Two cog wheels, a large and a small one, are fixed on the upper part of the after end of the torpedo, just in front of the screw propellers: the small wheel is fitted with a certain number of teeth, thirty for instance, which gears into an endless screw attached to the propeller in such a manner that one revolution of the propeller moves the wheel one tooth, therefore thirty revolutions would turn the wheel one complete revolution. The big wheel is fitted with much larger teeth than the small one, and by means of a pin on the latter wheel is moved round one tooth for every complete revolution of the small wheel, and clamped in this new position by a spring catch, which is also worked by the pin on the small wheel. In front of these wheels is a stud which works fore and aft in a slot, and attached to a spring which tends to draw it to the after end of the slot. This stud is connected by means of a wire rod to the valve that admits the compressed air to the engines; when the stud is in the fore part of the slot the valve is open, and when in the after part it is closed. _Adjusting Length of Range._--By means of a lever the spring of the stud is compressed, and the stud moved to the fore part of the slot; then the big wheel is moved round until a stud on its face is the required number of teeth above the lever. For every thirty revolutions of the propeller, and consequently one tooth of the big wheel, a certain known distance is traversed, which varies according to the pattern of the torpedo. _Adjusting Apparatus._--When the propeller has made the number of revolutions corresponding to the length of range required, and consequently has moved the big wheel the number of teeth it was set above the lever, the stud on the big wheel presses against the lever and so releases the spring in the slot, causing the slot stud to fly from the fore part to the after part of the slot, by which action the valve admitting the compressed air to the engines is closed, and consequently the engines cease to work. Attached to the axle of the big wheel is a small brass arm, which is connected by means of a brass rod to the safety wedge, and is so arranged that after the required number of revolutions of the propeller, the safety wedge will be drawn out; or it may be drawn out at the instant of the torpedo leaving the tube, carriage, &c. Also by means of an additional lever at the fore part of the torpedo, which is connected by means of a wire rod to the valve that admits the air to the engines, and by arranging the attachment of the safety wedge to the brass rod from the big wheel, so that on the wedge being withdrawn it is released from that brass rod, on the torpedo having completed its run, the action of closing the valve which admits the air to the engines causes the additional lever to force the wedge into the safety position.