During operations vessels may be tracked by the vertical or by the horizontal method of position finding. If by the former, either the command “Fire” may be given when the vessel is on the cross wires of the instrument set at the range and azimuth of a mine, or the time from any point to the instant of passing over a mine may be found by means of the prediction ruler ([see below]) and the command “Fire” be given at the proper instant, as indicated by the stop watch. For the horizontal base system the latter method must be used.
Prediction ruler ([fig. 15]).—This is a 10-inch white celluloid slide rule with a beveled edge. The slide is graduated in “Yards in 15 seconds,” and on the left and right of the runway, respectively, are a “Fire at time” and a “Yards to mine” scales. The beveled edge is graduated from the center outward in both directions with “0” in the center of the scale and “500” at either end. Each 50 and 100 has its value engraved on the scale.
Method of using.—Plot the position of the target for a 15-second interval. With the beveled edge find the distance the target has passed over during the interval; and also determine the distance from the last plotted position to the mine. Move the slide until the graduation corresponding to the “Yards in 15 seconds” is opposite the graduation corresponding to the “Yards to mine,” and read the “Fire at time” scale opposite the arrow on the slide. The reading will be the number of seconds from the last plotted position to the mine which the vessel is approaching. A stop watch is started at the time of the last observation on the target, and at the expiration of the time obtained from the “Fire at time” scale the command “Fire” may be given.
Observation firing.—The mine commander’s station is connected with the casemate by telephone. At the command “Observation firing” sent to the casemate, the casemate operator will see that all automatic switches are up, and that all firing switches are open. He will then close the double circuit breaker, and switches 4 and 9, which will energize the busses of the power panel. At the command “Group ——, mine ——,” the operator will close switches 3 and 8 on the power panel, thereby putting both D. C. and A. C. power on the operating boards. At the command “Ready,” given from the mine commander’s station at the proper time, the operator will stand ready to trip the corresponding automatic switch. At the command “Fire” the automatic switch will be tripped and the firing switch will be closed. Without delay, after the mine is fired, the firing switch and the power switch will be opened, the automatic switch closed up, and the mine switch opened on the mine block.
If the mine is struck before the command “Fire” is given, the automatic switch will fall, and the mine should be fired by closing the firing switch unless there are positive orders to the contrary.
Contact firing.—For contact firing the mine system will be set so that a signal will be sent to the casemate and the mine will be fired when the latter is struck by a passing vessel. This is the normal method of firing in actual service. At the command “Contact firing,” which may be given for all groups, or certain individual ones, the casemate operator will see that all automatic switches are up, power and mine switches closed, and firing switches open; he will then close the double circuit breaker, and switches 4, 9, 3, and 8 on the power panel. This puts both D. C. and A. C. on the operating boards. He will then close the firing switches on all the boards or on such as may have been indicated. When a mine has been fired, the corresponding mine block will be cut out.
If it is desired to delay the firing of a mine after being struck, the command “Delayed contact firing” is given. The operations are the same as for contact firing except that the firing switch is closed by the operator a short time after the mine has been struck or when directed to do so. After the mine has been fired the firing switch will be opened, and the corresponding mine block will be cut out.
APPENDIX NO. 1.
EXPLOSIVES.
The latest adopted explosive for submarine mines is trinitrotoluol, also called trotol. The commercial names for this explosive are trinol, trotyl, and triton.
Wet guncotton is used extensively for submarine mines and in emergency other commercial high explosives may be employed, preferably dynamite.