and their relation to the Velocities of Currents. 195 
30. As an interesting coincidence, it may be mentioned that 
Poisson, in his memoir before referred to*, gives the results of 
four experiments on the velocity of waves made by Biot. In 
this case the waves were produced by suddenly withdrawing from 
the water a partially immersed solid of revolution. The velocity 
of the wave was found to vary with the form of the body, and 
with the radius of its section at the water’s level. In one case, the 
body being an ellipsoid, and the radius in question equal to 2 of 
an inch, the velocity was about 54 inches per second ; in another 
case, where the body was a sphere and the radius of the section 
1+ inch, the velocity was 7:87 inches per second. The waves 
experimented upon by Weber had a far greater velocity: they 
were produced by allowing a column of liquid suspended in a 
tube to descend suddenly into the general mass ; and their velo- 
cities varied from 17 to 34: inches per second. 
31. In the foregoing experiments, the velocity > was deter- 
mined by causing an immersed cylinder to move with a given 
velocity in still water. A few experiments were next made with 
a view of ascertaining the value of X when a cylinder is simply 
immersed in a current of known velocity. According to art. 10, 
the sine of half the angle 20 between the branches of the ripple 
caused by immersing a cylinder in a current is inversely propor- 
tional to the velocity v of the current, and directly proportional 
to the velocity >) in question. In fact it was there shown that 
tan 6 
\/ ete ge ree (48) 
To determine the velocity v at any point of the surface of a 
current, a Wollaston’s current-meter was used. As is known, 
this instrument consists of a screw which is made to rotate by 
the force of the current. The mstrument must be immersed to 
the depth of two inches at least, in order that the screw may be 
completely covered by the water; and when so immersed, the 
rotation of the screw can be communicated at any instant to a 
divided wheel, and the communication as suddenly broken. The 
space described by the current during the interval between 
making and breaking this communication—an interval which 
can be measured by means of an ordinary watch with a seconds’ 
hand—is at once read off on the wheel, the instrument having 
been. previously carefully graduated. 
The angle 0 was determined by means of a simple instrument, 
to which we may Sive the name of ripple-meter. It consisted of a 
glass plate (BCD E, PI. IV. fig. 3) 5 inches square, through which, 
at a point A, a hole ;4,th of an inch in diameter was drilled in 
* Mémoires de l’Acad. Roy. des Sciences de VInstitut. Année 1816, 
vol, i, p. 173, a 
A=v sin 0=v 
