DETERMINATION OF THE RATE OF VIBRATION OF TUNING-FORKS. 
5 
before being attached to the drum; as the division of a long strip of paper by means 
of compasses was found tedious and somewhat inaccurate, it was effected by rolling a 
toothed wheel along the paper, thus obtaining fine equidistant marks through which 
lines were ruled in Chinese white by means of a drawing pen. For convenience of 
counting, every fifth and tenth line was made longer than the others, and after attach¬ 
ing the strip to the drum, figures were placed opposite every tenth line. The two 
strips which are most generally used for the determinations have 486 and 985 lines 
respectively. 
Opposite the graduated paper on the drum is fixed a microscope placed in a 
horizontal position ; the instrument is suspended from a shelf above the table on which 
the chronograph rests, and the shelf also supports the clock. In the sub-stage a 2-inch 
objective is placed which produces an image of the graduations at the focus of the 
object glass of the microscope, the portion of the scale used being illuminated by a 
lamp and condensing lens. At the common focus of the lenses is placed the tuning- 
fork, the stem of which is surrounded by a piece of sheet lead and fixed in a vice 
attached to the table, the upper part of one of the prongs being in such a position that 
about one-third of the field of view is cut off by the prong, Plate 3, fig. 6. The 
lower part of tire fork is surrounded by a glass case made of four strips of glass glued 
to a square piece of wood through which the stem of the fork passes. A hole is bored 
in one of the plates of glass and through this passes a thermometer graduated to '2 C., 
the bulb being placed as close as possible to the fork without touching it. A plan of 
the arrangement is shown in Plate 3, fig. 8. 
In the eyepiece of the microscope a horizontal hair and a vertical scale are fixed 
(Plate 3, fig. 6). A double-bass bow is suspended from the shelf by two parallel 
strings so that the operator may keep the fork in vibration while observing through the 
microscope. When fork and drum are at rest the white lines on the drum are seen 
across the field of the microscope but cut off on one side by the fork (Plate 3, fig. 6). 
When the drum rotates the lines can no longer be seen, but if at the same time the 
fork is in vibration wave-figures are produced extending from its edge. When the 
drum is rotating with such a velocity that a line passes over a distance equal to the 
interval between two adjacent lines during one vibration of the fork, a stationary wave 
(Plate 3, fig. 7) is seen of a length equal to the distance between two lines, and it only 
remains therefore to count the lines which pass in a known period of time to determine 
the number of vibrations of the fork. It was not found possible to obtain a perfectly 
uniform rotation of the drum by the regulator alone, as the slight irregularities of the 
clock-work and pulleys were sufficient to cause unequal rates of rotation. In order to 
obtain a final regulating power a piece of string connects the axis of the drum with a 
pulley, g (Plate 3, fig. 8), which can be turned by the operator’s left hand. The 
motion of the wave upwards or downwards indicates that the drum is going too fast or 
too slow, and a gentle check, or assistance given by the hand to the axis of the pulley, 
is enough to keep the rotation uniform. 
