Change of locality, though ever so rapid, can never reveal itself to our senses when it takes place in a vacuum, or when the enveloping atmosphere travels at the same rate as the moving body.
Though we are incessantly whirled around the Sun at the rate of about seventy thousand miles an hour, which of us is conscious of the slightest motion?
In such a case, as far as sensation is concerned, motion and repose are absolutely identical.

Neither has any effect one way or another on a material body.

Is such a body in motion?
It remains in motion until some obstacle stops it.

Is it at rest?
It remains at rest until some superior force compels it to change its position.

This indifference of bodies to motion or rest is what physicists call _inertia_.
Barbican and his companions, therefore, shut up in the Projectile, could readily imagine themselves to be completely motionless.