00 AGITATION OF THE ATMOSPHERE CONSIDERED. 



sphere, circulating at pleasure, and of any desired quality of heat and mois- 

 ture, become of incalculable value. Admitting damp to be the greatest 

 enemy that tender plants have to encounter during winter ; that a current 

 of air dispels that damp as effectually, and much more safely, than fire heat 

 (the least excess of which is always hurtful and often fatal), the conclusion 

 is, that plants in a damp state are really more benefited by the application 

 of fire heat, from the commotion it creates in the air, than from any trifling 

 addition it may make to the temperature. Hence the great utility of Mr. 

 Penn's apparatus, with which the same quantity of fuel will create a tenfold 

 current, giving at all times the power of maintaining sufficient heat to keep 

 plants in a state of health without the possibility of injuring them. Some 

 persons suppose that plants will thrive better in a lower circulating medium 

 than they will do in a higher stagnated one (that is, that they will do as 

 well in a current of air heated to 60, as they would where it is stagnated 

 and heated to 70) ; then I reply that we know that plants of a more hardy 

 nature will bear much more heat with the air in a state of circulation than 

 they will when it is stagnant. Therefore, with an atmosphere so truly 

 under our control as that produced by Mr. Penn, we may reasonably ex- 

 pect an approximation in the habits of plants, that will render the division 

 of structures, however desirable under any circumstances, less a matter of 

 absolute necessity than it has hitherto been. It is, I think, not improbable 

 that this may be the case to an extent that will render greenhouse grapes 

 equal to the present forced fruit." (Gard. Mag. vol. vi. 2d series, p. 64rl. 



273. Pits and cucumber frames, which are kept at a high temperature 

 during winter, frequently have the air within surcharged with moisture to 

 such a degree at that season as to endanger the health of the plants. The 

 ordinary remedy for this evil is to admit a portion of the external air during 

 bright sunshine ; but a safer mode, if it can be adopted, is to admit the ex- 

 ternal air through tubes heated by being bedded in dung or tan, or by being 

 placed in contact with the flues or hot- water pipes by which the pit is heated. 

 By this means, the admitted air has its capacity for moisture greatly in- 

 creased, and it will absorb and change the steam contained in the atmosphere 

 of the pit and the dew-drops on the grass and framework into elastic invisible 

 vapour. Where hot water is used as the sole means of heating pits, if Mr. 

 Penn's system be adopted, the air will be kept constantly in motion, and 

 very little danger will arise from the damp, as will be afterwards shown 

 when we come to treat of the construction of pits. 



274. In all plant structures change of air and ventilation are least ne- 

 cessary when the plants are beginning to grow, and most so when they are 

 coming to maturity. The reason is, that at this latter period plants are 

 more abundantly covered with leaves than at any other ; and these leaves 

 being fully expanded, more air is required to enable them to perform their 

 respiratory functions. It is also found that increased ventilation and a drier 

 air are of great advantage to the maturation of the fruit ; but by dryness of 

 the air must be here understood not so much the absence of invisible elastic 

 vapour as of steam, or watery exhalations not held in a state of combi- 

 nation. " When grapes begin to colour," says Mr. Duncan, a scientific and 

 experienced gardener, " it is of as much importance to obtain a dry atmo- 

 sphere as it was, previously, to have a moist one ; because the change 

 effected in grapes while ripening is produced under the full influence of 

 light, heat, and dryness : and it is well known that grapes grown in dry 



