Mr. Séjourné, a French engineer, who has been connected with the construction of numerous tunnels by the Belgian method where he was in position to secure comparative figures, has given the following rules for calculating the cost of tunnels. Assuming _A_ to represent the cost of excavating a cu. yd. in the open air, the cost of excavating the same quantity underground in driving headings will be from 9 _A_ to 11 _A_, and in enlarging the profile it will be about 5 _A_. The cost of constructing single-track tunnels varies with the thickness of the lining, and may be calculated by the following formulas: Without lining, _C_ = 5.5 _A_. With roof arch only, _C_ = 6.4 + 6.4 _A_. With lining 18 in. thick, _C_ = 9.4 + 7 _A_. With lining 2 ft. thick, _C_ = 11 + 8 _A_. In these formulas _C_ is the cost per cu. yd. of excavation, including the masonry. For double-track tunnels the amounts given by the above formulas may be used by reducing them about 7¹⁄₂% or 8%. The second method of estimating the cost of tunnel work consists in assuming as a unit the unit cost of tunnels previously excavated under similar conditions. Mr. La Dame gives the following unit prices for a number of tunnels driven through different materials: +-------------------+--------+-------------+--------+----------------+ | NATURE OF SOIL. |TUNNELS,| EXCAV. PER |COST PER| MAX. AND MIN. | | | NO. OF | CU. YD. |LIN. FT.| PER LIN. FT. | +-------------------+--------+-------------+--------+----------------+ |Granite-gneiss | 56 |$3.07 @ $3.85| $100. |$61.46 @ $190.40| |Schist | 39 | 1.38 @ 1.53| 75.42| 43.11 @ 70.68| |Triassic | 3 | ... | 90.85| 84.75 @ 93.33| |Jurassic | 69 | 1.23 @ 1.38| 77.86| 35.24 @ 157.2 | |Cretaceous | 34 | 0.61 @ 0.77| 59.60| 27.37 @ 92.25| |Tertiary and modern| 39 | 0.33 @ 0.61| 105.80| 51.52 @ 188.36| +-------------------+--------+-------------+--------+----------------+ In the following table is given a list of tunnels excavated through different soils, from the most compact to very loose materials, and driven according to the various methods which have been illustrated. DOUBLE-TRACK TUNNELS.