“It is beyond doubt that the auriferous [gold] beds of the Rand, in South Africa, will continue in depth far beyond a point where high temperature will render mining operations impossible. Where is this limit likely to be? Experiments have discovered a rise of 1° Fahrenheit for every 203 feet of vertical depth. If we assume that the maximum air temperature in which individuals can do a shift’s work is 100° F, we find that the limit of work by temperature is 12,000 feet vertical.”

Scientific American, September 1899

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