504 Scientific Intelligence. 



shell with a hard point is incident normally on such armor, both 

 the projectile and the plate are seriously damaged, but the former 

 does not penetrate the latter. Even when a thinner plate is 

 pierced by the shell the projectile is usually smashed to pieces in 

 the act. To overcome this lack of penetrating power it has 

 become the custom, in recent years, to provide the point of the 

 shell with a cap of soft steel. In this event, the projectile 

 punches a clean hole both in its cap and through the armor plate. 

 In some instances the shell is so nearly intact as to admit of its 

 being used over again. The cap seems to form a ring around the 

 nose of the shell at just the right instant, preventing any lateral 

 deformation or flow of the tip, thus concentrating the blow over 

 a small area and enabling the projectile to pierce the armor. — 

 Nature, February 15, 1912, p. 531. h. s. u. 



8. A Photograpliie Study of Vortex Mings in Liquids. — 

 For purposes of demonstration, vortex rings are usually made 

 in air by sending puffs of smoke through a circular opening in the 

 front of a suitable box. It has been shown quite recently, how- 

 ever, by Edwin F. Nokthrup that more beautiful and instructive 

 results can be obtained by using liquids of small viscosity instead 

 of air. The essential parts of the apparatus used and a few of 

 the phenomena produced may be briefly described as follows : 



The rectangular experimental tank was chiefly made of plate 

 glass and its length, depth, and width were respectively 151 cm , 

 59-5 cm , and 12 cm . The vortex rings were projected by means of 

 a cylindrical can or "gun" 7*7 cm in diameter and 6'6 cm axial 

 length. One end of this can consisted of a flexible diaphragm 

 of phosphor bronze which could be given an impulse by the 

 plunger of an electromagnet. The muzzle end of the gun was 

 partly closed by any one of a set of metal discs which contained 

 one or more holes of various shapes and dimensions. x\ circular 

 hole l cm in diameter and at the center of the disc was usually 

 employed to produce single vortex rings. To produce double 

 rings two holes each of 0'85 cm diameter were provided with their 

 centers at a distance of l*275 cm from the axis of the cjdinder. The 

 tank was generally filled with slightly acidulated water whereas 

 the can contained strongly alkaline water which was deeply 

 colored with phenol phthalein. By this scheme the necessity of 

 refilling the tank after one or two "shots" was avoided because 

 the deep red vortex rings disappeared completely as soon as they 

 lost their form and became dispersed. 



Before investigating photographically the behavior of the rings, 

 Northrup observed the following noteworthy properties of the 

 vortices. The speed of the rings was initially about two meters 

 per second and, when free to do so, they moved in straight lines 

 with their planes perpendicular to the direction of propagation. 

 They did not appropriate suspended particles of slightly greater 

 density than the medium in which they moved. When a tightly- 

 stretched sheet of chiffon cloth was interposed in the path of the 

 vortex, the ring would pass through the net without being 



