and their relation to the Velocities of Currents. 197 
that, with one and the same immersed body, the value of X varies 
somewhat with the veloclty of the current; but there can be 
little doubt that, under more favourable circumstances, this 
variation would be found to be far less than that indicated by 
the Table. 
In many cases no ripple whatever was produced by the immer- 
sion of the cylinder of ivory attached to the ripple-meter; and in 
all such cases the current-meter indicated a velocity less than 7 
inches per second; the quickest of such currents in fact had 
only a velocity of 25 feet per minute, or 5 inches per second. 
This corroborates the explanation given in art. 11, where it was 
foreseen that a body immersed in a current whose velocity was 
less than that of the wave, would allow the water to flow past 
it without visibly rippling its surface. 
The phenomenon represented by the second figure of art. 10, 
where the branches of the ripple turn their concavities towards 
each other, and which was shown to be a necessary consequence 
of Weber’s statement, that the velocity of the wave diminishes as 
its radius increases, was never observed. A slight concavity, 
however, might easily have escaped detection. 
32. The velocity \ being once determined for any ripple-meter, 
we can of course by its means determine, conversely, the velocity 
of a current, provided the latter exceeds the limit A. For this 
purpose I made use of a more convenient, though perhaps less 
accurate ripple-meter, with a description of which I will conclude 
the present paper. AB and A Cn fig. 4 represent two strips of 
brass, each 3 inches long, and made toturn round A. Theends 
B and C of these strips also turn on axes at the extremities of 
two brass stirrups B F and C G, through which passes a wooden 
scale D EK divided into twentieths of an inch. The cylinder, of 
the same dimensions as before, by whose immersion in the cur- 
rent ripples are produced, is pushed through an aperture in the 
joint A. The stirrup BF being fixed at the zero of the scale, is 
held there by a clamping screw F, and the stirrup C G is made 
to slide along the scale until the strips AB, AC are parallel 
to the branches of the ripple. This adjustment once made, the 
distance BC, as read off from the scale, being directly. propor- 
tional to 2 sin @, is clearly inversely proportional to the velocity 
of the current (arts. 10 and 31). 
The decrease in velocity from the centre to the banks of a 
stream is clearly indicated by this little instrument. As an illus- 
tration, I give the results of a few observations made on a mill 
stream at Brimscombe near Stroud. A wocden plank was thrown 
across the stream, which was about 7 feet 6 inches wide, and 
upon it, commencing at one bank, marks were made 9 inches 
apart. By kneeling on the plank, the ripple-meter was im- 
