y the curve with the straight line represents the ratio of the
velocity of the fork to the velocity of recoil. If a be the amplitude
of vibration, considered constant, v the velocity of the fork at the
midpoint of its path, r the velocity of recoil, [alpha] the angle made
by the tangent to the curve with the straight line at the point of
intersection, and t the line of a complete vibration; then, v =
2[pi]a/t; r = v/tan [alpha].

Jervis-Smith.

F. Jervis-Smith's tram chronograph (_Patents_, 1894, 1897, 1903) was
devised for measuring periods of time varying from about one-fourth to
one twenty-thousandth part of a second (_Proc. Roy. Soc._, 1889, 45,
p. 452; _The Tram Chronograph_, by F. Jervis-Smith, F.R.S.). It
consists of a metal girder having a T-shaped end. This carries two
parallel steel rails, the edges of which lie in the same vertical
plane. The girder, which is slightly inclined to the horizontal plane,
is geometrically supported, being carried at its end, and at the
extremities of the T-piece, on a V-groove, trihedral hole and plane. A
carriage or tram furnished with three grooved wheels runs on the
rails, and a slightly smoked glass plate is attached to its vertical
side. The tram in the original instrument was propelled by a falling
weight, but in an improved form one or more spiral springs are
employed. All time traces are made immediately after the propelling
force has ceased to act. The tram is brought to rest by a gradually
applied brake, consisting of two crossed leather bands stretched by
two springs; a projection from the tram runs between the bands, and
brings it to rest with but little lateral pressure. When, for certain
physiological experiments, a low velocity of traverse is required, a
heavy fly-wheel is mounted on the tram and geared to its wheels. A
pillar also mounted geometrically, placed vertically in front of the
carriage, carries the electromagnet style or signals and tuning-fork
which can be brought into contact with the glass by means of a lever.
Also styli are used which depend for their action on the displacement
of one or more wires under tension or torsion carrying a current in a
magnetic field, the condition being such that no magnetic lag due to
iron armatures and cores exists. Two motions of a slide on the pillar,
viz. of rotation and translation, allow a number of observations to be
made. The traces are