and a knob 558 for manually turning the thread member 555 the threads engaging the fitting 553 of first hemicylindrical tube 421 are set to urge first hemicylindrical tube 421 away from second hemicylindrical tube 423 at the same time that the same turning of the first threaded member engages fitting 553 of first hemicylindrical tube 423 set to urge first hemicylindrical tube 423 away from second hemicylindrical tube 421 this means that the turning of first threaded member 555 in one direction urges the first and second hemicylindrical tubes 421 and 423 evenly away from each other and alternatively the turning of first threaded member 555 in the opposite direction urges the first and second hemicylindrical tubes 421 and 423 evenly toward each other likewise a second threaded member 557 has a pair of threaded areas in which the threads are oppose pitched the threads engaging the fitting 553 of first hemicylindrical tube 421 are set to urge first hemicylindrical tube 421 away from second hemicylindrical tube 423 at the same time that the same turning of the first threaded member engages fitting 553 of first hemicylindrical tube 423 set to urge first hemicylindrical tube 423 away from second hemicylindrical tube 421 but in an oppose orientation than the threads of first.