2179. To effect the renewal or cooling down of the air, without manual labour, various contrivances have been adopted. Dr. Anderson and J. Williams made use of oblong bladders made fast at one end, and with the other attached by means of a cord to a movable pane or small sash. The bladder being filled with air at the common temperature allowed for the house, and hermetically sealed, the window remains at rest; but as the air of the house becomes heated, so does that of the bladder, which consequently swells, and assumes the globular form; its peripheries are brought nearer together, and of course the sash or pane pulled inwards. In a small house this scheme may answer perfectly well for the prevention of extreme heat. Another mode is by using a rod of metal, such as lead, of the whole length of the house, and one end being fixed to the wall, on the other is attached a series of multiplying wheels, the last of which works into one, which in various ways may open valves or sashes. As the expansion of lead is considerable, the effect of twenty degrees of increase, with proper machinery, might perhaps guard against extremes, as in the other case. A column of mercury, with a piston-rod and machinery attached, has also been used; and a ring on a barometrical principle is suggested by Sylvester. There is also a very ingenious mode invented by Kewley. For details at greater length on all the departments of the construction of hothouses, see Remarks, &c. 4to. 1817.