108 Dr. Tyndall on some Phcenomena of a Water-Jet. 



approach the point where the jet enters from all sides, and are 

 there swallowed np. Sometimes a single bubble will remain 

 dancing and quivering for nearly live minutes under the jet, thus 

 exhibiting an inversion of the experiment seen in some of the 

 filter-makers^ windows in London, where a little ball is supported 

 by an ascending jet ; the ball is kept up in opposition to its gra- 

 vity, and the bubble is kept down in opposition to its lightness. If 

 a layer of oil of proper thickness be poiired upon water, and a jet 

 of the former enter, underneath the jet a basin is formed in which 

 the bubbles dance, while a billowy motion is imparted to the sur- 

 face all round. 



§2. 



When a small steady jet is intersected by the surface of the fluid 

 into which it enters, on the funnel side of the contracted section 

 there is no cavity formed, but the reverse ; the water climbs up 

 the jet to a certain height, and thus forms a little conical emi- 

 nence at the base of the descending column. This is best ob- 

 seiTed by placing a small beaker full of water in a basin, and 

 allowing the jet to enter it. WTien the eye is brought to a level 

 with the surface of the water, the eminence is distinctly visible. 

 This is manifestly due to capillary attraction, the jet behaving 

 in this respect like a solid metallic wire. 



If the water instead of passing through a rigid tube pass 

 through a flexible one, the jet issuing therefrom is unsteady, even 

 before it attains its maximum contraction ; and the same generally 

 occurs when the M'ater passes through a glass tube at a high ve- 

 locity/. In the latter case the water in contact with the sides of 

 the tube is impeded by friction, and thus forms, as it were, a 

 flexible canal through which the interior portion passes. A 

 reference to this fact will perhaps explain the experiment adduced 

 by M. Magnus in refutation of Venturi, where it was found, 

 that, on bringing the tube-end within a millimetre or so of the 

 suiface of the water, bubbles still entered. 



§3. 



The entrance of the vein into the fluid may be obsei'ved in an 

 indirect manner by the light of a candle, or better still, of a 

 lamp with a flat wick, the edge being turned towards the vein. 

 If the water be contained in a white basin, the shadows of the 

 bubbles are thrown upon the bottom ; and any alteration that 

 occu.rs at the point where the vein enters, is in this way rendered 

 much more distinct than if viewed directly. If the vein be 

 motionless, a defined shadow will be thrown upon the bottom of 

 the basin, and the base of the shadow will rest upon a veiy beau- 



