PLANT-HOUSES. 



485 



361 



in the liouse, when the air of the 

 house is at the highest temperature 

 required; and, consequently, when 

 any increase of temperature takes 

 place, the air in the hladder will ex- 

 pand and completely distend it, so 

 as to change its shape from that of 

 an oval to that of a globe ; shortening 

 the long axis and lengthening the 

 short one. The effect of shortening 

 the long axis is to pull in the hinged 

 pane of glass, and thus to admit the 

 external air. This is, without doubt, 

 a very rude mode, and will not regu- 

 late the temperature with any degree 

 of nicety ; but for ordinary purposes, 

 and where economy is a great object, 

 it is quite sufficient. Another mode 

 consists in applying a hollow brass tube, of 5 or 6 feet in length, against the 

 lower part or valve of a window that opens outwards, or against the lower 

 end of the vertical rod of a cap which opens upwards. Thus, in fig. 361., if 

 we imagine the rod / to be a hollow 

 brass tube, touching the floor {g) at 

 one end, and the spindle {h) of the 

 ventilating cap at the other ; and 

 that, when the rod is in this state of 

 contact with both the floor and the , 

 spindle, the house is at the maximum 

 temperature required ; it is evident 

 that any increase of heat, by expand- 

 ing, and consequently lengthenmg, 

 the brass rod, would raise the venti- 

 lating cap ; and the extent to which 

 this might be done, would be greatly 

 increased by placing the brass rod on 

 the end of a lever. Brass rods, when 

 required to open the sashes or ven- 

 tilators of a green-house, may also be 

 applied in a great variety of other 

 ways, which any ingenious mechanic 

 will readily discover. The last modfe 

 which we shall mention is one which 

 has been adopted with success by an 

 eminenthorticulturist, John Williams, 

 Esq., of Pitmatson. This ventilator 

 operates by the expansion and con- 

 traction of air in an air-tight vessel, 

 {fig. 362. a), communicating with a 

 cylinder and piston (&, c, d), which, 

 by means of a rod (<7), operates on the 



