PLYMOUTH SOUND: ITS TIDAL CURRENTS. 275 
PLYMOUTH SOUND—ITS TIDAL CURRENTS. 
BY. MR. J. C. INGLIS, C.E. 
. (Read November 22nd, 1877.) 
Tae waters in Plymouth Sound are continually ebbing and flowing 
in more or less defined directions and quantities. A general view of 
these will, it is hoped, be found a not uninteresting study. There 
are many difficulties in the way, many doubtful points have still 
to be cleared up, especially bottom currents; points, which from 
their nature do not affect the navigation of the Sound, and have 
thus escaped observation, but which are of the first importance in 
a complete view of the movements of the waters in the Sound. It 
has been attempted here to give in as few words as possible a 
general idea of the currents rather than an exact description at 
any one time. 
In accounting for the surface currents at parts, it will be neces- 
sary to examine the geography of the bottom of the Sound. In 
doing this it will be apparent that the interesting nature of Plymouth 
Sound does not end at the low-water mark. The accompanying map, 
prepared from the large chart of the Sound, shows this formation. 
A deep and comparatively narrow trough extends from between 
Mount Batten and Drake’s Island, where it abruptly commences, 
in the rear of the island and along the Hamoaze as far as Saltash 
Bridge. Such an excavation or depression in solid rock is very 
exceptional, and in two places the depth is over twenty fathoms at 
low-water; viz., at Devil’s Point and off Eastern King, the deepest 
points in the Sound. Before the twenty fathom line in the open 
channel is reached, we have to go two and a-half miles outside the 
Breakwater. Such a depression necessarily has a very great in- 
fluence on the tidal phenomena of the Sound, and indeed is the 
key to the most important movements which take place. The 
water flows along it with considerable velocity, more than once 
impinging on its steep sides, and causing where it does so the 
