THE EARTH. 177 



depths of the ocean, for instance, a very slow and imperceptible 

 motion of the whole body of water will suffice to raise its sur- 

 face several feet high ; but if the same increase of water is to be 

 conveyed through a narrow channel, it must rush through it 

 with the most impetuous rapidity. Thus, in the English Chan- 

 nel, and the German Ocean, the tide is found to flow strongest 

 in those places that are narrowest ; the same quantity of \rdter 

 being, in this case, driven through a smaller passage. It is often 

 seen, therefore, pouring through a strait with great force ; and, 

 by its rapidity, considerably raised above the surface of that part 

 of the ocean into which it runs. 



This shallowness and narrowness in many parts of the sea, 

 give also rise to a peculiarity in the tides of some parts of the 

 woiM. For in many places, and in our own seas in particular, 

 the greatest swell of the tide is not while the moon is in iis 

 meridian height, and directly over the place, but some time after 

 it has declined from thence. The sea, in this case, being ob- 

 structed, pursues the moon with what despatch it can, but does 

 not arrive with all its waters till long after the moon has ceased 

 to operate. Lastly, from the shallowness of the sea, and from 

 its being obstructed by shoals and straits, we may account for 

 the Mediterranean, the Baltic, and the Black Sea, having no 

 sensible tides. These, though to us they seem very extensive, 

 are not however large enough to be aifected by the influence of 

 the moon ; and as to their communication with the ocean, 

 through such narrow inlets, it is impossible, in a few hours' 

 time, that they should receive and return water enough to raise 

 or depress them in any considerable degree. 



In general, therefore, we may observe, that all tides are much 

 higher, and more considerable, in the torrid zone, than in the 

 rest of the ocean ; the sea in those parts being generally deeper, 

 and less aifected by changeable winds, or winding shores.' The 

 greatest tide we know of, is that at the mouth of the river Indus, 

 where the water rises thirty feet in height How great, there, 

 fore, must have been the amazement of Alexander's soldiers at 

 80 strange an appeararice ! They who always before had been 

 accustomed only to the scarcely perceptible risings of the Medi- 

 terranean, or the minute intumescense of the Black Sea, when 



1 Bufi'on. vol. ii p. 187. 



