35 ^ 
W. Neilson Jones. 
by its own weight, the air being forced at constant pressure through 
the stomata. The more widely open are the stomata the more 
rapidly does the index move. 
As the index passes the terminals ft', the electric circuit 
including the recorder is completed and a record registered on the 
drum. A record is similarly made when the index reaches the 
terminals tt. Thus the time taken for the index to pass between the 
two pairs of terminals t't' and tt is recorded on the drum. When 
