152 



June 13, 1839f 



JOHN W. LUBBOCK, Esq., V.P. and Treas., in the Chair. 



The ballot for Edwin Guest, Esq., was postponed in consequence 

 of the number of Fellows required by the Charter not being present. 



The following papers were read, viz. : — 



" Researches on the Tides. Tenth Series. On the Laws of Low 

 Water at the Port of Plymouth, and on the permanency of mean 

 water." By the Rev. W. Whewell, B.D., F.R.S., Fellow of Tri- 

 mty College, Cambridge. 



In this memoir the author investigates the question, how far the 

 mean water, that is the height of the tide midway between high and 

 low water, is permanent during the changes which high and low 

 water undergo. That it is so approximately at Plymouth having 

 been already ascertained by short series of observations, it was de- 

 sirable to determine the real amount of this permanency by induction 

 from longer series of observations. A period of six years was chosen 

 for that purpose ; and the method of discussing these observations 

 was the same, with slight modifications, as in former researches. 



The height of low water, cleared from the effects of lunar parallax, 

 and very nearly so from those of lunar declination, and compared 

 with the height of high water, similarly cleared, enabled the author 

 to ascertain whether the mean water also was affected by the semi- 

 menstrual inequality. The results of the calculation show that the 

 height of mean water is, within two or three inches, constant from 

 year to year : and that, for each fortnight, it has a semi-menstrual 

 mequality amounting to six or seven inches; — the height being great- 

 est when the transit is at 6h. and least when at 11 h.,— the imme- 

 diate cause of this inequality being, that the semi- menstrual inequality 

 of low water is greater than that of high water : this inequality, 

 however, is probably modified by local circumstances. 



These researches have also verified the theoretical deduction, that 

 the height both of low and of high water being affected by the moon's 

 declination, their mean height partakes of the variations in this latter 

 element, in successive years, consequent on the change of position 

 of the moon's orbit. At Plymouth the increase in mean low water 

 amounts to about two inches for each degree of increase in the de- 

 clination. In the high water this change is less marked. 



The parallax correction of the height of low water is obtained 

 from all years alike, by taking the residue of each observation, which 

 remains w^hen the semi-menstrual inequality is taken away, and ar- 

 ranging these residues, for each hour of transit, according to the 

 parallax. The declination correction is obtained in a manner analo- 

 gous to the parallax correction, from each year's observations, with 

 some correction for the variation in the mean declination of the moon 

 in each year. 



" Researches on the Tides. Eleventh Series. On certain Tide 



