196 T. A. Hirst on Ripoles, 
order to insert a solid cylinder of ivory about 1 inch long. Pass- 
ing through the centre of A and parallel to the sides of the glass, 
a fine line Aa, an inch long, was scratched on its surface with a 
diamond point ; and through the extremity a of this lme another, 
mn, was drawn perpendicular to the former, and graduated on 
each side from a into tenths of an inch. In using this ripple- 
meter, the plate of glass was held close and parallel to the sur- 
face of the current, so that the ivory pin, by becoming partially 
immersed, might cause a well-defined ripple, Ac, visible through 
the glass; the plate was then turned until A a bisected the angle 
between the branches of this ripple, when of course the ratio ~ 
gave at once the tangent of the required angle @. In this case, 
as in that of the rotating jet, a number of secondary ripples are 
also visible: after a little practice, however, and when the cur- 
rent was not too slow (the angle b A c too obtuse), it was not dif- 
ficult to estimate, approximately, the value of tan @ corresponding 
to the principal ripple. 
The following Table contains a few of the best results of many 
experiments made on streams. In it the first column gives the 
values of tan @ as read off on the ripple-meter ; the second column 
shows the corresponding values of the velocity v as indicated in 
feet per minute by the current-meter; and the third column 
contains the respective values of A, calculated according to the 
formula (43), in inches per second. 
tan @. Ve r. 
"993 44 6:2 
774 554 6:8 
705 59 6°8 
577 67 67 
*545 71 6-8 
*392 96 70 
388 105 76 
The observations, of which the above are some of the most 
trustworthy results, were made at different periods on streams 
in Hampshire and Gloucestershire. When we take into consi- 
deration the fact that a stream could rarely be found where the 
velocity at any one point remained constant, that in all @ases 
this velocity was determined with the curreng-meter at a point 
2 or 3 inches below the surface, and lastly, that the method of 
estimating the angle formed by the branches of the ripple can 
only lead to approximate results, the general agreement of the 
values of X with those obtained from the rotatory experiments is 
as close as could be expected. The results appear to indicate 
