466 
We shall now discuss a few more particulars of the arrangement 
and the use of the apparatus. 
€ 
3. Determination of velocity. In order to get an insight into the 
course of the velocity in the movement along the guides the position 
of DE (see fig. 1) was determined corresponding with 16 different, 
equidistant positions of the fly-wheel. The graph indicating the con- 
nection between the positions of the fly-wheel and the deviations, 
has about the shape of a sinusoid, but the two halves of the curve 
are not symmetrical, and in particular, the course of the graph in the 
neighbourhood of the two boundary values is not exactly the same, 
as already appears from a consideration of fig. 1, when A is imagined 
to move along the dotted circle. 
The velocity-time curve can be graphically derived from the path- 
time curve. At the maxima the velocity is practically constant over a 
distance of 20 em, of which only 10 cm. are used. As was already 
stated the maximum velocity amounts to 10005 for 184 revolu- 
tions per minute, and proportionally the calculated velocity 
can be derived for another number of revolutions. Whether 
really the maximum velocity should be taken into account in the 
calculation, depends further also on the position of the moving 
column at the moment that the shutter, before the objective of 
the telescope, transmits light. Im some cases this position did not 
correspond to that of the maximum velocity, which circumstance 
was of course taken into account in the interpretation of the photos. 
In the most accurate experiments the maximum velocity of the 
column was directly measured (for the method used see one of the 
following communications), which renders us independent of the 
supposition that the fly-wheel possesses a constant angular velocity. 
It appeared in the experiments that the machine ran more uniformly 
when the fly-wheel rotates clockwise (seen from the side of the larger 
fly-wheel) than in the opposite direction. Of course this favourable 
direction was always used. 
4. Shutter. Only at the moment that the machine has its 
greatest velocity may the light be admitted to the photographic 
plate. The following arrangement was made for this purpose. The 
axis of the fly-wheel is provided with a toothed wheel, which 
engages with a second toothed wheel with double the number of 
teeth. An insulated brass ring with cams is fitted on each 
side of this second wheel. The cams on the two rings are placed 
