1. CONVEYING MEANS INCLUDING A HOUSING, A FIRST CHAMBER IN SAID HOUSING, A SECOND CHAMBER IN SAID HOUSING, AIR PRESSURE GENERATING MEANS COMMUNICATING WITH SAID FIRST AND SECOND CHAMBERS TO DELIVER AIR UNDER PRESSURE TO SAID FIRST CHAMBER AND TO WITHDRAW AIR FROM SAID SECOND CHAMBER IN A SUBSTANTIALLY CLOSED SYSTEM, A CONVEYING CONDUIT EXTENDING THROUGH SAID HOUSING, NOZZLE MEANS COMMUNICATING WITH SAID FIRST CHAMBER AND POSITIONED TO DIRECT AIR FROM SAID FIRST CHAMBER INTO SAID CONDUIT, SCREEN MEANS IN SAID SECOND CHAMBER AND POSITIONED ADJACENT SAID NOZZLE MEANS, SAID SCREEN MEANS COMMUNICATING SAID CONDUIT WITH SAID SECOND CHAMBER, SAID NOZZLE MEANS INCLUDING A FRUSTO-CONICAL BAFFLE, SAID BAFFLE HAVING A CENTRAL APERTURE OF A DIAMETER CORRESPONDING TO AND AXIALLY ALIGNED WITH THAT OF SAID CONDUIT, A FRUSTO-CONICAL FLANGE CARRIED BY SAID CONDUIT AND EXTENDING IN SPACED, PARALLEL RELATIONSHIP WITH SAID BAFFLE TO FORM A CONICAL NOZZLE PASSAGE COMMUNICATING SAID FIRST CHAMBER WITH THE AREA WITHIN SAID CONDUIT.