the 34 automatic 34 position once either of such positions has been selected by the operator however the operator must manually hold the switch 256 in the third or 34 fire 34 position wherein the automatic mode of the controller 246 is overridden so that the operator controls the lengths of blast time from the machine whether or not any granular material will issue with the water jet blast depends upon the position of the rocker switch 204 at this time the control box 36 is also provided with a three position selector switch 258 for allowing the operator to select any one of three different spacings between the slots formed in the ground by the jets from the moving machine the switch 258 causes the controller 246 to maintain the selected spacing distance between the starting points of successive slots for example one choice is a spacing of 1 5 inches from the starting point of one slot to the starting point of the next slot another choice is 3 0 inches between starting points a third choice is 4 5 inches between starting points and a fourth choice is 6 0 inches a depth selecting switch 260 is also provided on the control box 36 to provide an input to the controller 246 the depth selection switch 260 may be placed in one of several different positions which establish the duration of blast time from the machine when the machine is stationary the duration of blast time has a direct relationship with the.