56 Lord Rayleigh on the Crispations of Fluid 
impulses with the hand, and the weights were adjusted until 
the period corresponded to one mode of free vibration of the 
pool of mercury. When the adjustment is complete, a very 
small vibration of the plank throws the mercury into great 
commotion, and unless the vessel is deep there is risk of the 
fluid being thrown out. The question now to be decided is 
whether, or not, the vibrations of the mercury are executed in 
the same time as those of the plank. 
On March 18 the plank was adjusted so as to excite that 
mode of vibration of the mercury in which there are two nodal 
diameters. Two other diameters bisecting the angles between 
these give the places of maximum vertical motion. At one 
moment the mercury is elevated at both ends of one diameter 
and depressed at both ends of the perpendicular diameter ; 
half a period later the case is reversed. The frequency of the 
fluid vibrations could be counted by inspection, and was found 
to be 30 (complete) vibrations in 15 seconds, or exact] v two 
vibrations per second. The vibrations of the plank were 
counted by allowing it to tap slightly against a pencil held in 
the hand. In five seconds there were 21 complete vibrations, 
i. e. 4i vibrations per second, almost exactly twice as many 
as was found for the mercury. The measurements were re- 
peated several times; and the general result is beyond question. 
On another occasion the mode of fluid vibration was that in 
which there is but one nodal diameter, the fluid being most 
raised at one end of the perpendicular diameter and most de- 
pressed at the other end. The frequency of fluid vibration 
was 30 / 22 = 1-36; while that of the plank was 27 / 10 = 2-7. 
Here again the fluid vibrations are proved to be only half as 
quick as those of the support. 
The mechanics of the question are considered in a commu- 
nication to the Philosophical Magazine for April 1883, to 
which reference must be made. Merely to observe the phe- 
nomenon, it is sufficient to take a porcelain evaporating-dish 
containing a shallow pool of mercury 2 or 3 inches in dia- 
meter, and, holding it firmly with both hands, to impose upon 
it a vertical vibratory motion. After a few trials of various 
speeds it is possible to excite various modes of vibration, in- 
cluding those referred to in connexion with the plank. The 
first (with two nodal diameters) is more interesting in itself, 
and is more certainly due to a vertical as opposed to a hori- 
zontal vibration of the support. The gradually shelving bank 
presented by the dish adds to the beauty of the experiment 
by its tendency to prevent splashing. 
Dr. Matthiessen, in the papers referred to, records a long 
series of measurements of the wave-lengths of crispations cor- 
