ox THE TIDE3. 557 



dependent of them, and happening only once a day; then, if a point be so 

 situated in the canal which we have been considering, that the effects of the 

 two equal semidiurnal tides may be destroyed, those of the daily tides only 

 will remain to be combined with each other ; and their joint result will 

 be a tide as much greati^r than either, as the diagonal of a square is 

 greater than its side; the times of high and low water being intermediate 

 between those which belong to the diurnal tides considered separately. 

 Thus, in the port of Batsha, the greater tide probably arrives at the third 

 lunar hour directly from the Pacific ocean, and at the ninth from the gulf of 

 Siam, having passed between Sumatra and Borneo; so shat the actual time 

 of high water is at the sixth lunar hour. The magnitude of this compound 

 tide is by no means inconsiderable; it sometimes amounts to as much as 

 13 feet. (Plate XXXVIII. Fig. 523, o24.) 



Besides the variations in the height of the sea, which constitute the tides, 

 the attractions of the sun and moon are also supposed to occasion a retardation 

 in its rotatory motion, in consequence of which it is left a little behind the 

 solid parts of the earth ; and a current is produced, of which the general 

 direction is from east to west. This current comes from the Pacific and 

 Indian oceans, round the Cape of Good Hope, along the coast of Africa^ 

 then crosses to America, and is there divided and reflected southwards to- 

 wards the Brazils, and northwards into the Gulf stream, which ti'avels round 

 the gulf of Mexico, and proceeds north eastwards into the neighbourhood 

 of Newfoundland, and then probably eastwards and south eastwards once 

 more across the Atlantic. It is perhaps on account of these currents that 

 the Pted Sea is found to be about 25 feet higher than tlte Mediterranean: 

 their direction may possibly have been somewhat changed in the course of 

 many ages, and with it the level of the Mediterranean also; since the floor 

 of the cathedral at Ravenna is now several feet lower with respect to the sea 

 than it is supposed to have been formerly, and some steps have been found 

 in the rock of Malta, apparently intended for ascending it, which are at 

 present under water. 



The atmosphere is also liable to elevations and depressions analogous to 

 those of the sea, and perhaps these changes may have some little effect on 

 the winds and on tlie weather; but their influence must be very incousider- 



