 plants to exercise their respiratory func- 

 tions, provided the atmosphere is unmixed 

 with accidental impurities, but to carry 

 off noxious vapours generated in the 

 artificial atmosphere of a glazed house, 

 and to produce dryness, or cold, or both. 

 If ventilation is merely employed for 

 the purpose of purifying the air, as is 

 often the case in hothouses and in dung- 

 pits, it should be effected by the intro- 

 duction of fresh air, damp and heated. 

 If it is only for the purpose of lowering 

 the temperature, as in greenhouses, or 

 in the midst of summer, the external air 

 may be admitted without any precau- 

 tions." — Lindley's Theory of Hort. 



" Vertical openings in upright walls, 

 if they communicate immediately with 

 the external atmosphere, are objection- 

 able, because they are liable to be acted 

 on by the prevailing wind, which, blow- 

 ing sometimes in one direction, sometimes 

 in another, and sometimes not at all, 

 renders their action uncertain, and not 

 to be depended on. Generally speaking, 

 the exterior terminations of all ventilat- 

 ing openings, whether for ingress or 

 egress, should be in a horizontal, rather 

 than in a vertical plane." — Walkers 

 Useful Hints on Ventilation. 



The earlier constructed hothouses in 

 this country, being copied from those of 

 the Dutch, had fixed roofs : how they 

 were ventilated, we have scarcely any 

 record left us. As soon, however, as 



