The area over which these subsidences take place is about two square miles. Some years ago the property in Northwich was valued at £311,885, but the depreciation on it was valued at one third, or £102,945—the annual loss being £5,147. When the matter was brought before the House of Commons it was stated that damage had been done to no less than 892 buildings. But the number to-day, if it could be estimated, would be infinitely larger. These 892 buildings comprised five public buildings, 15 manufacturing works, 21 slaughter-houses and stables, 34 ware-houses and workshops, 41 public-houses, 140 shops, and 636 houses and cottages.
In ten years the pumping up of brine had excavated from beneath beneath Northwich a space large enough to form a ship canal from Northwich to Warrington 150 feet wide and 30 feet deep. And a well-known authority declares that the subsidences during the present century form an excavation very much more extensive than was required for the Manchester and Liverpool Ship Canal. For the subsidences correspond with the amount of salt taken from the earth.
| May & Co. photo.] | [Northwich. |
Every ton of white salt consumes one ton of rock salt, and a ton of rock salt represents a solid cubic yard. As 1,200,000 tons of white salt are made every year at Northwich it follows that at least 1,200,000 cubic yards of solid foundation are removed from beneath Northwich each year. This is equal to an annual uniform subsidence of 248 acres one yard thick. No wonder that Northwich has fits!
Taking the fits as proved, we will now look more closely beneath the pie-crust of Northwich. The best way to do so is to get into a big tub which will just hold two people and go down the shaft of a salt mine, lowered by a windlass. First of all you pass through 32 feet of soil and drift, and then about 92 feet of what would commonly be called rock. Then below these 124 feet you come to the first bed of rock salt, which averages about 75 feet in thickness. Passing through this you come to 30 feet more of rock, and below again is found another bed of rock salt, which averages in thickness about 90 feet. It is the lower bed of rock salt which is mined. The bottom of the mine down which I went was 330 feet below the surface, but the atmosphere was delightful, being cool and dry and not in the least oppressive. A magnificent chamber, 25 feet high and 17 acres in extent, had been dug out of the salt, and its extent could easily be gauged by the help of the candles which had been lit all round the mine. Massive pillars of salt of 10 or 12 feet square are left at intervals of 25 yards to support the roof.
The rock is got largely by blasting. A hole is drilled, and into the bottom of the hole a small powder ball is put. Loose powder is placed in a piece of straw and the straw is lighted. In a few seconds it burns down to the powder ball, and the rock salt which has lain so quietly in its bed for æons breaks up, and in process of time may find itself in any quarter of the globe.
| T. Birtles, photo.] | [Warrington. |
No damage is done to the surface by the mining of this lower bed of rock salt. It is too deep for that. The subsidences are all connected with the upper bed of salt. These upper beds used to be worked because the lower beds were not known, and when they were neglected they fell in, and in this way the large sheets of water of which I have spoken were formed above the earth's crust.