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Monday y Sd April 1854. 

 Dr CHRISTISON, V.P., in the Chair. 



The following Communications were read : — 



1. On a New Hygrometer, or Dew Point Instrument. 

 By Professor Connell. 



This instrument consists essentially of a little spherical bottle of 

 thin brass, polished externally ; a small exhausting syringe ; a ther- 

 mometer with ground brass stopper ; and a brass clamp. The bottle 

 has a diameter of 1/^ inch, and is capable of holding half an ounce. 

 Its neck is attached to the syringe by means of a lateral screw, and 

 is three-fourths of an inch high, and about three-tenths of an inch 

 wide. The syringe is about five inches long, and has a diameter of 

 eight-tenths of an inch. The stopper attached to the thermometer 

 fits air-tight into the upper part of the neck of the bottle. The 

 clamp is intended for securing the instrument to the sill of an open 

 window, or to a table or other fixture in a room. Three drams of 

 ether are then slowly introduced into the little bottle, and the ther- 

 mometer inserted. The syringe is worked slowly at first, and the 

 speed gradually increased, when the thermometer will immediately 

 begin to fall from the cold produced by the evaporation ; and the 

 exhausting process is continued until dew is seen to be deposited on 

 the external surface of the little bottle. The temperature indicated 

 by the thermometer is then noted, the process of exhaustion stopped, 

 and the temperature again noted when the dew disappears from the 

 brass surface. The mean of these two observations may be taken as 

 the dew point. To prevent the spreading of the heat produced by 

 the friction of the piston, to the little bottle, the termination of the 

 syringe which screws into the neck of the bottle is constructed of 

 ivory ; and as it is found that the vapour of the ether acts on valves 

 of the usual oiled silk, they are constructed of goldbeater's leaf, four 

 plies of it being used for each valve. A reduction of temperature, 

 varying under different circumstances of temperature from 20° to 

 40"", has been produced by the instrument ; and should it ever be 

 found that extreme cases of united cold and dryness of atmosphere 

 shall occur, which are not within the power of the present size of 

 the instrument, there is little doubt that a sufficient increase of re- 



