1835.] Philosophical Transactions. 467 



time of the previous transit of the moon, in order to obtain the 

 interval. He infers that the tide is not affected by distant and 

 general irregularities, but that it is irregular only so far as it is 

 influenced by causes which operate in the neighbourhood ; as, for 

 example, the effect of the wind in connexion with the form of the 

 land. By obtaining means for different localities, the effect of the 

 disturbing causes may be estimated. 



An examination of the results of the time of high water, conducted 

 by erecting a series of equi-distant ordinates to represent the intervals 

 of the moon's transit and high water, and drawing a continuous line 

 through the extremities of these ordinates, shews that the curves 

 present, in general, the form of that deduced by Mr. Lubbock, from 

 the London observations ; and, in most instances, the tides of a single 

 place present the features of agreement with the theory which Mr. 

 Lubbock has shewn to obtain with such remarkable exactness in the 

 London tides : that is, the ordinate of the curve has, in the course of 

 the fortnight, a minimum and a maximum magnitude, so that the 

 curve assumes the form a>. The amount of flexure is not, however, 

 the same at all places, as appears from comparing the observations at 

 Brest, Plymouth, and London. Hence the fallacy of attempting to 

 deduce the mass of the moon from the phenomenon of tides, as advo- 

 cated by Laplace. 



The force of the moon determines only the amount of the semi- 

 menstrual inequality. This inequality has a common form, though 

 differing in amount at each place. If, therefore, we introduce a 

 local serai-menstrual inequality, in addition to the general one ; the 

 discrepancies of the curve might be reconciled. These curves appear 

 flatter at promontaries, and become more so as the tide wave pro- 

 ceeds. Another cause, viz. the meeting of the tides may, however, 

 possess some influence in producing this shape. The tide waves must 

 meet at some indeterminate point : as, for instance, on the coast of 

 Kent, and at this point the tide is later than it is if we proceed along 

 the coast, either east and north, or south and west. Still the meeting 

 of the tides is not a single point, but, in reality, takes place along 

 the whole coast, from the Isle of Wight to the Downs, and perhaps 

 to the coast of Suffolk. 



The diurnal difference of the height of the tides ranges from two 

 or three inches to one foot. This difference may be traced as far as 

 Portland Bill, but from this point the tides are not affected by it. 

 The tide hour varies very rapidly on rounding the main promon- 

 tories of the coast, and where the cotidal lines drawn to correspond 

 with such conditions, are brought near together, the place of high 

 water moves slowly, so that it is high water at one point while at a 

 neighbouring point the water is considerably below its greatest height, 

 which will produce a difference of level and a rapid stream tide. 



On certain peculiarities in the double Refraction, and Absorption 

 of Light, exhibited in the oxalate of chromium and potash. By Sir 

 David Brewster. 



This salt occurs in flat irregular six-sided prisms, the two broadest 

 faces being inclined to each other like the faces of a wedge, whose 



2 H 2 



