TIDAL AND LEVELLING OBSERVATIONS. 197 



already for some years past been engaged upon tidal observations 

 in the Gulf of Cutch and on their reduction in England. 



Inquiries were at once set on foot to ascertain the most suitable 

 ports for tidal stations. The necessary conditions were, firstly, a 

 site immediately over or close to the sea, where the depth was not 

 less than from 10 to 15 feet at lowest tides ; secondly, the presence 

 of a port officer to exercise a general supervision over the operations 

 and correct the clocks of the several self-registering instruments 

 whenever necessary, either by direct determination of time or by 

 getting the true time from the nearest telegraph office ; thirdly, the 

 feasibility of periodical inspection of the instruments at intervals 

 of not less than six months generally, and more frequently when 

 there might be no superintending officer resident on the spot. 



During the following year Captain Baird visited Bombay, 

 Karachi, Karwar, Aden, Beypur, Paumben, Madras, and Viza- 

 gapatam, making due aiTangements at each port. A man was 

 placed in charge of the instruments in each place, and taught how 

 to manipulate them ; he was also taught to read the graphic 

 delineations of tidal height, and to enter the value for each hour 

 in a tabular report which was sent off to Poona daily. 



The barrels of the tidal gauges are five feet in length, and thus 

 capable of registering on the scale of nature all tides of which the 

 amplitudes between extreme high and low water do not exceed 

 five feet. For tides of greater amplitude a gear of wheels is supplied 

 to each instrument, enabling the scale to be varied. 



The aualysis of tidal observations is a matter of much complexity, 

 and from the outset Colonel Walker determined to adopt the 

 procedure recommended by the Tidal Committee of the British 

 Association for the investigation of the tidal constituents, and to 

 employ Mr. Edward Eoberts, F.R.A.S., of the Nautical Almanac 

 Office, — who, under Sir William Thomson's supervision, had reduced 

 and analyzed the tidal observations placed at the disposal of the 

 British Association for the advancement of Science — in working out 

 the yearly tables of tidal prediction from the values of the tidal 

 constituents with his tide-predicting machine. 



In the system of analysis adopted by the Tidal Committee of 

 the British Association, the successive heights of the tide for each 

 hour at each port are grouped in a large number of combinations, 

 so that the exact effect of each grouping is ascertained with great 

 precision. The following explanation may serve to elucidate the 



