An Apparatus for Ventilating Hot-Houses. 503 



which is a balance weight (e). An index (/), shews the 

 elevation and depression of the float. When the tempe- 

 rature of the house is increased, the air in the cylinder will 

 immediately expand, and pressing upon the water in the 

 bottom, will elevate that which is in the tube, and with it, 

 of course, the float will be elevated. The movement of the 

 wheel (d), whose axis runs through the valve (g) immediately 

 opens the valve in proportion to the extent of the revolution. 

 The heated air which collects in the perforated air tube (A) 

 is thus suffered to escape. 



The length and shape of these tubes may be varied accord- 

 ing to circumstances ; if the communication with the external 

 air is to be made at the top of the house, it will be necessary 

 to put a conical cap on the top of the air tube (i), acting upon 



