TIDES AND OCEANIC HIGHWAYS. 353 



conservator of its harmony. The theory of the tides is exceedingly simple in its prin- 

 ciples, but the most complex of all physical problems .in its details. 



If we suppose a zone of water to surround the globe in every part, and to be subject to 

 no external influence, it is obvious, that the waters would uniformly maintain the same 

 level, and be arranged upon its surface at an invariable depth, assuming the form of the 

 circle E. It is easy, however, to perceive what effect, according to the law of gravity, 



the presence of a body like the moon will produce 

 upon the zone of water, a fluid so susceptible of 

 mobility, yielding to the slightest impressions. As 

 a solid body draws toward it any other body, by a 

 force which varies with its distance from the point 

 attracted, then if m be the moon, the particles of 

 water at a will feel more powerfully the effect of 

 her attraction than those on either side, or than the particles of land at the bottom of the 

 water, b, which are at a greater distance, and the result will be a bulging out of the 

 watery zone, as in the diagram, immediately next the lunar globe. But while high water 

 is thus produced at , , it is also produced at the same time in the opposite hemisphere 

 at c, d, owing to the varying distance of c and d from m, while at e and f no such diver- 

 sity of attraction can be experienced, and the effect is, the reduction of the watery zone 

 to the form of an ellipsoid, instead of that of a sphere. There is some difficulty to many 

 minds in conceiving of the moon's influence raising the waters directly under her, and 

 producing at the same time the like effect upon the waters on the opposite side of the 

 earth. But a little consideration will at once remove it ; for, obviously, the earth's centre, 

 E, will be more powerfully drawn towards the moon than the waters at c d, and recede 

 from them, producing the same effect as though they receded, or rose up from the centre 

 of the earth. Now, as the waters cannot rise in one place without falling in another, 

 they are depressed at e and f when elevated at a and d ; and as the moon is a month in 

 making her circuit round the earth, there would be two elevations and depressions of the 

 waters in the month in each place, if the earth remained stationary upon its axis. But 

 by the diurnal rotation, the moon passes every day the superior and the inferior meridian 

 of every part of the globe, producing daily two seasons of high and low water. The 

 tidal swell of the ocean, or high water, at any given point, is thus occasioned by the 

 point in question being in the direct line of the lunar attraction, or the moon being upon 

 the meridian of its position ; and the ebb of the tide, or low water at the point, is caused 

 by the moon being upon a circle which cuts the meridian at right angles. But the facts 

 of the case do not exactly tally with what we should expect from the theory, for the 

 times of high and low water are not coincident with the fulfilment of the conditions 

 stated. Thus, if the moon passes the meridian of Ushant, on the north-west coast of 

 France, at twelve o'clock in the day, high water will not take place until about three 

 hours afterwards, or three o'clock in the afternoon. The explanation of this is, that the 

 impulse received by the waters of the sea in being brought by the rotation of the earth 

 under the influence of the lunar attraction, continues to act for some time after the rota- 

 tion has carried them away from it, so that there is still a tendency to ascend, as the effect 

 of the impulse received, though the immediate action of the planet is not so strong 

 as it was. 



Besides the action of the moon in elevating the waters of the ocean, there is that of 

 the sun to be considered. The solar attraction is not so influential a cause in the pro- 

 duction of the tides as the lunar, because, though a much larger body, the sun is at far 

 greater distance from the earth. Still the sun acts upon the ocean in precisely the same 

 manner, though in a feebler degree, and his influence is sufficiently energetic to produce 



A A 



