as DYNAMICAL GEOLOGY. - [Boox IIT _ 
while the rise and fall of the tide there amounts to 40-feet. In like 

a 
‘- 
4 
manner the tides which enter the Bay of Fundy, between Nova — 
Scotia and New Brunswick, get more and more cooped up and 
higher as they ascend that strait, till they reach a height of 
70 feet. 
While the tidal swelling is increased in height by the shallowness — 
and convergence of the shores, it gains at the same time force and ~ 
rapidity. No longer a mere oscillation or pulsation of the great ocean, 
the tide acquires a true movement of translation, and gives rise to © 
currents which rush past headlands and through narrows in powerful 
streams and eddies. The rocky and intricate navigation of the west’ 
of Scotland and Scandinavia furnishes many admirable illustrations 
of the rapidity of these tidal currents. The famous whirlpool of 
Corryvreckan, the lurking eddies in the Kyles of Skye, the breakers 
at the Bore of Duncansbay, and the tumultuous tideway, grimly 
named by the northern fishermen the Merry Men of Mey, in the Pent- 
land Firth, bear witness to the strength of these sea rivers. At the 
last mentioned strait the current at its strongest runs at the rate of 10 © 
miles an hour, which is fully three times the speed of most of our 
large rivers. | 
(2.) Currents.—Recent researches in ocean temperature haye. 
disclosed the remarkable fact that beneath the surface layer of water 
affected by the temperature of the latitude there lies a vast mass of 
cold water, the bottom temperature of every ocean in free communi- 
cation with the poles being little above and sometimes actually below 
the freezing point of fresh water. In the North Atlantic a tempera-: 
ture of 40° Fahr. is reached at an average depth of about 800 fathoms, 
all beneath that depth being progressively colder. In the equatorial 
parts of that ocean the same temperature comes to within 300 fathoms: 
of the surface. In the South Atlantic, off Cape of Good Hope, the 
mass of cold water (below 40°) rises likewise to about 300 fathoms 
fromthe surface. This distribution of temperature proves that there 
must be a transference of cold polar water towards the equator, for in 
the first place the temperature of the great mass of the ocean is much 
lower than that which is normal to each latitude, and in the second 
place it is much lower than that of the superficial parts of the earth’s 
crust underneath. On the other hand, the movement of water from 
the poles to the equator requires a return movement of compensation 
from the equator to the poles, and this must take place in the super- 
ficial strata of the ocean. Apart therefore from those rapid river-like 
streams which traverse the ocean, and to which the name of currents 
is given, there must be a general drift of warm surface water towards’ 
the poles. ‘This is doubtless most markedly the case in the North 
Atlantic, where, besides the current of the Gulf Stream, there is a 
1 See in particular memoirs by Carpenter & Wyville Thomson, Proc. Roy. Soc. xvii. 
(1868), Brit. Assoc. xli. et seq., Proc. Roy. Geograph. Soc. xv. Reports to the Admiralty 
of the Challenger Exploring Expedition. Wyville Thomson’s “ Depths of the Sea,* 
1873, and “ Atlantic,” 1877. 

