Recently Professor Black showed a photograph of a baby seal swimming in a shallow pool at a speed greater than the speed of the surface waves. In terms of the hydraulic analogy the seal may be said to be a “supersonic” swimmer.

In lighter vein, it may be wondered whether supersonic flow extends to the insect world! The attached Schlieren photograph shows, for example, a blowfly in a flow at a Mach number of 1·4. The blowfly, suitably stiffened by a film of clear plastic to prevent it from shattering, was sting mounted in a shock tube containing freon as the working fluid.