486 



THE VILLA GAEDENEE. 



hinged pane or sash to be opened. The use of the water or other fluid is to 

 confine the air ; and, by that means, when the air expands or contracts, it 

 operates upon the piston. By means of an adjusting screw, the register may 

 be made to open at any required degree of heat. The air-vessel should 

 contain several gallons, according to the size of the valve or register to be 

 opened. When first used, the vessel must be heated suflSclently to expand 

 the internal air ; water is then to be poured in at the top of the cylinder, so 

 as to give the required motion to the float ; and about half an inch of fine oil 

 must be laid on the top of the water, to prevent evaporation. In a plant 

 cabinet, such an instrument may be conveniently placed under the stage, so 

 as to have the rod {g) directly under the ventilator or sash to be opened. 



SuBSECT. II. — Ornamental Greenhouses. 



533. A green-house is a house with a glazed roof and sides, in which plants 

 ai-e kept in pots ; usually on wooden stages in the centime, but sometimes on 

 the brickwork, casing the pipes or flues ; or on shelves at the back or sides 

 of the house. The green-house differs somewhat from the plant cabinet, 

 which is always an excrescence affixed to the house, and generally entered 

 from the staircase or landing-place of the first floor ; whereas the green- 

 house is always built on the ground floor, and may be either attached to the 

 house or not at pleasure. Some green-houses have no apparatus for heating ; 

 and those which have hot-water pipes, smoke flues, or any other mode of 

 heating, have only a sufficient apparatus to keep up a moderate heat, say 

 from 45° to 50°, When plant houses can be heated more than this, by artifi- 

 cial means, they are no longer called green-houses, but stoves. Green- 

 houses, without fire heat, are generally used for camellias, the Australian 

 acacias, some of the Australian climbers, and most of the newly-introduced 

 Chinese plants ; in fact, all that are called hardy green-house plants, and 

 which it is only necessary to preserve from the frost. Green-houses, with 

 fire-heat, are used for Mexican and Peruvian plants, and for those from the 

 warm parts of Australia. 



534. A small green-house, with ornamental glass, is shown in figs. 363. to 

 365. This house, which partakes of the nature of a plant cabinet, but is not 



363 



