'26 



THE POPULAR SCIENCE MONTHLY. 



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 follow- 

 ing is offered as an illustration of the principles involved, and in ex- 

 planation 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 cur- 

 rent, 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 aw T ay 

 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 natu- 

 rally diminishing as the distance from the orifice J increases. 



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, at- 

 tached to a platform JP, and a jet of water V projected vertically 

 upon it, the spring will be compressed until its resistance is equal to 



