118 Proceedings of Royal Society of Edinburgh. [sess. 
This plan of making the dish actually touch the fork was adopted 
in all the subsequent experiments, and yielded consistent results. 
The cause of the failure of the earlier methods is not perfectly 
clear. The average value of the wave-length in mercury for 256 
vibrations per second was found from the ten best photographs to 
be 0*108 cm., the values varying from 0*102 to 0*112 cm. If we 
take Quincke’s value for T, viz., 540 dynes per centimetre, this 
would correspond with a note of 445 vibrations per second. 
In the cases in which a dipper was used the explanation may be 
found in the circumstance that two impulses are given to the 
mercury for each complete vibration of the fork, one as the dipper 
enters, the other as it leaves, the liquid. If this is the true source 
of error it might be supposed that there is a somewhat similar cause 
acting when the vessel rests on the sounding-box, for in that case 
too it must receive two impulses for each vibration. On the other 
hand, when the dish presses lightly on the prong of the fork it 
would receive only one impulse for each vibration. But if the 
above explanation were complete one would expect to get ripples 
due to a note an octave above that of the fork, whereas those actually 
got correspond almost exactly to of the same octave, the fork 
giving the C below. I propose to examine this point more carefully 
hereafter. 
As finally arranged, the driving-fork was placed at a distance and 
was connected by loose wires with the fork — one of the series 
belonging to a Helmholtz vowel-sound apparatus — actually used to 
set up the ripples. This was placed on a stone slab in front of a 
window so that it could be illuminated by the light from the sky. 
The vessel usually employed was an ebonite developing dish, either 
of half-plate or quarter-plate size. This dish was supported near 
the fork in such a way as to touch one of the prongs at a point as 
low down as possible. The mercury or other liquid was then poured 
in till a depth was reached, beyond which any increase seriously 
diminished the distinctness of the ripples. If the contact with the 
dish be made at a point taken at random a very complex and 
unsteady series of ripples will usually be produced, but with a little 
care it is possible to find a point which yields two steady series of 
ripples crossing each other at right angles. Except with the eye 
nearly on a level with the surface of the liquid, the ripples can be 
