The large hole in the mine case covered by the cap is closed by a plug. The joint is made water-tight by a lead washer jammed between the plug proper and the case and by a coating of red lead or similar waterproofing material upon the screw threads. In the strong currents and deep water of some harbors more buoyancy than is possessed by the 32-inch case is required. This is obtained by inserting between the hemispheres a cylinder of 20-pound wrought iron which is stiffened by extra welded ribs for the larger sizes. Such cases are designated by the diameter in inches of a sphere having the same buoyancy. Thus, a No. 40 case is made by inserting a cylinder 32 inches in diameter and 20.4 inches in length between the two hemispheres of a No. 32 case; this is sufficient to make the displacement equal to that of a spherical case 40 inches in diameter. In the latest types the cylinders are made of corrugated mild steel of less thickness, which diminishes very materially the weights of the cases. The following table exhibits the dimensions and weights of buoyant mines, with trotol fuse cans, complete except the charges and moorings. The actual free buoyancy when planted will be the difference between the displacement and weight as given in the table, reduced by the weight of the charge and of the moorings and cables: PLAIN CASES. ----+-----------+--------+--------+---------+---------------------- | |Computed|Measured| Length | No. | Displace- | weight,| weight,| of | Remarks. | ment. | empty. | empty. |cylinder.| ----+-----------+--------+--------+---------+---------------------- | _Pounds_ |_Pounds_|_Pounds_| _Feet_ | 32 | 635 | 308 | 311 | 0.00 | All are about | | | | | 33½ inches in | | | | | outside diameter; | | | | | the extreme | | | | | length in each | | | | | case is 4.3 feet | | | | | plus the length 33 | 695 | 364 | | .17 | 34 | 762 | 395 | | .35 | 35 | 829 | 427 | | .54 | 36 | 904 | 462 | | .75 | 37 | 982 | 498 | | .96 | 38 | 1,064 | 538 | | 1.20 | 39 | 1,149 | 578 | | 1.43 | 40 | 1,242 | 621 | 625 | 1.70 | 41 | 1,341 | 665 | | 1.96 | 42 | 1,436 | 712 | | 2.24 | 43 | 1,540 | 788 | 759 | 2.53 | 44 | 1,652 | 842 | | 2.77 | One extra welded rib. 45 | 1,767 | 876 | | 3.17 | Do. | | | | | | | | | | { Lot of 1879; one