I am standing on the threshold about to enter a room. It is a complicated business. In the first place I must shove against an atmosphere pressing with a force of fourteen pounds on every square inch of my body. I must make sure of landing on a plank travelling at twenty miles a second round the sun-a fraction of a second too early or too late, the plank would be miles away. I must do this whilst hanging from a round planet, head outward into space, and with a wind of aether blowing at no one knows how many miles a second through every interstice of my body.