524 Mr. H. S. Allen on the Motion of 



for any desired fall. In practice failures sometimes occurred, 

 mainly in consequence of the contact at T having to be made 

 under water, the mercury surfaces there being easily con- 

 taminated. 



The general appearance of the apparatus may be gathered 

 from the figure (Plate I. fig. 1). The vessel in which the 

 fall took place may be seen on the extreme left. In order 

 to prevent the transmission of vibrations to the liquid from 

 the electromagnetic engine (which cannot be seen in the 

 figure), this vessel rested on a stand separated from the 

 table supporting the rest of the apparatus. The aluminium 

 shutter is partly visible between the front of the vessel and 

 the cardboard shade. The inside of this shade, as well as 

 other parts of the apparatus exposed to the light, was painted 

 a dull black. Immediately in front of the lantern are the 

 two storage-cells for working the release. On the right of 

 the picture is the vessel in which the timing sphere falls. 

 The falling weight and its release are on the side of the table 

 not shown in the figure. 



The experiments were carried out in a cellar in the 

 Cavendish Laboratory. 



Results. 



An inspection of the reproductions in Plates I. & II. will show 

 the character of the photographs obtained. Fig. 1 (PI. II.) in 

 which a glass scale is photographed, shows the actual size of 

 the aperture in the screen. Special care was taken that the 

 upper and lower edges of this aperture should be sharply defined 

 and horizontal, so that they might serve as fixed lines from 

 which to measure the vertical displacements of the spheres. 

 Figs. 2-3 (PI. I.) & 2-4 (PL II.) illustrate the manner in which 

 successive images of the aperture are formed on the moving 

 plate. It will be noticed that at least three flashes occurred 

 during the time the shutter remained open. Since the shutter 

 opened from above downwards, the lower part of the first image 

 and the upper part of the third are generally cut off. In some 

 of the photographs the horizontal velocity of the plate has not 

 been great enough to separate the images completely. 



When the falling sphere happens to be in the path of the 

 beam of light, we obtain two or more images of the shadow. 

 An examination of one of these images will show that the 

 opposite edges are most sharply defined in a direction 

 sloping downwards from left to right. This is a consequence 

 of the finite duration of a flash. The direction of the sharp 

 edge is that of the velocity compounded of the separate 

 velocities of the ball and the plate. 



