PHYSICS. 339 



were first observed by Oberbeck, and studied more recently by Kots- 

 chan. If the vertical jet for example strikes the center of a disk in 

 the form of a regular polygon, the reflected jet divides into as many 

 liquid sheets as there are sides to the polygon. These sheets are at 

 first inflected as they leave the disk, but then recurve themselves, form- 

 ing volutes of great elegance directed toward the interior of the poly- 

 gon. The memoir is illustrated with numerous plates (Wied. Ann., 

 1885, XXVI, 530; J. Phys., November, 1886, If, v. 479.) 



Thomson and ISTewall have studied the formation of vortex rings by 

 drops falling into liquids. When a drop of ink falls into water from 

 not too great a height it descends through the water as a ring, in which 

 there is considerable rotation about the circular axis passing through 

 the centers of its cross-sections. The drops were observed by instan- 

 taneous illumination ; and it was seen that the drop enters the liquid as 

 a sphere, becomes flattened as it descends, and finally breaks into a ring 

 more than half an inch below the surface. To avoid complication 

 drops were let fall into liquids of the same kind. These liquids were 

 found to arrange themselves into four classes, distinguishable by the 

 character of the ring formed, and also by the ratio of the coefficient 

 of viscosity to the density. In Class i, ether, chloroform, and carbon 

 disulphide gave rings oidy very uncertainly, the drop breaking up and 

 spreading irregularly through the liquid. The ratio is not in this class 

 greater than 0.7. To Class ii belong water, alcohol, turpentine, paraf- 

 fine, etc. These have the ratio between i and 3, and give the best rings. 

 For Class iti the ratio is between 3 and probably 8 or 10. This class 

 Includes moderately viscous liquids, such as butyl alcohol, amyl alcohol, 

 fairly strong sulpliuric acid, and diluted glycerine. Class iv includes 

 all the most viscous liquids like strong solutions of sugar, potash, sul- 

 phuric acid, glycerine. The ratio has a value of 15 to 30, and no ring 

 is formed at all unless special precautions are taken to get large drops. 

 Capillarity plays no essential part in these phenomena. (Nature, Feb- 

 ruary, 1886, xxxTTi, 356.) 



Vautier has applied a graphic method to the direct measurement of 

 the velocity of efilux of liquids. In the flat, horizontal bottom of a 

 cylindrical vessel full of water is an orifice in a thin plate, for which 

 an ajutage can be substituted. Inside the vessel and in the line of its 

 axis is placed a tube containing an emulsion of an insoluble liquid of 

 the same density as water. This liquid is a mixture in suitable propor- 

 tions of nitrobenzine and oil of turpentine. The very fine bubbles of 

 this emulsion pass along the axis of the jet, of which they take the exact 

 velocity. The image of the jet, and therefore that of the bubbles, is 

 formed on a photographic plate, which by means of a suitable mechan- 

 ism is made to move at right angles to the trajectory of the jet. The 

 plate is exposed when the liquid begins to flow, so that it receives the 

 image of the jet during its passage. The plate, when developed, shows 

 one or more lines, according as one or more bubbles have passed during 



