lines n p q divide the distance between line m and lines into quarters radius of curvature r3 approximates the natural vibration shape of discharge reed valve 81 the relationship of r3 r5 r6 and r7 are such that r7 lt r6 lt r5 lt r3 a slight air gap is created between the curved cross section and the curve shaped reed valve 81 at its maximum amplitude when naturally vibrating with point c as the fulcrum as shown in phantom in fig8 in such a structure substantially the same advantages as those in the first preferred embodiment are realized moreover the impact force with which discharge reed valve 81 impacts inclined surface 351 is smaller than that of fig4 this is due to the fact that the inclined surface 351 of fig8 comprises a curve having varying radii of curvature which are smaller than the radius of curvature of inclined surface 251 of fig4 fig9 illustrates a third preferred embodiment of the present invention elements similar to those in the previous embodiments are represented by the same reference numerals instead of a curved cross section flat surface 451 has a linear cross section starting at point c and terminating at point h a slight air gap is created between the linear cross section and the curved shape assumed by reed valve 81 at its maximum amplitude i e when vibrating in natural frequency with point c as the fulcrum as shown in phantom substantially the same advantages as those in the first preferred embodiment are realized fig1 illustrates a fourth preferred embodiment of the present invention elements in fig1 similar to those in fig8 9 are designated with the.