"Here for instance is a kind of attraction which Professor Guthrie has made us familiar. A disk is set in vibtration, and is then brought near a light suspended body, which immediately begins to move towards the disk as if by an invisible cord. ...Sir W. Thomson has pointed out that in a moving fluid the pressure is least where the velocity is greatest. The velocity of the vibratory motion of the air is greatest near the disk. Hence the pressure of the air on the suspended body is less on the side nearest the disk... the body yields to the greater pressuire, and moves toward the disk. The disk, therefore, does not act where it is not. It sets the air next to it in motion by pushing it, this motion is communicated to more and more distant portions of the air in turn, and thus the pressure on opposite sides of the suspended body rendered unequal, and it moves toward the disk in consequence of excess pressure. The force is therefore the force of the old school—a case of vis a tergo—a shove from behind."