SPKING AND NEAP TIDES. 



moon, the solar tide is westward of the lunar one ; and, conse- 

 quently, the actual high water, which is the result of the 

 combination of the two waves, will be to the westward of the 

 place where it would have been if the moon acted alone, and the 

 time of high water will therefore be accelerated. In the second and 

 fourth quarters the general effect of the sun is, for a similar 

 reason, to produce a retardation in the time of high water. This 

 effect, produced by the sun and moon combined, is what is 

 commonly called the priming and lagging of the tides. 



The highest spring tides occur when the moon passes the 

 meridian about an hour after the sun ; for then the maximum 

 effect of the two bodies coincides. 



The subject of the tides has received much attention from 

 several scientific investigators in Europe. The discussions held 

 at the annual meetings of the British Association for the 

 Advancement of Science, on this subject, have led to the 

 development of much useful information. The labours of 

 Dr. Whewell have been especially valuable on these questions. 

 Sir John Lubbock has also published a valuable treatise upon it. 

 To trace the results of these investigations in all the details 

 which would render them clear and intelligible, would greatly 

 transcend the necessary limits of this notice. We shall, however, 

 briefly advert to a few of the most remarkable points connected 

 with these questions. 



10. The apparent time of high water at any port in the after- 

 noon of the day of new or full moon, is what is usually called the 

 establishment of the port. Dr. Whewell calls this the vulgar 

 establishment, and he calls the corrected establishment the mean 

 of all the intervals of the tides and transits of half a month. 

 This corrected establishment is consequently the luni-tidal 

 interval corresponding to the day on which the moon passes the 

 meridian at noon or midnight. 



The two tides immediately following another, or the tides of 

 the day and night, vary, both in height and time of high water, 

 at any particular place with the distance of the sun and moon 

 from the equator. As the vertex of the tide-wave always tends 

 to place itself vertically under the luminary which produces it, 

 it is evident that, of two consecutive tides, that which happens- 

 when the moon is nearest the zenith, or nadir, will be greater 

 than the other ; and, consequently, when the moon's declination 

 is of the same denomination as the latitude of the place, the tide 

 which corresponds to the upper transit will be greater than the 

 opposite one, and vice versa, the differences being greatest when 

 the sun and moon are in opposition, and in opposite tropics. This 

 is called the diurnal inequality, because its cycle is one day ; but 



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