_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.