The said small vessels or torpedoes may be provided with vertical rods to indicate their position to the operator at the station; these rods are shown at _L_, and they should be made hollow to allow the air in the water compartments or chambers to escape to permit the water to enter the same when the vessels _F_ are to be sunk; or other suitable provision may be made for the escape of the air from these compartments. The said vessels _F_ are preferably made cylindrical with conical ends, and are provided with suitable insulating and water-tight packing boxes, as shown at _F_^{1} for the cable _G_ to pass through at the stem and stern of each vessel. _The Lay Torpedo in Clearing Obstructions._--For this purpose the torpedo boat is provided with an apparatus, shown at Figs. 134 and 135, in combination with the electric cable, whereby the said boat is controlled and guided, and there is arranged in the boat _A_ a compartment _A_^{3}, from which extends down into the water a line or rod _U_, provided at its outer end with a hook or claw _U_^{1}, properly formed to take hold of any chain or bar with which it may come in contact. In the said compartment _A_^{3}, and upon the upper end of the said line or rod _U_, is placed a small case or cylinder _U_^{2} containing a charge of dynamite or other explosive material and a cartridge or fulminating cap, or a bottle of sulphuric acid, surrounded with a certain quantity of chlorate of potash and sugar. This case or cylinder _U_^{2} is shown detached and drawn to an enlarged scale at Fig. 135, and it will be seen that the said case is provided with a tube 1 containing a cartridge, or a phial filled with explosive substance at 2, and a ball or weight at 3. The said case is fitted to slide upon the said line or rod _U_, and when placed at the upper end thereof and not held or retained will slide to the lower end of the same. In the said compartment _A_^{3} is arranged at _U_^{4} an electro-magnetic apparatus, included in the circuit of the said cable, and connected with a bolt or catch which in its normal position holds the said explosive case and prevents its running down on the grappling line or rod _U_. This explosive case is also provided at its lower end with a grappling hook _U_^{5}.