a silver bead attached to the end of a rod produced, when set in vibration, the patterns engraved above by virtue of the momentary persistence of retinal impressions. (2) It is also demonstrated by the "Vibration Microscope" of Helmholtz mentioned elsewhere.

(3) Koenig's Manometric Flames.-This method consists

in transmitting the movements of sono-
rous waves through a thin membrane
of caoutchouc to a small reservoir of
ordinary gas connected with a flame.
A capsule of wood or metal is divided
in the middle by such a membrane,
forming two compartments. One of
these is continuous with the vibrating
mass of air, the other with the ordi-
nary gas mains, and with a burner
(Fig. 45). The membrane thus forming
part of the wall of the pipe, yields to
the alternate condensation and rare-
faction of the air, and transmits these
alterations of pressure to the stream
of gas.
The result is that the flame
flickers up and down in coincidence
with the vibration to be observed (Fig.
46). To render the flickerings distinct
from one another they are received on
a rotating minor. While the flame
burns steadily, there is thus formed a
continuous band of light. But if the
capsule is connected with a tube sound-
ing its foundation notes the flame takes
the form shown in the first figure
(Fig. 47). If the octave be sounded it
assumes that in the second.

If two tubes simultaneously give the fundamental and its octave, we obtain the following appearance (Fig. 48). If the interval be that of a third, the flame takes the more complicated shape shown below (Fig. 49).

Clarke and MacLeod's Method.A new method for investigating and determining velocities of rotation has recently been published in a paper read before the Royal Society in April 1877, which besides its

other useful applications, affords an excellent, probably the best, method of determining the period of tuning-forks. It

Fig. 46.-Apparatus for the comparison of the vibratory movements of two sonorous tubes.

"If a circle of dots equally spaced rotates in front of a tuning-fork provided with a lens or mirror; then if the fork is so arranged that it imparts to the image of the dot a

Fig. 47.-Manometric flames. Fundamental note, and the octave above the fundamental note.

movement at right angles to the motion of the double movement aided by the continuity of produce on the eye the impression of a wavy line.

latter, this vision will The form

of this wave line will depend on the ratio of the number of vibrations of a fork in any given time, to the number of dots which pass before it in the same time. With certain simple ratios, waves are produced whose forms are easily recognisable.

If one dot passes in the time of a double vibration, a single wave is produced. If two dots pass in the same time, a double wave is produced. The double figure seems the best suited for measuring velocities.

H

"Now if the exact ratios stated above obtain, the waves corresponding to them will appear to be stationary. If however the speed of rotation is a little too quick for the exact ratio,

the wave will have a slow progressive motion in the same direction as that of the dots, while if the speed of rotation is a little too slow, the wave will move slowly in the opposite direction." 1 Extracted from a pamphlet by the inventors. See also Proceedings of

the Royal Society for April, 1877,