204 Scientific Proceedings, Royal Dublin Society. 



On viewing the jet by intermittent light, such as that given by 

 a rotating disc with slots in it, when interposed between the eye 

 and the jet, or by the sparks from a Ley den jar charged from an 

 induction coil, we can see the true shape of the drops, and discern 

 their mode of formation. 



The device I adopted was to put the spark-gap behind a con- 

 densing lens, in front of which the stream of water flowed. By 

 means of a second lens an image of the stream was formed on a 

 ground-glass screen, which was very brilliant when viewed from 

 behind. If the screen were replaced by a sensitive plate (cutting 

 off extraneous light), a photograph could be obtained of the jet at 

 any instant by allowing a single spark to pass ; this can easily 

 be done by moving the hammer of the coil by hand. These 

 photographs afford a very convenient way of studying the jet. 



Towards the lower part of the clear column alternate swellings 

 and contractions may be seen, while at the extremity a drop forms 

 which extends itself horizontally and leaves the column with its 

 major a.xis horizontal ; not, as is generally thought, vertical. 

 There is then left a slender neck of liquid, which next detaches 

 itself, and, gathering together by surface-tension, forms a smaller 

 drop. (Plate XIX., fig. 1.) 



The larger drops as they fall do not at once assume the 

 spherical form, but execute vibrations, being alternately elongated 

 and compressed in the direction of their axis of symmetry. The 

 time of vibration r, of course, depends upon the nature of the 

 fluid and the size of the drop. 



Lord Eayleigh gives this formula * : — 



-i 





where V = volume of the vibrating mass, p the density of the 

 liquid, and T the surface-tension. The time of vibration of a 

 drop of water 2'5 cms. in radius is very nearly 1 second. 



There is nothing very regular in the appearance of such a jet; 

 the drops are moving with different velocities ; frequent collisions 

 occur, giving rise to the fine spray that may be seen in the lower 

 part of the stream. If a sudden blow be given to the stand 



1 See Lord Eayleigh's "Scientific Papers," vol. i., p. 392. 



