uniform motion enables the reader, after having seen the perforated
slips once or twice, to determine fairly well the time which elapses
between each pressure of the button.--_The Engineer._

[Illustration: WATCHMAN'S DETECTER]

* * * * *




INTEGRATING APPARATUS.


At a recent meeting of the London Physical Society, Mr. C. Vernon Boys
read a paper on "Integrating Apparatus." After referring to his
original "cart" machine for integrating, described at a former meeting
of the society, he showed how he had been led to construct the new
machine exhibited, in which a cylinder is caused to reciprocate
longitudinally in contact with a disk, and give the integral by its
rotation. Integrators were of three kinds: (1) radius machines; (2)
cosine machines; (3) tangent machines. Sliding friction and inertia
render the first two kinds unsuitable where there are delicate forces
or rapid variation in the function to be integrated. Tangent machines
depend on pure rolling, and the inertia and friction are
inappreciable. They are, therefore, more practical than the other
sort. It is to this class that Mr. Boys' machines belong. The author
then described a theoretical tangent integrator depending on the
mutual rolling of two smoke rings, and showed how the steering of a
bicycle or wheelbarrow could be applied to integrate directly with a