and their relation to the Velocities of Currents. 19] 
with equal accuracy the portion of this ripple between the jet 
and cusp coincided successively with the corresponding portions 
Aa,C, AaC of the theoretical ripples. To test the coincidence 
of the portion of the ripple outside the circle described by the 
jet (A X Ein fig. 7, Plate I.), a hole was drilled through the glass 
plate A A! at a distance of 24 inches from its centre, and a solid 
cylinder thrust through the same so as to be partially immersed 
in the water of the bath. The mill was turned as before by the 
jet A’ alone, the aperture at A being closed; and when the velo- 
city of rotation was properly regulated, it was found that the 
complete actual ripple produced by the immersed cylinder coin- 
cided very well with as much of the theoretical one as could be 
traced upon the glass plate. 
27. The slight divergence from perfect coincidence between 
the theoretical and actual ripples being sufficiently accounted for 
by the conditions under which the experiments were made—the 
limited extent of the bath, the disturbing effect of the portion of 
the axis O F immersed in the water, and the magnitude of the 
jet (which in theory was disregarded),—the accuracy of the prin- 
ciples upon which the foregoing theory is based may be consi- 
dered as sufficiently established, and we may proceed at once to 
give an account of the measurements from which an approximate 
value of the velocity \ with which the waves, producing the rip- 
ples in question, are propagated may be deduced. 
For this purpose the curves on the glass plate A A’ were obli- 
terated, and replaced by concentric circles whose radu increased 
by half inches from 1 to 5 inches. A constant rotation was 
secured in the manner described in art. 23, and its velocity de- 
termined in each case by counting the number n of rotations 
made in a given time ¢, the latter being measured by an accu- 
rate timepiece, the motion of whose index could be instantaneously 
arrested or renewed by simply pressing or releasing a spring. 
Since each jet issues at right angles to the horizontal arm B B’, 
it is clear that its effect, as far as the rotation of the mill is con- 
cerned, depends not only upon the height of the column of water 
in the tin tube, but also upon the inclination of the jet to the 
horizon ; for upon the former depends the velocity of efflux, and 
the uncompensated pressure against the part of the horizontal 
tube opposite to the orifice through which the jet issues, whilst 
upon the latter depends the magnitude of the horizontal compo- 
nent of this pressure, which component is alone effective in pro- 
ducing rotation. But every change in the velocity of efflux and 
in the inclination of the jet to the horizon produces a corre- 
sponding change in the distance, from the axis, of the point at 
which the jet falls into the water in the bath, and consequently 
in the radius @ of the circle described by the jet. This radius, 
