The rate of increase of temperature in the Lake Superior mines averages about one degree per 105 feet, but is greater in depth, about one degree Fahrenheit per 90 feet. While the mean air temperature in Calumet is about 39 degrees Fahrenheit, the mean ground temperature is 43 degrees (42.6 degrees Fahrenheit), owing to the blanketing effect of the snow in winter.
This increase of gradient is due to an increase of surface temperature about 10,000 years ago; for, by taking the gradient below 4,000 feet and producing it to the surface, we find a surface temperature of 32 degrees Fahrenheit, that which the ice bottom may have had. The time since the rise in temperature is found by seeing how much the scale of time may be enlarged in the probability curve connecting the temperature and time for a flow of heat due to a sudden rise of . . .