Physical geography. By Mary Somerville ...

202 PHYSICAL GEOGRAPHY. CHAP. XVI.
the Indian Ocean it also pursues a northerly course, and finally
washes the shores of Hindostan, the Bay of Bengal, and the Arabian Gulf; while in the Pacific, on the contrary, the waves diverge
from the equator towards the poles - but in all they partake also of
the westerly course of the moon.
Although such are the directions in which the tides unquestionably proceed along the shores of those seas, yet observations at
islands in the open sea and towards the centres of the oceans contradict the idea of corresponding progressive waves throughout the
entire area of those seas.
Upon the coasts of Britain and New Brunswick the tides are
high, from the local circumstances of the coast and bottom of the
sea; while in the centre of the ocean, where they are due to the
action of the sun and moon only, they are remarkably small. The
spring-tides rise more than 40 feet at Bristol, and in the Bay of
Fundy; in Nova Scotia, they rise upwards of 50 feet; the general
height in the North Atlantic is 10 or 12 feet, but in the open and
deep sea they are less; and at St. Helena they are not more than
3 feet, whilst among the islands in the Pacific they are scarcely perceptible.
The mean height of the tides will be increased by a very small
quantity for ages to come, in consequence of the decrease in the
mean distance of the moon from the earth; the contrary effect will
take place after that period has elapsed, and the moon's mean distance begins to increase again, which it will continue to do for many
ages. Thus, the mean distance of the moon, and the consequent
minute increase in the height of the tides, will oscillate between fixed
limits for ever.'
The tidal wave extends to the bottom of the ocean, and moves
uniformly and with great speed in very deep water, variably and
slow in shallow water; the time of propagation depends on the depth
of the water as well as on the nature and form of the shores. Its
velocity varies inversely as the square of the depth -a law which
theoretically affords the means of ascertaining the proportionate
depth of the sea in different parts; it is one of the great constants
of nature, and is to fluids what the pendulum is to solids-a connecting link between time and force.
The great oceanic wave that twice a-day brings the tides to our
shores, has occupied a day and a half in travelling from the place
where it was generated. The wave first impinges on the west coast
of Ireland and England, and then passes round the north of Scot.
land, up the North Sea, and enters the Thames, having made the
tour of Great Britain in about 18 hours.'
For the reason of this secular variation in the moon's distance, see page
42 of' The Connexion of the Physical Sciences.'
For illustration of the course of tidal waves, see plates, in Johnston's
Physical Atlas,' in folio.