2155. Various modes of aeration have been practised, some of them even before the full importance of the operation was understood; the object being at first merely to imitate the natural action of the atmospheric air in keeping the leaves in motion. One of the earliest and simplest plans, observes Dr. Lindley, in the elaborate article in the Gardeners' Chronicle which has been already quoted, 'was that practised by Mr. Knight himself; and this consisted in passing pipes, open at each end, through the heating materials of a hotbed, one end being in the interior of the frame, and the other exposed to the open air;' and in this way Mr. Knight 'succeeded in constantly renewing the atmosphere of the frame, and in keeping the leaves in motion, with, as he tells us, the happiest effect.' Among various other modes of aeration which are mentioned by Dr. Lindley, is that of Mr. Williams of Pitmaston, who 'keeps the south end of his melon frame open to the outward air night and day, except that it is covered over with a screen of fly wire painted black, and continued in the inclination of the roof. This screen receives the rays of the sun from 10 A. M. to 3 P. M. all the summer long; it becomes heated to 80ï¾¦ or 100ï¾¦, and consequently heats the air that passes between its interstices. By raising the sashes at the back, a very powerful current of air is established, though the thermometer ranges from 80ï¾¦ to 90ï¾¦ below the leaves in a sunny day; and, in short, the atmosphere is as hot as is experienced in the southern parts of Italy, with almost as much ventilation as if the plants were growing in the open air.' (Ibid. and Journ. of the Hort. Soc., vol. i. p. 43.) Another plan mentioned by Dr. Lindley is that contrived by Mr. Leaf's gardener at Streatham; and this plan 'consists in passing a zinc pipe, thickly perforated with small holes, from end to end of the vinery, and exactly beneath the range of hot-water pipes which heat the structure. In the outer wall, communicating with this perforated pipe by means of a broad funnel, a register valve is fixed, by which the admission of air can be regulated with the utmost nicety, or the supply may be shut off altogether. This valve is fixed a little below the level of the perforated pipe. The action of this contrivance was evident enough, from the motion communicated to the foliage of the vines; and its effects were apparent in the unusually healthy and vigorous appearance they bore until the period of the fruit ripening.' (Gard. Chron. for 1846, p. 268.) Penn's mode of heating (see ï¾º 2148.) was very efficacious in keeping the air in motion; but its fault was, that it did not supply a current of fresh air. The Polmaisc system, on the other hand, derives its principal advantage from its affording the plants a constant supply of warmed fresh air (see ï¾º 2149.); and this is probably the nearest approach to the atmosphere of tropical climates that can be obtained in this country.