278 JOURNAL OF THE PLYMOUTH INSTITUTION. 
Our consideration is more particularly directed on the present 
occasion to the currents due to the tidal rise and fall of the water, 
and not to the laws of that rise or fall. Necessarily the currents 
are entirely regulated by the extent of this rise and fall, which, as 
we all know, depends in the main on astronomical agencies. The 
direction and force of the wind influence the height of the 
tides, and the direction and velocity of the currents, here as else- — 
where to a great extent; three or more feet of water have been 
banked up in the Sound from this cause alone. The pressure of 
the atmosphere also has its influence, which at this port is very 
noticeable, possibly from our proximity to the open sea. <A low 
barometer brings in more water, and a high less. Commander 
Walker, late Queen’s Harbour-Master for this port, a most assiduous, 
able, and successful observer, has estimated from a long series of 
observations made at the Dockyard, Devonport, that for every inch 
of mercury fallen sixteen inches more water is brought in, than 
would otherwise happen at the same time under the other con- 
ditions, and vice versa; sudden changes being accompanied with a 
difference at the rate of twenty inches. 
Thus with the direct action of the wind on the water, the effects 
of freshes from the Tamar and the Plym, and astronomical and 
meteorological influences, all more or less varying in intensity, we 
can understand the exceedingly changeable nature of the tidal 
currents in the Sound. 
The first indication of a rising tide outside the Breakwater is 
November, 1877, and published in the Western Morning News: ‘‘ It appears 
to have been stated that no survey of the Sound has taken place since the 
Breakwater was built. I beg therefore to inform you that myself and two 
other officers were employed by the Admiralty, between the years 1856-60, 
to survey the coast and harbours, &c., between the Start Point and Rame 
Head, including Plymouth Sound. The latter being so important a place 
was most carefully examined, and laid down on a large scale. The soundings 
.were taken in parallel lines one hundred feet apart from shore to shore, the 
sounding-boat being guided by two flags in a line trigonometrically fixed on 
the shore; the lead-line was marked to feet, used by an experienced seaman, 
under the eye of officers long accustomed to surveying work, the rise and fall 
of tide being registered every ten minutes, in order to reduce the soundings 
to a uniform level. I know the late Queen’s Harbour-master (Commander 
Walker) had an idea that the Sound was silting-up, and we were aware of its 
being the general impression; but on comparing the best old charts of the 
Sound with our careful survey, no proof could be found to establish the 
fact.” 
