THE TIDES. 275 



The Corrected Establishment. — The mean luni-tidal interval, or corrected 

 establishfuent of each place, differs from the vulgar establishment, or time 

 of high water for new and full moon; for the time of high water at syzigy 

 is affected by the semi-menstrual inequality belonging to the moon's posi- 

 tion one or two days earlier, and is therefore later by about thirty minutes 

 than the mean interval would give it.* 



(228.) Semi-menstrual Inequality. — The interval of tide and moon's 

 transit is affected by a considerable inequality, which goes through its 

 period twice in the space of one month; it may be considered as depending 

 upon the moon's distance from the sun in right ascension, or, which is the 

 same thing, on the solar time of the moon's transit. The difference of the 

 greatest and least intervals at London is !•" 28™. f 



(229.) The Age of the Tide. — The Tide does not depend upon the passage 

 of the moon upon that particular day or hour, but from some previous 

 transit ; hence the Tide is observed to take place at London at two o'clock 

 on the days of new and full moon ; therefore, as the Tide of London is 

 found to be determined by the position of the sun and moon upon two 

 days and a half before it occurs, I'' 26™ is the corrected estahlishmeiit for 

 London, as explained in (227).]: 



(230.) Difference of the Tivo Diurnal Tides. — It has been remarked in 

 various places, by separate observers, that the evening Tide is higher than 

 the morning Tide in one part of the year, and lower at another. This is 

 thus explained by Newton. From the vernal to the autumnal equinox, the 

 sun has North declination ; and as the moon's orbit is never much inclined 

 to the sun's, a line drawn from the earth's centre to the moon would meet 

 the earth's surface, on the side towards the sun, in North latitude. Now, 

 such a line is the axis of the tide-spheroid, supposing the Tide to be 

 always under the moon ; and the Tide taking place when the moon in the 

 meridian is higher, as the place is nearer to the vertices or points where 

 the axis of the tide-spheroid meets the earth's surface. Hence, in this 

 case, the Tides which occur on the side of the earth next the sun, or the 

 day Tides, would be larger for a place in North latitude than the Tides on 

 the opposite side. For a similar reason, the night Tides would be higher 

 in winter. 



(231.) Height of Mean Water. — The mean between high and low water 

 is found to be constant and permanent, however much may be the differ- 

 ence of high and low water. It has been found, from a great number of 

 observations on the South coast of England, not to vary more than 2 or 3 

 inches ; therefore all heights ought to be referred to the mean level of the 

 sea, instead of the vague and uncertain data of high or low water. 



The refined surveying operations have demonstrated one singular fact, 

 which could only have been elicited in the laborious and exact processes 

 carried on during the Ordnance Survey. It is, that the mean level of the 

 sea, as we assume it, is not a level, from whatever cause it may arise, and 

 it is difficult to assign one. It is found that the mean level of the sea 

 around Ireland is lower on the South than it is on the North coast 



• 



Phil. Trans. (Whewell), 1836, page 292. t Pliil- Trans., 1834, page 19. 



t Phil. Trans., 1839, page 154. 



