The coal-output of the world, speaking roughly, doubled every ten years between 1840 and 1900, in the form of utilized power, for the ton of coal yielded three or four times as much power in 1900 as in 1840.

粗略地说，从 1840 年到 1900 年，世界煤炭产量每 10 年翻一番，以所用功率的形式表示，因为 1900 年每吨煤产生的功率是 1840 年的三到四倍。