1885.] On the Formation of Vortex Rings by Drops. 419 



rough outline of the theory we adopt. It will be seen that the most 

 important property of the liquid concerned is its viscosity — the 

 viscosity must he such that when the drop has become disk-shaped 

 there should be enough vortex motion in it to cause it to break up ; if 

 the viscosity is too small the vortex motion in it will not have had time 

 to spread far by the time the drop has become disk-shaped, and so the 

 drop will continue to flatten and get into thin sheets with streaks of 

 vortex motion in it instead of breaking up into a ring, whilst if the 

 viscosity is too great the vortex motion will be dissipated before the 

 drop becomes disk-shaped. 



We shall now give the experiments which led us to form the 

 conclusions. We began by investigating the change in the shape of 

 the drop before it became disk-shaped. 



Shape of the Drop before it becomes a Ring. 



To study the change of shape of the drop as it falls through the 

 liquid we have had recourse to instantaneous illumination, and have 

 .used for this the bright spark formed at breaking in a mercury cup a 

 circuit containing an electromagnet. It was necessary that we should 

 be able to illuminate the drop at any point of its fall, and it was 

 obviously convenient to make the actual fall of each drop start a set 

 of processes which should result in the spark. The figure shows 

 diagrammatically the arrangement we have used. 



The drop is formed at the end of a small piece of glass tubing 



Fig. 3. 



