seen that the legs 146 148 and the legs 152 154 are electrically connected by the switch elements 136 when the carrier 124 moves to its upper position illustrated in fig7 the legs 144 146 and the legs 150 152 are electrically connected by the switch elements 136 referring to fig1 3 and 6 the arrangement of the firing circuit 22 and particularly the wiring of the switch units 70 is illustrated for purposes of simplicity the showings of fig1 and 8 are described hereinafter as including three blasting caps 40 although it should be understood that as many gun sections 30 may be provided as desired the hot wire 28 is illustrated in fig1 as extending through the passage 58 to the switch 112 of the upper switch unit 70 as shown in fig6 the hot wire 28 is connected to the terminal leg 152 of the upper switch 112 an electrical connection 156 extends from the terminal leg 150 of the upper switch unit 70 to the terminal leg 152 of the lower switch unit 70 as shown best in fig3 the electrical connection 156 includes an insulated wire 158 connected between the terminal leg 150 and the conductive.