Royal Society. 317 



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 induc- 

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

 chosen for that purpose ; and the method of discussing these obser- 

 vations was the same, with slight modifications, as in former re- 

 searches. 



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 

 inequality 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 aff"ected 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 when 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 

 Observations made in the Indian Seas." By the Rev. W. Whewell, 

 B.D., F.R.S., Fellow of Trinity College, Cambridge. 



This paper contains the results of the examination by the author 

 of certain series of tide observations made at several places in the 

 Indian Seas, which were forwarded to the Admiralty by the Hon. 

 East India Company. These localities were Cochin, Corringa River, 

 Surat roads in the Gulf of Cambay, Gogah, on the opposite side of 

 the same gulf, and Bassadore, in the Island of Kissmis in the Persian 

 Gulf. 



" On the Electrolysis of Secondary Compounds." In a letter ad- 

 dressed to Michael Faraday, Esq., D.C.L., F.R.S., Fullerian Pro- 

 fessor of Chemistry in the Royal Institution of Great Britain, &c.. 



