216 MAGNUS ON THE DEVIATION OF PROJECTILES, 



When the cylinder does not rotate, the decrease of atmo- 

 spheric pressure is equal on both sides. On the contrary, when 

 it rotates, the velocity, and consequently the decrease of pressure 

 is greater on the side which moves in the same direction as the 

 current than on the other, where these motions have opposite 

 directions. The experiment showed, that an increase, and not 

 a decrease of pressure occurred on the side which rotated against 

 the current, so that here the pressure perpendicular to the di- 

 rection of the current is greater than it would have been if no 

 motion had taken place. 



From Savart's investigations, it is known that when two jets 

 of liquid, issuing from circular orifices of equal diameters, meel 

 each other with equal velocities, and so that their axes are in 

 the same right line, all motion is not thereby annihilated, but 

 the liquid moves laterally, and forms a circular disc perpen- 

 dicular to the direction of the jets. In general, when two fluid 

 masses thus strike against each other, instead of a mutual anni- 

 hilation of motion, as in non-elastic solid bodies, a lateral 

 motion is produced, whose du-ection and velocity depend upon 

 the ratio of the masses which strike against each other, as well 

 as upon their respective velocities. 



In the experiment with the cylinder, the air by rotation 



Hence the decrease of lateral pressure essentially de[)ends upon the expansion 

 of the mass in motion, or upon the fact, that through each more distant trans- 

 verse section a greater number of particles of the fluid move. 



That the decrease of lateral pressure depends, as is here stated, upon the 

 expansion of the fluid mass, can be easily confirmed by causing air or a liquid 

 to move with a certain velocity through a horizontal tube of uniform diameter. 

 If the fluid enters the same through an orifice whose transverse section is 

 equal to that of the tube, so that the stream does not expand itself, then also 

 no decrease of lateral pressure occurs. For if from the horizontal tube 

 another narrow one descend vertically into a vessel of water, the water does 

 not ascend in the latter during the motion in the horizontal tube. If, how- 

 ever, the orifice through which the fluid enters the horizontal tube be less 

 than the transverse section of the same, and the vertical tube reaches in- 

 wardly to where the fluid expands itself, the water is seen to ascend. If the 

 fluid issues into the horizontal tube from a wide vessel under considerable 

 pressure, a decrease of lateral pressure is observable close to the orifice, even 

 when the diameter of the jet there is equal to that of the horizontal tube. 

 This, however, depends also upon an expansion, that is to say, the fluid 

 entering the horizontal tube contracts itself, and afterwards, beyond its point 

 of greatest contraction, again expands. 



