See ee ae 
Se 
ON WAVES. 451 
6 inches to 6 feet in length, and it was observed that every wave 
broke exactly when its height above the antecedent hollow was 
equal to the depth of the water. At another time when the di- 
rection of the waves was oblique to the edge of the water, the 
breaking crest moved along from one end of the shore towards 
the other, uniformly and gradually as the wave advanced to the 
point of breaking depth, resembling the few de joie of a file of 
soldiers. 
When a wave that has been breaking on ashallow part of the 
water comes suddenly into deeper water, the form ceases to be 
crested, see Plate II.; and the wave subsides into the figure 
due to the depth. 
The phenomena of waves breaking on the shore were accu- 
rately obtained in the experiments No. 107—132, page 492. 
Plate II. figs. 6 and 7. 
Third Series of Observations. 
On the Tide Wave of the River Dee in Cheshire.-—The ob- 
ject of this series of observations was the comparison of the 
tidal wave moving in a given channel with the great primary 
wave of translation previously examined by Mr. Russell. 
To this object the river Dee is peculiarly suitable. Plate VI. 
fig. 1. gives a plan of that river at low water. The upper por- 
tion of the channel of the river is artificial. The waters of the 
river were turned into a new course about the middle of the last 
century. Of this course about 54 miles forms a_ perfectly 
straight canal, along which a large and rapid tidal wave is trans- 
ferred with great velocity. The two points A and B on the 
plan were selected as stations of observation. The distance be- 
tween A and B was carefully measured ; transverse sections of 
the river were made, and soundings were taken throughout the 
whole length of the channel. 
The distance between Aand B . . . . =5:275 miles. 
The mean depth of the channel at low water = 3:0 feet. 
The bed of the river has a slope nearly . =3°8 feet. 
The opposite sides of the river are parallel embankments 
about 500 feet apart at high water mark, but nearly half that 
breadth is occupied by groins, as shown in the sections of the 
river, figs. 2,3, and 4, Plate VII., and the intervals between them 
are filled up with high banks of sand. 
The tides selected to be observed were those which differed 
most in magnitude, and which were least affected by disturbing 
influences. They were made when the weather was settled, 
when there was no sensible wind, and when the river was as 
nearly as possible in its natural state. One entire tide wave was 
obtained on the 7th of September, and two others on the 9th 
262 
