source s within the meaning of the invention u03b1 is defined as the angle of divergence of the jet and the maximum velocity of the jet in question is the velocity measured on the axis a at the height of the source s and two adjacent jets are considered as having combined when their respective frontiers intersect the blower ring according to the invention has been tested according to various alternative embodiments shown diagrammatically in fig4 to 6 fig4 corresponds to a ring of the perforated annular tube type constituted by a simple torus of rectangular cross section 24 of which the bottom wall is pierced by a series of different orifices 25 the rings c1 and c2 correspond to this first type fig5 and 5a show a ring having nozzles or more precisely twin nozzles as can be seen on the section 4a this jet is thus produced by a nozzle 26 supplied by a tube 27 in contrast to the aforementioned case the supply to each group of two nozzles is therefore individualised the rings c4 and c5 correspond to this type finally fig6 shows an alternative form of embodiment derived from fig4 where a nipple 28 becomes seated opposite each orifice or hole the ring c3 has been constructed according to this model the characteristic features of the rings tested are as follows table i______________________________________ distance number between perforation ringtype of holes centres mm mm mm ______________________________________c1 290 7 43 1 5 686c2 86 25 2 5 686c3 86 25 2 5 686c4 86 25 2 5 686c5 43 50 1 3 686______________________________________ the ring c1 is a standard ring according to the teachings of.