156

THE VIBRATING

TRACT 35.

the angle of vibration, as measured by the simple equal de-grees of a circle ; or into unequal parts accordingto the chords,or to the versed sines of the arcs, to measure either the ve-locity of the vibration, or the force and strength of the pow-der : the arch in my instrument had all those three scales ofdivisions on it. The divisions in these scales, answering tothe angle of any recoil, are pointed out by a concentric in-dex, fitted on the; axis of vibration, I33; means of a round holeer socket, with which it embraces pretty closely the roundpart of the axis of the stem, but capable of being turned ea-sily about, it by the hand. By means of a spring, the roundend or socket of this instrument is pressed sideways, alongthe direction of the axis, always moderately tight against thesocket of the stem, which is firmly brazed to the same axis ;thus connecting the index' and the stem slightly together; bywhich means, these two always vibrate in conjunction withthe arch, unless when the index is stopped by some obstacle^When the machine is at rest, and the index brought to pointto the beginning of the divisions on the arch, an additionalpiece fixed on the index bears against a stop-bar, fixed acrosstin; frame of the machine; so that, when the powder is fired,the gun and arch together vibrate backwards, leaving theindex at rest, bearing still against the stop, and the divisions®f the arch passing by it, till the gun has recoiled to the ut-most extent that the force of the explosion can impel it:thee, returning again, it brings the index along with it (be-cause of their friction in consequence of the pressure of thespring) still pointing to the proper recoil division on the arch,showing the extent of the vibration ; which, on gently stop-ping the vibrations, is easily read off, and noted down.

6. The circumstances which are peculiar to this eprou-vette, and by which it differs from all others, as far as knownto me, are as follow.

1st. The convenient manner of placing the arch, whichmeasures the recoil,below the axis of the machine.

Sind’. The divisions on this arch being made, not only,according to equal degrees, but also according to the chords