The problem was that the weight of the columns of water in the shafts was not always the same. Particles of rock and earth were constantly washing out of the clamshell buckets as they were hauled up through the water shafts, and as a result a fine silt was held in suspension and this, in a column of water seven feet square by, say, thirty-five feet high, could and did increase the specific gravity of that column to a remarkable degree. But when that shaft was not in use for some reason or other, the silt would settle, the water would become less thick and would weigh less. So great would be
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