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 latter, this double movement aided by the continuity of vision will produce on the eye the impression of a wavy line. 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.

Fig. 48.-Manometric flames simultaneously given by two tubes at the octave.

If one dot passes in the time of

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 1 Extracted from a pamphlet by the inventors. See also Proceedings of the Royal Society for April, 1877.

III. (1) Photographic Methods.-Professor Blake of Broner University has recently contributed to the American Journal of Science an ingenious method of photographing vibrations.

Jaiversity

valuable contrivance; but it is obvious that as with a CAN

It is not necessary to enter into all the details of

fork vibrating at a standard rate, velocities of rotation can be accurately determined, so with a steady standard of rotation, the error of a tuning-fork from its theoretical vibration-number can be immediately detected.

Fig. 50.-Professor Blake's method of photographing vibrations.