Dr. Tyndall on some Pfusnomena of a Water -Jet. 109 



tiful caustic figure*. It is so easily produced experimentally, 

 that I forbear giving a drawing of it. The figm-e is due to the 

 refraction of the light by the cui-ved eminence on which the little 

 pillar of water is based. If we set a rod of any kind, a lead 

 pencil for instance, perpendicular in the water, we have the same 

 phsenomenon; the opake rod will appear cut in two by the 

 tigure ; and when the rod is raised, the figure Avill appear bright- 

 est immediately before it leaves the water. 



Sometimes a dark bag will appear suspended from the end of 

 the shadow. The same is observed with the rod when it is dry, 

 or when pushed dmvnwards through the fluid. In this case, 

 instead of capillary attraction, we have a recession of the fluid ; 

 and a hollow, instead of an eminence, is formed. On drawing 

 the rod upwards, the figure immediately starts into existence. 

 In the case of the jet, whenever the bag makes its appearance, 

 the contracted section is near the surface of the water ; and the 

 pulsations of the jet at this point are sufficient to beat away the 

 eminence and to establish a cavity in its stead. This is the case 

 which corresponds to the view of M. Magnus ; for were the siu-- 

 face now put in motion, bubbles would be formed. That the 

 bubble is due to lateral motion, is rendered very clear by this 

 mode of experiment : when no air enters, the surface is tranquil ; 

 but whenever a bubble is formed, a shock occurs which spreads 

 undulations all around. The bubbles which float on the surface 

 exhibit caustic figures similar to that at the base of the vein's 

 shadow ; when the water falls from a sufficient height, the flu.t- 

 tering of the figures receding from it is exceedingly pretty, the 

 vibration of the surface giving them all the appearance of yellow 

 butterflies plying their expanded wings f- 



Savart has traced the breaking of the jet to an intermittent 

 outflow, which probably has its origin in the vessel from which 

 the jet issues ; this will explain the word ' pulsations ' as used 

 above. To the same cause is probably due the rings which the 

 solid })ortion of the vein presents sometimes in entering the water, 

 and the little spiral thread which is obsei-ved quivering along it. 

 To the unassisted eye, the jet after it passes its contracted section 

 a])pears by no means broken, but merely in a state of quick 

 viliration. ' It is well known that the impression of an object 

 remains upon the eye about the eighth part of a second. Sup- 

 po.sing, therefore, the vein to be actually composed of drops, if 



* 1 may lit-re draw attention to the beautiful series of these figures at 

 present beiiiK puhhsheil by Knj^el and Schellbiich in BerUu. 



t Tlie hglit must not fall from above, l)ut as nearly horizontal as possible. 

 A convenient way of producing the l)ul)l)les is siuijjly to \nmY the water out 

 of a teapot, at the same time moving the hand which holds the teapot over 

 the surface. 



