a Sphere in a Viscous Fluid. 523 



foil capable of rotation about a horizontal axis. The depression 

 of this platform makes contact between two mercury cups 

 atT. 



The method of procedure is as follows : — The trap at T is 

 adjusted, the rocking-key is placed so as to give contact at "R, 



Fig. 2. — Diagrammatic View showing the Method of Release. 



B is the battery comprising a couple of two-volt storage-cells. 



■C is the small electromagnet by which the ball to be photographed is 



suspended. 

 D is an electromagnet similar to C by which the ball working the release 



is suspended. 

 E is a larger electromagnet which releases the weight for moving the 



plate horizontally. 

 F is an electromagnet for releasing the Thornton-Pickard shutter. 

 P, P', Q are mercury keys ; R, S is a rocking-key ; and T is a trap which 



makes contact when the ball falls from D. 



and contact is made at P, P', and Q. The balls are sus- 

 pended from the electromagnets at and D, through which 

 the current now runs. The room having been darkened, the 

 photographic plate is inserted in its sheath, and the strip of 

 magnesium ribbon is ignited. When it is well alight contact 

 is destroyed at Q, and the exposure proceeds automatically. 

 For breaking contact at Q sets free the balls simultaneously; 

 the ball falling from D makes contact at T, and a current is 

 sent through the electromagnet E so that the suspended 

 weight is released. This weight in its fall strikes the rocking- 

 key so as momentarily to give contact at S instead of R. The 

 current of one cell now passes through P, S, and F, and the 

 Thornton-Pickard shutter is opened. 



When the correct height of D above T has been determined 

 for some one position of C, we have only to raise or lower 

 and D by the same amount to ensure a successful exposure 



