Crispations of Fluid resting upon a Vibrating Support. 51 
The vibrations of the bar were maintained electromagnetically, 
as in tuning-fork interrupters, with the aid of an electro- 
magnet placed under the centre, the circuit being made and 
broken at a mercury-cup by a dipper carried at one end of the 
bar. By calculation from the dimensions*, and without 
allowance for the load at the centre, the frequency of (com- 
plete) vibration is 33. Comparisons with a standard tuning- 
fork gave more accurately for the actually loaded bar a fre- 
quency of 31. 
The vibrating liquid standing upon the plate presents ap- 
pearances which at first are rather difficult to interpret, and 
which vary a good deal with the nature of the liquid in 
respect of transparency or opacity, and with the incidence of 
the light. The vibrations of the liquid, whether at the rate of 
31 per second, or, as in fact, at the rate of 15^ per second, are 
too quick to be followed by the eye ; and thus the effect ob- 
served is an average, due to the superposition of an indefinite 
number of components corresponding to the various phases of 
vibration. 
The motion of the liquid consists of two sets of stationary 
vibrations superposed, the ridges and furrows of the two sets 
being perpendicular to one another, and usually parallel to the 
edges of the (rectangular) plate. Confining our attention for 
the moment to one set of stationary waves, let us consider 
what appearance it may be expected to present. At one 
moment the ridges form a set of parallel and equidistant lines, 
the interval being the wave-length. Midway between these 
are the lines which represent at that moment the position of 
the farrows. After the lapse of ^ period, the surface is flat; 
after another \ period, the ridges and furrows are again at 
their maximum development, but the positions are exchanged. 
Now, since only an average effect can be perceived, it is clear 
that no distinction can be recognized between the ridges and 
the furrows, and that the observed effect must be periodic 
within a distance equal to half a wave-length of the real 
motion. If the liquid on the plate be rendered moderately 
opaque by addition of aniline blue, and be seen by diffused 
transmitted light, the lines of ridge and furrow will appear 
bright in comparison with the intermediate nodal lines where 
the normal depth is preserved throughout the vibration. The 
gain of light when the thickness is small will, in accordance 
with the law of absorption, outweigh the loss of light which 
occurs half a period later when the furrow is replaced by a 
ridge. 
The actual phenomenon is more complicated in consequence 
* 'Theory of Sound,' § 171. 
E2 
