40 EEPOKT— 1886. 



report ; and he was instructed by the Committee to proceed with the 

 constmction of the instrument, which has been completed, and is placed 

 before the Section and described in a separate paper. 



The Committee respectfully request to be reappointed, with a grant 

 of 25Z. 



Report of the Committee, consisting of Professor Gr. H. Darwin, 

 Sir W. Thomson, and Major Baird, for preparing instructions 

 for the practical work of Tidal Observation ; and Fourth 

 Report of the Committee, consisting of Professors Gr. H. Darwin 

 and J. C. Adams, for the Harmonic Analysis of Tidal Observa- 

 tions. Draivn up by Professor G. H. Darwin. 



I. Record of Work during the past Tear. Datum Levels. 



Major Baird's manual of tidal observations is now printed, and will be 

 sold by Messrs. Taylor & Francis, Fleet Street. 



The Indian tidal results of all previous years, and those given in the 

 ^^arious Reports to the British Association, have been reduced by Major 

 Baird to the standard form recommended in the Report of 1883. To these 

 have been added the results derived by the United States Coast Survey, 

 and the whole has been published in the ' Proceedings of the Royal 

 Society,' No. 239, 1885, in a paper by Major Baird and Professor 

 Darwin. 



In the course of the Indian tidal operations a discussion has arisen as 

 to the determination of a datum level for tide-tables. The custom of the 

 Admiralty is to refer the tides to ' the mean low-water mark of ordinary 

 spring tides.' This datum has not a precise scientific meaning, but, at 

 ports where there ai'e but few observations, has been derived from a mean 

 of the spring-tides available. At some of the Indian ports this datum has 

 been found by taking the mean of all spring-tides on the tide diagram for 

 a year, with the exception of those which occur when the moon is near 

 perigee. The diurnal tides enter into the determination of tbe datum in 

 an undefined manner. It follows that two determinations of this datum 

 level, both equally defensible, might differ sensibly from one another. 



A datum level should be sufficiently low to obviate the frequent 

 occurrence of negative entries in a tide-table, and it should be rigorously 

 determinable from tidal theory. It is now proposed to adopt as the datum 

 level at any new ports in India, for which tide-tables are to be issued, a 

 datum to be called ' the Indian spring low- water mark,' and which is to 

 be below mean sea-level by the sum of the mean semi-ranges of the tides 

 Mg, S2, Ki, O ; or, in the notation used below, 



below mean water mark. 



This datum is found to agree pretty nearly with the Admii'alty datum, 

 but is usually a few inches lower. The definition is not founded on any 

 precise theoretical considerations, but it satisfies the conditions of a good 

 datum, and is precisely referable to tidal theory. 



If, when further observations are made, it is found that the values of 

 the several H's require correction, it is not proposed that the datum level 

 e;hall be altered accordingly, but when once fixed it is to be always 

 adhered to. 



