The Winsford district, taking the beds of rock-salt at an average thickness of 65 yds., which is 5 yds. less than the figure given by Dickinson, we have 1,932,902,400 tons.
As the whole of the white salt has been manufactured from brine derived from the rock-salt, it represents so many tons of rock-salt pumped up. Now, as the specific gravity of rock-salt is 2·125, a cubic yard contains 32 cwts. This being the case, we find the cubic yards of rock-salt pumped up annually in each district to be, viz.—
In Winsford District—
687,000 × 20 / 32 = 429,375 cubic yds.
In Northwich District—
587,000 × 20 / 32 = 366,875 cubic yds.
In Middlewich District—
14,000 × 20 / 32 = 8,750 cubic yds.
In Sandbach District—
78,000 × 20 / 32 = 48,750 cubic yds.