GEOTHERMAL GRADIENT DETERMINATIONS 53 
Consolidated Copper Company, are cooperating with the writer in a program 
of work which may extend over a period of years. The Michigan College is 
generously supplying the financial aid and the Calumet and Hecla Company 
the skilled technical assistance necessary to the success of the work. The 
measurements in all cases are to be guided by heat conduction considerations 
to the end that they may represent as closely as possible the actual virgin 
temperature at the spot, unaffected by mining operations. The work must 
accordingly proceed slowly, waiting frequently for favorable opportunities 
for temperature measurement; but while it has been going only about a year 
some preliminary measurements of value have been obtained as well as other 
results which may be of interest to the physicist. 
METHOD OF TEMPERATURE MEASUREMENT 
General procedure 
The measurements so far have been confined to the Calumet and Hecla 
mine in which most of the Agassiz measurements were also made. A some- 
what different procedure has been adopted, however. In the Agassiz work 
thermometers were sealed for weeks at the bottom of drill holes 10 feet deep, 
in shafts or passages which had, themselves, been exposed to ventilation in 
some cases for many months. A little calculation on the basis of heat con- 
duction theory will serve to show that the results under such conditions are 
very likely to be influenced by the ventilation, and only in the most favorable 
cases can be depended on to give virgin rock temperatures. 
In the present work the measurements have all been carried out in new 
workings, i.e., drifts or other cuttings which have been advancing steadily 
a number of feet a week and which are well removed from other parts of the 
mine. At such a “temperature station” a hole is drilled a few feet back from 
the breast and in rock whose face has been exposed only a few days. The hole 
is usually 7 feet deep, but in some cases special 14 foot holes are run. Tem- 
peratures are taken with two or more thermometers located at the bottom of 
the hole. It may be remarked that, save for the water used in drilling, which 
quickly drains out, the hole has been found practically dry in all cases. 
Thermometers 
After careful consideration of the advantages and disadvantages of elec- 
trical temperature measuring instruments, mercury thermometers have been 
finally chosen for this work, at least for the type of measurements being made 
at present. They have been specially made by Henry J. Green, reading 
0—40°C in 1/10°, and frequent zero tests and comparisons with one of the 
number calibrated by the Bureau of Standards give maximum errors of the 
order of 0.04°C. They are mounted in bakelite tubes 1 inch in diameter and 14 
inches long with the thermometer bulb specially insulated thermally. This is a 
vital matter to which a great deal of attention has been given. It is necessary 
that after pulling from a drill hole the mercury thread show no change for 
something like a minute, so as to give the operator plenty of time to make his 
readings. At the same time there must be combined with this lag or “delay 
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