106 Dr. Tyndall on some Pfumomena of a Water- Jet. 



to appeal'. Tliis is the case. If a jet be projected sideways, 

 its horizontal portion exhibits no sensible contraction ; and if 

 projected vertically upwards, the contraction becomes negative, 

 or in other words, the jet expands. The Trafalgar Square foun- 

 tains are an exjieriment in point, though probably no great dif- 

 ference will in their case be discernible ; for when the velocity 

 with which the water leaves the orilice is considerable, the areas 

 of its different sections become nearly alike. 



To return to the bubbles. Venturi, as remarked before, ima- 

 gines them to be carried downwards by adhesion to the jet. M. 

 Magnus, if I understand him aright, refers their production to 

 two simultaneous causes : first, to the cavity formed at the point 

 where the jet and fluid meet ; and secondly, to a motion of the 

 surface of the latter, which throws a roof over the cavity and thus 

 incloses the air. The true birthplace of the bubbles is undoubt- 

 edly here indicated ; it will perhaps be pennitted me to furnish 

 one or two additional proofs of this, and to look a little nearer 

 into then" origin and phsenomena. 



§1- 



A thin glass bottle was half-filled with water and corked air- 

 tight. Through the cork a small tube passed dowTiwards until 

 its end dipped beneath the surface of the water ; the bottle being 

 placed over a lamp, soon boiled; steam was generated, which, 

 reacting upon the surface of the water, forced it to issue through 

 the tube in a jet. The portion of the tube outside the bottle 

 was so bent that the jet descended vertically*. According as 

 the pressure increased, the contracted section receded from the 

 end of the tube ; and when at a certain distance therefrom a vessel 

 of water was so placed that its surface intersected the jet a little 

 above the contracted section, — no bubbles entei'ed. The lamp was 

 now lowered ; the tension of the steam being thus reduced, the 

 contracted section, in consequence of the diminished pressure, 

 approached nearer to the end of the tube, and finally appeared 

 above the surface of the water. From this time forward bubbles 

 were produced, and increased in number as the contracted section 

 ascended. 



This experiment appears to disprove the notion of Venturi ; 

 for were the bubbles due to adhesion, the greater the velocity of 

 the jet the greater ought to be the quantity of air carried down ; 

 but this is directly the reverse of the fact, the velocity in the above 

 instance being greatest when no bubbles at all were produced. 

 Nor was their absence at that time due to the tranquillity of the 

 sm*face ; for even when the latter was stirred briskly with a glass 

 rod, not a single bubble entered. 



* The end of the tube was drawTi out, and the water before it left it had 

 to pass through a uniform narrow bore about 4 inches in length. 



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