REPORT ON THE TIDES. 
195 
The continents of Africa and South America may be con¬ 
sidered as immense dams in the course of the tide-wave which 
completely change its direction, so that it is high water at the 
same time on the opposite shores of the Atlantic. The rudi¬ 
ments of the cotidal lines which would obtain in the case of a 
perfect spheroid probably exist round the south pole, interfered 
with, as they must be, by the great continent of ice in those re¬ 
gions. Owing to the obstructions I have mentioned, it is high 
water nearly at the same instant at the Cape of Good Hope, 
off the Straits of Gibraltar, off the coast of Scotland near the 
Murray Frith, and in the river Thames. The wave takes six 
hours in proceeding from the Land’s End to theNorthForeland, 
being at the rate of about 70 miles an hour, and in a direction 
contrary to the course of the luminary. If the ocean completely 
covered the solid nucleus of the earth, it would only be high 
water at the same instant at places of which the longitude dif¬ 
fered by 180°; and at the equator the tide-wave would travel 
at the rate of about 500 miles per hour. The motion of the 
crest of the tide-wave must be carefully distinguished from that 
of the particles of water themselves, which forms a current the 
velocity of which seldom exceeds a few miles per hour : these 
currents are modified by others due to changes of temperature. 
The analytical investigation of the motions produced by changes 
of temperature, and of the propagation of heat in fluids, is one 
of extreme difficulty, and has not been yet attempted. In order 
to approach this important question with any chance of success, 
it seems necessary to consider the problem in the first instance 
in its most simple form, and one in which the results of theory 
can easily be compared with those of observation. 
Works of navigation and sailing directions supply much in¬ 
formation with respect to the velocity and direction of the cur¬ 
rents ; while the time of high water appears to have been care¬ 
fully ascertained at very few points only on the earth’s surface. 
Yet the phenomena of the tides are of extreme interest. La¬ 
place says, “ Les marees ne sont pas moins interessantes a 
connoitre, que les inegalites des mouvemens celestes. On a 
neglige pendant longtemps de les suivre avec une exactitude 
convenable, a cause des irregularites qu’elles presentent; mais 
ces irregularites disparaissent en multipliant les observations.” 
There is indeed no branch of Physical Astronomy in which so 
much remains to be accomplished. 
N 
Q 
/V 
