102 EDITH BELLAMY SHREVE 



leaf. Around the other pair of cork tips are wound several 

 turns of silk thread. The leaf passes between the two sets 

 of corks so that the junction e touches its surface. The silk 

 threads then touch the other surface of the leaf and act as a 

 support for holding the clip in place. The clip is represented 

 in position on a leaf at D. E shows the other thermo-couple 

 hanging free in the air of a thermos bottle. At F is represented 

 a portable telescope galvanometer of high sensitivity, one volt 

 passing through a resistance of 40 megohms or more, causing 

 a deflection of 1 scale division. The instrument which I used 

 has withstood a year of very rough field service. 2 At b is shown 

 a two-way switch with connections made so that the galvanom- 

 eter is always closed excepting for the second when the thermo- 

 couple circuit is opened for a reading. This key thus dampens 

 the galvanometer swings so that the readings may be taken 

 without loss of time. All wires are insulated excepting at the 

 thermo-j unctions. 



To find the temperature of a leaf it is necessary to know only 

 the temperature of the junction in the thermos bottle and the 

 throw of the galvanometer, provided the galvanometer has 

 been previously calibrated. The calibration is made by placing 

 the clip A in another thermos-bottle which is fitted with a stop- 

 per and mercury thermometer. The temperatures of the air 

 in the two bottles are then made to differ by any convenient 

 amount, for example 10°C. The key is then pressed and the 

 throw of the galvanometer read, and the deflection for 1° 

 calculated. The galvanometer is so sensitive that a difference 

 of 0.1 °C, between the two junctions can be detected; conse- 

 quently there is no need to carry ice in the thermos bottle. It 

 is necessary to place the junction / inside a thermos bottle be- 

 cause the lag of a mercury thermometer is too great to register 

 the frequent and rapid changes of temperature that take place 

 in the free air in the field. The air in the thermos bottle may, 

 of course, be either above or below the temperature of the out- 

 side air. I found it convenient in field work to start the morning 



2 The instrument is made by the Leeds & Northrup Company, Philadelphia. 



