prior to the flow of the liquid drawn from chamber 243 and the discharge under pressure of this liquid to injector 207 fig1 to 17 illustrate other alternative embodiments of the means capable to prevent in various forms the operation of the apparatus as long as the latter is not brought to a proper position with injector 217 being in the immediate vicinity of the site where the jet of liquid should be directed in the example shown in fig1 the intermediate member 212 is formed with a transverse hole 310 for the mounting of radial pins 311 each being subjected to the action of a spring 312 bearing against a protruding head 313 and tending to push these pins back to the outside of the slot by disengaging the shell 314 of a plunger 315 provided with a seal 316 in its outer surface and slidably mounted in a housing 317 of the intermediate member 212 plunger 315 includes an end pin 318 adapted for blocking in the closed position ball 221 of the valve which closes thus the end of duct 218 the same plunger is subjected to the action of a spring 319 bearing on a fixed plaquette 320 mounted in the portion in register of the end piece 211 with interposition of a seal 321 in the alternative embodiment in consideration the intermediate member 212 is associated with a sliding thumb piece 322 formed with an outer shallow formation 323 allowing the user to slide it freely on the outer surface of the member.