THE BALL-PARADOX. 727 



the force of the jet. In that position the ball resembles a sphere ex- 

 posed to atmospheric pressure above and below. 



Now, let an horizontal jet of water II be thrown against the ver- 

 tical jet V, as shown in Fig. 3, and its action, opposed to the spring S, 

 must cease at once, and the unbalanced pressure of the spring raise 

 the ball into the position shown in Fig. 3. 



The situation of the ball in Fig. 3 is now equivalent to that in 

 Fig. 1, where the air-jet J, passing over one side of the ball, calls 

 into action the pressure D at right angles to the jet. Were the 

 sphere B in Fig. 1 placed in the centre of the jet, no such action could 

 result ; and, if the ball were placed there, its gravity would be un- 

 balanced until the ball fell to one side of the jet, and the supporting 

 power of the air evoked. 



Fig. 3. 



To prove that a current of air moving parallel with any surface 

 destroys the atmospheric pressure at that point, take a visiting-card 

 and bend the ends at right angles with the card, so as to turn up one 

 quarter of an inch at each end. Now place the card near the edge of 

 a smooth table, supported by the two ends like a little bench, and 

 try the effect of blowing violently between the card and the table in 

 a direction parallel to both. The current will destroy the atmospheric 

 pressure beneath the card, and the unbalanced pressure above will 

 force the middle of the card downward. Or, take another card and fit 

 a quill or straw tightly into a hole cut in the centre. Try to displace 

 another card laid loosely over the first by blowing upward against it. 

 If a pin is stuck through the centre of the second card, into the open- 

 ing of the quill, to keep it from sliding off, it will be found that no 

 effort will blow the upper card from the first, as the current of air 

 passing out between the cards destroys the atmospheric pressure be- 

 tween them, calling into play a force upon their outside surfaces that 

 presses them tightly together. The arrangement of the cards is 

 shown in Fig. 4. 



It is thus evident that a pressure perpendicular to any surface can 

 be displaced by one acting parallel to it. 



