Letter from William George Penney, on a paper 'The supersonic flow of compressible fluid through axially symmetric tubes of uniform and varying section' by R K Tempest to D C Martin, Assistant Secretary of the Royal Society Reports back on three papers he has received. Has enclosed a lengthy commentary on Tempest's paper. Some small alterations should be made but if the author doesn't wish to make these changes, it should still be published. [Published in Proceedings of the Royal Society A, 1950]. Endorsed on recto as received 7 October 1949. 6 October 1949