Following on from the first in the series "Approaching the Speed of Sound", this picks up with a model aircraft in a wind tunnel and proceeds to demonstrate how air density can affect the speed of the plane. This is especially risky in the "transonic" range where stability is largely compromised. Using colour illustrations, we are presented with bow waves and shock waves, and a simple explanation is provided as to the distinctions between subsonic, transonic and ultimately supersonic airflows and how that impacts on the ability of the plane to move quickly, with out excessive turbulence. It helpfully uses boats on water and other more practical illustrations to stop this becoming a lecture in aerodynamics, but it does rather let itself get bogged down in Mach terminology and language that began to wash over my head a bit. Still, it does present an interesting look at how (wave) drag and shock must be overcome if safe, and comfortable, supersonic travel is to be achieved.