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XIV. PJumomena of a Water-Jet. Btj John Tyndall, Ph.D.* 



THE recent investigations of Professor IMagnus on the motion 

 of fluids t, have imparted an interest to some very common- 

 place occurrences, and disclose among others the singular fact, 

 that the bubbles formed by the pouring out of a glass of water — 

 an occurrence of hourlj'^ observation all over the world — had up 

 to the time of his inquiries remained without explanation. Ven- 

 turi had endeavoured to account for them by assuming that the air 

 adheres to the water and is carried by it downwards ; but a little 

 attention to the experiment compels us to discard this notion, 

 and to seek, as M. Magnus has done, an explanation elsewhere. 

 If a funnel be taken capable of holding about a pint of water, 

 and if a cork iixed upon one end of a narrow tube be fitted into 

 the funnel at the junction of the shank with the conical portion, 

 so that the narxow tube shall pass dowmwards through the shank, 

 we have a convenient apparatus for making many experiments 

 in connexion with this subject ; by regulating the diameter of 

 the tube, a jet of any required thickness may be obtained. 



The funnel thus arranged being placed at a convenient height 

 above a beaker glass and charged, a clear vein of fluid will pass 

 downwards into the glass. The upper part of the vein is steady 

 and limpid, and presents the appearance of a solid glass rod ; the 

 vein decreases in diameter as we descend, and after passing the 

 section where its contraction is greatest |, its steadiness ceases 

 and a quivering motion is exhibited. Savart has attributed this 

 to the resolution of the vein into detached masses. Following 

 the jet downwards, we find that these masses become more and 

 more attenuated ; and were the height sufiicient, they would 

 finally appear as a kind of water-dust, an example of which on 

 a large scale is furnished by the Staub-bach near Lauterbrunnen 

 in Switzerland. Travellers usually attribute the breakmg up of 

 the Staub-bach to atmospheric resistance, but the latter has com- 

 paratively little to do in the matter ; were the surrounding space 

 a vacuum, the same would be exhibited. 



The acceleration of gravity appears sufficient to account for 

 the contraction of the vein. The water in passing through the 

 tube has to contend with capillary attraction and the friction of 

 the sides ; on escaping into the air it is relieved from these, and 

 obeys the common law of falling bodies. The velocity of the 

 particles an inch below the end of the tube is greater than their 

 velocity close to the end, so that the vein must either contract 

 or be broken. The outline of the vein becomes thus a curve, to 

 which its axis is an asymptote. From this it would follow, that 

 if the action of gravity were annulled, the contraction ought not 

 * {/ominunicatcd liv the Author, 

 t Sec p. 1 of the present vohune of tliis Journal. 

 X Throughout the memoir this is called the contracted section. 



