ft. in. 1. Tertiary and Cretaceous Strata, to a depth of 100 yards, average progress 1 8 2. Cretaceous Strata, without flints " 250 " " 2 1 3. Cretaceous Strata, with flints " 250 " " 1 4 4. New Red Sandstone " 250 " " 1 10 5. New Red Sandstone " 500 " " 1 5 6. Permian Strata " 250 " " 2 0 7. Coal Measures " 200 " " 2 3 8. Coal Measures " 400 " " 1 8 ---- ------ General Average 275 1 9 ---- ------ When the cost of materials and labour is known, that of the boring may be approximately estimated from the above averages. Should hard limestone or igneous rock be met with, the rate of progress may be less than half the above general average. Below 100 yards, not only does the rate of progress rapidly increase, but the material required diminishes in like proportion, so that for superficial borings no surface erections are needed, and the cost sinks to two or three shillings a yard. COST OF BORING. The cost of boring when executed by contract has already been treated of at page 80. The following formula will furnish the same results as the rule there given, but with the least possible labour of calculation; _x_ = 0·5_d_(·187 + ·0187_d_); _x_ being the sum sought, in pounds, and _d_ the depth of the boring in yards. _Example._ Let it be required to know the cost of a bore-hole 250 yards deep. Here 125{·187 + (·0187 × 250)} = £607·75. TEMPERING BORING CHISELS. 1. Heat the chisel to a blood red heat, and then hammer it until nearly cold; again, heat it to a blood red and quench as quickly as possible in 3 gallons of water in which is dissolved 2 oz. of oil of vitriol, 2 oz. of soda, and 1/2 oz. of saltpetre, or 2 oz. of sal ammoniac, 2 oz. of spirit of nitre, 1 oz. of oil of vitriol: the chisel to remain in the liquor until it is cold. 2. To 3 gallons of water add 3 oz. of spirit of nitre, 3 oz. of spirits of hartshorn, 3 oz. of white vitriol, 3 oz. sal ammoniac, 3 oz. alum, 6 oz. of salt, with a double handful of hoof-parings, the chisel to be heated to a dark cherry red. GASES IN WELLS.