THE EARTH. 



75 



fore, the waters farthest from the moon, hav- 

 ing less weight, and being lightest, will be 

 pressed on all sides, by those that, having 

 more attraction, are heavier; they will be 

 pressed, I say, on all sides ; and the heavier 

 waters (lowing in, will make them swell and 

 rise in an eminence directly opposite to 

 that on the other side of the globe, caused 

 by the more immediate influence of the 

 moon. 



In this manner the moon, in one diurnal re- 

 volution, produces two tides; one raised im- 

 mediately under the sphere of its influence, 

 and the other directly opposite to it. As 

 the moon travels, this vast body of waters 

 rears upward, as if to watch its motions ; and 

 pursues the same constant rotation. How- 

 ever, in this great work of raising the tides, 

 the sun has no small share ; it produces its 

 own tides constantly every day, just as the 

 moon does, but in much less degree, because 

 the eun is at an immensely greater distance. 

 Thus there are solar tides, and lunar tides. 

 When the forces of these two great lumina- 

 ries concur, which they always do when they 

 are either in the same, or in opposite parts 

 of the heavens, they jointly produce a much 

 greater tide, than when they are so situated 

 in the heavens, as each to make peculiar 

 tides of their own. To express the very 

 same thing technically ; in the conjunctions 

 and oppositions of the sun and moon, the attrac- 

 tion of the sun conspires with the attraction of 

 the moon ; by 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 syzi- 

 gies, and least in the quadratures. 



This theory well understood, and the as- 

 tronomical terms previously known, it may 

 readily be brought to explain the various ap- 

 pearances of the tides, if the earth were co- 

 vered with a deep sea, and the waters unin- 

 fluenced by shoals, currents, straits, or tem- 

 pests. 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 shallowness of some places, 

 and the narrowness of the straits in others, 

 there arises a great diversity in the effect. 



U 



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 imperceptible mo- 

 tion 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 impetuous rapidity. 

 Tims, in the English Channel, and the Ger- 

 man Ocean, the tide is found to flow strong- 

 est in those places that are narrowest ; the 

 same quantity of water being, in this case, 

 driven through a smaller passage. It is of- 

 ten seen, therefore, pouring through a strait 

 with great force ; and, by its rapidity, consi- 

 derably raised above the surface of that par! 

 of the ocean into which it runs. 



This shallowness and narrowness in many 

 parts of the sea, give also rise to a peculiari- 

 ty 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 i? 

 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 despatch 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 be- 

 ing 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 exten- 

 sive, are not however large enough to be af- 

 fected 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 they should receive and re- 

 turn water enough to raise or depress them 

 in any considerable degree. 



In general 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 deep- 

 er, and less affected 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. 



Bufibn, vol. ii. p. 1 87- 



