as shown on page 210 of the work cited the crest 33 of the female thread is wider than the standard and is most easily obtained by using a tap drill arranged to be engaged in the tapped hole 31 is a correspondingly threaded stud 35 having the american standard v form thus when the stud is screwed into the hole the wide flat crest 33 will define with the neighboring flanks of the stud thread a helical passage 36 thus the gas is directed in a helical path from the plenum chamber 17 through the passage 36 to exit into the mass 10 of particles notwithstanding truncation of the thread as at 33 it is still possible to attain a tight fit between the plate 14 and stud 35 to preclude backing out of the latter under vibration to reduce or extend the lengthof the helical passage 36 the stud is rotated clockwise or counterclockwise for this purpose a recess 41 is provided in the stud this may be a plain slot phillips or allen recess or other a modified embodiment is depicted in fig3 and 4 in this case a helical passage 36a is constituted as before however the stud is now the shank 35a of a machine screw the head whereof is indicated at 37 and the.