1877.] Sir G. B. Airy on the Tides at Malta. 485 



December 6, 1877. 

 Sir JOSEPH HOOKER, C.B., President, in the Chair. 



The President announced that he had appointed as Vice-Presidents : — 



The Treasurer. 

 Mr. Abel. 

 Dr. Parr. 



Prof. Henry Smith. 

 Dr. Allen Thomson. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. "On the Tides at Malta." By Sir G. B. Airy, K.C.B., 

 Astronomer Royal. Received July 14, 1877. 



(Abstract.) 



A self-registering tide-gauge had been erected in the harbour of La 

 Valetta, and records had been made with it continuously from 1871, 

 March 31, to April 28, and with interruptions through other days, under 

 the care of Admiral Sir Cooper Key, Naval Commander at Malta; and 

 the register sheets were placed in the hands of the Astronomer Royal. 

 They form a band about 60 feet long. 



The first step required to make the observations available was to 

 eliminate the effects of frequent non-tidal undulations, whose period is 

 approximately 2 L m . This was done by carefully bisecting the intervals 

 between adjacent elevations and depressions in about 1100 instances, and 

 drawing a pencilled curve through the points of bisection. 



It then became necessary to consider the best plan of reduction. It 

 was determined to commence by comparison, in respect of times, with 

 the predicted tides at London, as given in the Admiralty Tide-Tables, 

 which are generally recognized as accurate, and are free from diurnal tide. 

 The times of London high- water were therefore laid down on the register- 

 sheets (without regard to difference of longitude) ; each tidal day was 

 divided into 48 equal parts, and for each dividing point the ordinate of 

 the tidal curve was measured. The number of measures is about 1900. 

 These measures were collected in groups of 6 measures for each group 

 (or 8 groups for each tidal day), and the mean of each group is exhibited 

 in a Table. 



The method of treating these means of group-measures is then ex- 



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