164 TRANSPIRATION AND ASCENT OF SAP ch. 



elements of the junctions are of different materials viz., 

 brass and copper. It was found that the different tem- 

 peratures of the opposite sides of the galvanometer in an 

 ordinary laboratory could cause quite an appreciable 

 deflection. To remove this, the galvanometer was placed 

 in a thermostat, arranged to maintain a temperature of 

 about 21 (Fig. 28, T). For this purpose one of Hearson's 

 incubators was used. A hole was cut in the wooden door. 

 Through this a beam of light illuminated the galvanometer- 



Fi.i. 28 



mirror, and was reflected back to the scale (S). The inner 

 glass door was found not to injure the sharpness of the image 

 of the cross- wire sufficiently to be objectionable. It was 

 necessary to stand the galvanometer (Fig. 28, G) on a stout 

 glass plate on the copper floor of the thermostat, which 

 otherwise slowly sagged under the pressure of its feet. The 

 thermostat during observations, extending over a year, was 

 maintained at temperatures which varied very slowly 

 between 20*2 and 21 5, so that at any moment the parts 

 of the galvanometer must have been very closely at the 

 same temperature. 



This constancy of temperature was probably also ad- 



