80 CARNEGIE INSTITUTION OF WASHINGTON. 



While numerous records of the temperature of the soil have been kept 

 at various localities, they have largely been of such a nature as not to 

 be comparable one with another. The work of this committee was 

 inaugurated wdth the fundamental purpose of securing a series of 

 soil-temperature data for a period of several j^ears under conditions 

 as nearly identical as possible. The aim has been to secure thermo- 

 graph records from bulbs placed at the same depths, in level, well- 

 drained soil, free from the shade of trees or buildings, and under a 

 ground-surface kept free of sod or weeds, but without continuous 

 cultivation. Very light and very hea'vy soils have been avoided, except 

 in cases where no others were to be found in the vicinity of the stations. 

 Marked differences of soil temperature are known to accompany 

 differences of soil texture in the same locality, but it was deemed 

 inad\'isable to attempt to place all of the thermograph bulbs under 

 identical soil conditions by transporting the same type of soil to all 

 stations. Such readings would result in obtaining a series of physical 

 constants that would have only an indirect relation to the natural 

 vegetation and agricultural activities of the regions in which the 

 various stations are located. 



During 1917 over 30 stations were maintained by the valued coop- 

 eration of as many men, and nearly complete records for the year 

 were secured at most of these stations. The distribution of the instru- 

 ments is such that data are being obtained from all parts of the United 

 States and from portions of Canada. At several stations bulbs are 

 installed at 12 and at 3 inches, while at the majority of the stations 

 they are maintained at 3 inches only. Still more intensive investiga- 

 tions of soil temperature are being carried out by several of the coop- 

 erators who were interested in the subject prior to the inauguration 

 of the survey. It is hoped that the series may be maintained for 

 several years and that it will then be possible to draw a map of the 

 isotherms of soil temperature for northern North America. When 

 this work, has been carried through with our knowledge of the tem- 

 perature of the soil, it may be placed on a geographical basis, as has 

 long ago been done for the temperature of the air and other condi- 

 tions of critical importance to plant and animal life. 



Apparatus for determining the Temperature of Leaves, hy Edith B. Shreve. 



A method for the determination of leaf temperature has now been 

 brought to a stage where it can be used satisfactorily for finding the 

 surface temperature of leaves without wounding them. The time 

 required to obtain a given temperature is less than 10 seconds. The 

 essential part of the apparatus is a thermo-couple made of No. 40 

 "Ideal" and copper wires, one junction of which is so fastened to a 

 light spring chp that it is held in position on the surface of a leaf 

 without shutting the leaf off from the influence of the surrounding 



