Page:Popular Science Monthly Volume 10.djvu/746

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when really revolved by the force of the air, has an uncertain axis of rotation until it has been turning for some time. The jet of air will also sustain a larger ball of lighter gravity behind the glass one, the former hanging on the lower side of the jet behind the heavier and smaller one.

These phenomena have excited so much attention that the following is offered as an illustration of the principles involved, and in explanation of the various points noted above:

The current of air sustains the ball by removing the atmospheric pressure from the ball where it strikes it; the unbalanced pressure of the air on the opposite side then forcing the ball toward the current, as shown by the arrow marked D in Fig. 1. The friction of the jet J against the ball tends to throw it in the direction indicated by C. To balance these two forces, we have the action of gravity, shown at G, which, being a constant factor, must be exactly neutralized by the forces named for the ball to remain suspended.

This adjustment, nice as it is, can be easily effected by placing the ball near enough to the jet at first; for, the pressure D being ample to sustain the ball in any position (else the experiment cannot be performed), the force of the jet will inevitably drive the ball away to a point where the power in that direction is just able to balance the force of gravity—the ball being evidently lifted, in spite of gravity, so long as it moves in the direction of the jet, and the force C naturally diminishing as the distance from the orifice J increases.

PSM V10 D746 Demonstration of fluid dynamics.jpg
Fig. 2.

To make it clear that it is the ordinary atmospheric pressure that supports the ball, let us imagine it exposed to two forces (as shown in Fig. 2), acting on opposite sides of the sphere. The atmospheric pressures that act on all other sides of the sphere are ignored, as they perfectly balance one another.

If a ball B (Fig. 2) is secured to the top of a spiral spring S, attached to a platform P,, and a jet of water V projected vertically upon it, the spring will be compressed until its resistance is equal to