Although earth and rock are much harder substances than water, the depths of the earth have been much easier for man to reach than the depths of the water. For, although a solid is harder to penetrate initially than a fluid, once penetrated, there is a hole which offers no subsequent resistance, whereas a fluid always exerts a pressure, increasing with depth. So it happens that, although man has been down in the earth for many thousands of feet, no diver had ever until recently been down more than about 30 fathoms (180...
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