TIDAL AND LEVELLING OBSERVATIONS. 207 



1885-86, the instruments at the latter station being provided at the 

 cost of the Durbar. Diamond harbour, Amherst, and Moulmein were 

 discontinued on completion of the usual period of registration, -which 

 in the case of Moulmein -was extended to six years owing to certain 

 peculiarities in the tides of that port. 



An important improvement was introduced into the tide tables for 

 18S7, by the employment of a scientific datum, which rendered it 

 possible to fix finally the datum for each of the observatories. 

 Previously to that the datum line of soundings adopted by the 

 Admiralty was the " mean low water for ordinary spring tides," but 

 as the term was not scientifically accurate with reference to tidal 

 theory, a new datum line, called " The Indian spring low- water 

 " mark," was definitely adopted after discussion with Professor 

 Darwin and Captain Wharton, R.N., the Hydrographer, and a table 

 was prepared to show the datum finally decided upon.* 



In 1886 the Amherst observatory was closed and the instruments 

 sent to Akyab, where a new station was erected, and on the night of 

 the 29th September in the same year the tidal observatory at Dublat, 

 near the mouth of the Hugli, was swept away with all its contents 

 by a heavy wind and sea, and none of the instruments or records could 

 be recovered. Fortunately the clerk was absent and no loss of life 

 occurred. 



The diagrams of the self -registering tide gauges at False Point and 

 Dublat (Saugor island), and at Diamond harbour and Kiclderpore on 

 the Hugli, for the 11th March and 9th April 1885 show unmis- 

 takably that considerable tidal disturbances took place on those dates 

 at the stations named. Prom a consideration of the facts, which are 

 reviewed by Major Bairdin a special memorandum,-)- it would appear 

 that a submarine depression caused the disturbance of the 11th 

 March, and that this was followed about a month later (9th April) by a 

 submarine upheaval which caused a very considerable wave to pass up 

 the Hugli. There are indications that the latter may have occurred 

 at the Sand Heads, opposite Balasore. 



The stations at Negapatam and Elephant Point were closed in 1888, 

 and two fresh ones at Tuticorin and Prince's Dock, Bombay, were 

 started. In 1889 the observatories at Colombo and G-alle having 



* The datum is defined as the sum of the semi-ranges of the principal lunar (M 9 ) 

 and principal solar (S 2 ) semi-diurnal tides, and of the uni-solar (K L ) and the lunar 

 diurnal (Oj) tides below mean sea level, that is to say : — ■ 



A — [H of M 2 + H of S 3 + H of K ; + H of 0^ above the zero of the gauge. 



f See page lxxiv of Appendix to Indian Survey Report for 1886-87. 



