OF FLUIDS ON VIBRATING ELASTIC SURFACES. 
335 
of the vibrating plate exerted upon the fluid was principally employed. These 
were apparently permanent elevations, at regular intervals, strongest at the plate, 
projecting directly out from it over the surface of the water, like the teeth of 
a coarse comb gradually diminishing in height, and extending half or three 
quarters of an inch in length. These varied in commencing at the glass, or 
having intervening ridges, or in height, or in length, or in number, or in 
breaking up into violently agitated pimples and drops, & c. according as the 
plate dipped more or less into the water, or vibrated more or less violently, 
or subdivided whilst vibrating into parts, or changed in other circumstances. 
But when the plate (sixteen or seventeen inches long) dipped about one sixth 
of an inch, then four of these linear heaps occupied as nearly as possible the 
same space as four heaps formed with the same plate in the former way (83) 
and accompanied with the same sound. 
114. By fixing a wooden lath (69) perpendicularly downwards in a vice, 
plates of any size or form could be attached to its lower end and immersed 
more or less in water ; and by varying the immersion of the plate, or the length 
of the lath, or the place against which the exciting rod ( 71 ) was applied, the 
vibrations could be varied in rapidity to any extent. 
115. On using a piece of board at the extremity of the 
lath, eight inches long and three inches deep, with pieces of 
tin plate four inches by five, fixed on at the ends in a per- 
pendicular position to prevent lateral disturbance at those 
parts, very regular and beautiful ridges were obtained of any desired width, 
fig. 28. These ridges, as before, formed only on the wood, and were parallel to 
the direction of its vibration. They occurred on each side of the vibrating 
plane with equal regularity, force and magnitude, but seemed to have no con- 
nection, for sometimes they corresponded in position, and at other times not ; 
the one set shifting a little, without the others being displaced. 
116. It could now be observed that the ridges on either side the vibrating 
plane consisted of two alternating sets ; the one set rising as the other fell. 
For each fro and to motion of the plane, or one complete vibration, one of the 
sets appeared, so that in two complete vibrations the cycle of changes was 
complete. Pieces of cork and lycopodium powder showed that there was no 
important current setting in the direction of the ridges ; towards the heads 
2 x 
MDCCCXXXI. 
