In connection with gliding flight enough measurements of “ lift ” and “ drag ” have been made to enable us to calculate the conditions for success of an aeroplane fitted with wings of standard sections; but no attempt has been made, as far as the present writer is aware, to ascertain what would happen if a flying machine were fitted with wings of standard section and these were flapped in a rhythmical manner. Would it support and propel itself? Several authors, including M. F. Fitzgerald and Colonel J. D. Fullerton in the previous issue of this journal, have discussed various portions of this problem; but instead of appealing to wind-tunnel determinations the latter has used such expressions as pSUV or pSUV2 for the pressure on a wing of area S, where p is the density of the air and V, U are the component velocities along and at right angles to its surface.