THE EARTH. 105 



peculiar tides of their own. To express the very same thing techni- 

 cally ; in the conjunctions and oppositions of the sun and moor, the 

 attraction of the sun conspires with the attraction of the moon ; bv 

 which means the high spring-tides are formed. But in the quadratures 

 of the sun and moon, the water raised by the one is depressed by the 

 other ; and hence the lower neap-tides have their production. In a 

 word, the tides are greatest in the syzigies, and least in the quadra- 

 tures. 



This theory well understood, and the astronomical terms previously 

 known, it may readily be brought to explain the various appearances 

 of the tides, if the earth were covered with a deep sea, and the waters 

 uninfluenced by shoals, currents, straits, or tempests. But in every 

 part of the sea, near the shores, the geographer must come in to cor- 

 rect the calculations of the astronomer. For, by reason of the shal- 

 lowness of some places, and the narrowness of the straits in others, 

 there arises a great diversity in the effect, not to be accounted for 

 without an exact knowledge of all the circumstances of the place. In 

 the great depths of the ocean, for instance, a very slow and impercep- 

 tible motion of the whole body of water will suffice to raise its surface 

 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 im- 

 petuous rapidity. Thus, in the English Channel, and the German 

 Ocean, the tide is found to flow strongest in those places that are nar- 

 rowest ; the same quantity of water 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 world. For in 

 many places, and in our own seas in particular, the greatest swell of 

 the tide is not while the moon is in its meridian height, and directly 

 over the place, but some time after it has declined from thence. The 

 sea, in this case, being obstructed, pursues the moon with what dis- 

 patch it can, but does not arrive with all its waters till long after the 

 moon has ceased to operate. Lastly, from this 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 how- 

 ever large enough to be affected by the influence of the moon ; and 

 as to their communication with the ocean, through such narrow inlets 

 it is impossible in a few hour's time that they should receive and r 

 turn water enough to raise or depress them in any considerable degree. 



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

 er, and more considerable, in the torrid zone, than in the rest of the 

 ocean ; the sea in those parts being generally deeper, and less affect- 

 ed 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, therefore, must have been the 

 amazement of Alexander's soldiers at so strange an appearance ! Thev 



BufFon, vol. ii. p. 187 



