216 



THE TIDES. 



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more feeble, there is a still greater interval between the transit of the sun and 

 occurrence of the solar tide. 



But besides these circumstances, the tide is affected by other causes. It is 

 not the separate effect of either of these bodies, but to the combined effect of 

 both, and at every period of the month, the time of actual high water is either 

 accelerated or retarded by the sun. In the first and third quarters of the moon, 

 the solar tide is westward of the lunar one ; and, consequently, the actual high 

 water which is the result of the combination of the two waves will be to the 

 westward of the place it would have 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 com- 

 bined, 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 of late years received much attention from sev- 

 eral scientific investigators in Europe. The discussions held at the annual 

 meetings of the British association for the advancement of science, on this sub- 

 ject, have led to the development of much useful information. The labors of 

 Professor Whewell have been especially valuable on these questions. Sir 

 John Lubbock has also published a valuable treatise upon it. To trace the re- 

 sults of these investigations in all the details which would render them clear and 

 intelligible, would greatly transcend the necessary limits of this discourse. We 

 shall, however, briefly advert to a few of the most remarkable points connected 

 with these questions. 



The apparent time of high water at any port in the afternoon of the day of 

 new or full moon, is what is usually called the establishment of the port. Pro- 

 fessor Whewell calls this the vulgar establishment, and he calls the corrected es- 

 tablishment the mean of all the intervals of the tides and transit of half a month. 

 This corrected establishment is consequently the luni-tidal interval correspond- 

 ing 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 produ- 

 ces 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, conse- 

 quently, when the moon's declination is of the same denomination as the lati- 

 tude 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 it varies greatly 

 at different places, and its laws, which appear to be governed by local circum- 

 stances, are very imperfectly known. 



We have now described the principal phenomena that would take place 

 were the earth a sphere, and covered entirely with a fluid of uniform depth. 

 But the actual phenomena of the tides are infinitely more complicated. From 

 the interruption of the land, and the irregular form and depth of the ocean, 

 combined with many other disturbing circumstances, among which are the in- 

 ertia of the waters, the friction on the bottom and sides, the narrowness and 

 length of the channels, the action of the wind, currents, difference of atmo- 

 spheric pressure, &c., &c., great variation takes place in the mean times 

 and heights of high water at places differently situated ; and the inequali- 

 ties above alluded to, as depending on the parallax of the moon, her posi- 



