56 L. R. INGERSOLL 
have been made by a method developed by the writer* some years ago. The 
specimen of rock, which must be in the form of a slab say 6 cm thick and 
25 or more cm square, is mounted in a paraffined wood frame which ther- 
mally insulates the edges, leaving only the faces exposed. A fine thermocouple 
is located in a small hole drilled into the center of the slab, everything being 

Rock temperature (°Fahrenheit) 





7 
77Average surface 
temperature 
0 {000 2000’ 3000" 4000" “S000” Fauee 
Depth below surface feed 
Fig. 2. Geothermal measurements in the Calumet and Hecla mines. Average temperature 
gradient, 0-5500 ft. depth, 1°F in 108.5 ft. or 0.00922°F /ft., or 1°C in 59.5 meters, or 0.0164° 
C/m. Gradient, 3500-5500 ft. depth, 1°F in 103.1 ft. or 0.00970°F /ft. 
carefully waterproofed. When such a slab has been left in a constant temper- 
ature region for 24 hours and is then suddenly plunged into stirred ice water 
so that the faces are quickly lowered to and kept at zero, the diffusivity can 
be readily calculated from the time it takes for the center temperature to 
fall half way from its original value to the zero point. Two specimens of trap 
rock tested in this way gave values for the diffusivity of 0.0077 and 0.0074 
respectively. Since the former specimen was from the vein and the latter 
* For a brief account of this method see L. R. Ingersoll and O. A. Koepp, Phys. Rev. 24, 
92 (1924). For the underlying theory see Ingersoll and Zobel, “Mathematical Theory of Heat 
Conduction” (Ginn) p. 105 ff. 
200 
