metering hole thus the velocity of the coolant is reduced as it passes toward the gas stream and it is substantially uniformly distributed across the opening the flow area of this opening 58 is greater than the flow area of the metering hole but should not exceed 7 times the metering hole area such excess area would result in too low an incoming velocity fig8 illustrates the flow area 52 just at the edge of the overlaying plate 34 while fig9 illustrates the location of the downstream wall 42 at a location where the opening 46 is overlaying the downstream wall hypothetical extension 60 of the downstream wall is arranged to clear the upstream edge 62 of the first plate 34 at the downstream end of trapezoidal 46 the forming of depression 40 with the relatively flat surface 42 results in a pair of bent lines 64 substantially defining the outer edge of the downstream wall 42 these bend lines are at an angle 66 of 25 degrees with respect to the line 68 fig1 and 11 illustrate an alternate embodiment where the overlaying sheet portion 48 of first sheet 34 is bent downwardly into chamber 50 forming a cantilevered section 70 an impingement opening 72 is located to impinge against this overlaying portion 70 this also provides for the diffusion of the incoming air so that it moves in substantially parallel flow lines at relatively low velocity through opening 74 the relatively high velocity of the incoming meter airflow is reduced in an area where it is surrounded by bounding surfaces accordingly the flow is decreased and diffused without the ingestion of hot gas from the main gas stream which is otherwise caused by low static pressures in the jet while the diffuser openings in.