When the grappling hook _U_^{ 1}, on the lower or outer end of the line or rod _U_, engages with any obstruction the boat will be stopped, and this stoppage will be indicated on the keyboard. The operator by this indication is apprised of the stoppage of the boat by an obstruction, and by sending a current through the cable by means of a switch provided for this purpose on the keyboard he can immediately release the explosive case _U_^{ 2}, which runs down the line or rod _U_, and engages by its grappling hook _U_^{ 5} with the hook _U_^{ 1}. The line or rod _U_ is then disengaged from the boat _A_, and the explosive case _U_^{ 2} turns or falls over. As it turns over the ball or weight 3 contained in the tube 1 drops on the said phial 2, fractures it, and thereby allows the acid to mix with the explosive or fulminating charge and explode the case _U_^{ 2}. This explosion will rupture or destroy the obstructing chain or bar, so that the ironclad ships or other vessels can pass freely and safely into the harbour or beyond the point where it was intended to stop them.
_Used to clear away Mines and Electric Cables._--For this purpose there is an implement _V_ provided, Fig. 136, somewhat of an anchor form, but with four or any desired number of arms _V_^{ 2} extending outward at a suitable angle from its shank _V_^{ 1}. In the neck of each of these arms are fitted two small plain or toothed discs _V_^{ 3}, which are so arranged as to present their teeth to any object lying in the angle or corner formed by and between the arms _V_^{ 2} and shank _V_^{ 1} of the said implement, as shown at _W_.
In using this implement it may be attached to a line or cable coiled in the torpedo boat, which, in this case, is used without being charged with explosive material, and is sent in advance of any ship that has to enter or pass through the suspected water. This line must be arranged in combination with a detaching apparatus controlled by electro-magnetic apparatus included in the circuit of the cable which connects the torpedo boat with the keyboard at the operating station.
By sending a current from the station the operator releases the said implement or its line from the detaching hook or holding device. The said implement then sinks to the bottom; then the said boat returns to the ship, paying out the said line as she so returns. The end of this line is then taken by a steam tug or other vessel, and the said grappling implement is thereby dragged along through the water over which the ships are to advance, thus breaking any wires or cables that may be in its course. This operation is shown at Fig. 137, in which _A_ is the towing boat, _K_ the controlling cable, _V_ the said implement, _V_* the line attached to the implement _V_, _X_ _X_ submerged mines, and _X_^{ 1} _X_^{ 1} are the mine cables.