200 ' TIDAL AND LEVELLING OBSERVATIONS. 



ports, include all that produce any sensible effect, and lie has been 

 most successful in finding numbers for the wheels in the gear-work 

 which represent relatively the periods of each of the constituent 

 tides. The machine can be run for a period representing a whole 

 year, practically without error. Thus, for the chief tidal con- 

 stituent, if the hand represented mean lunar time at starting, its 

 error relatively in time at the end of a run of 365 days would not 

 amount to a quarter of a minute. 



The saving effected by the machine can be imagined when it is 

 stated that the curves for a whole year can be run off in about two 

 hours, and then only require to be read off, whilst the calculations 

 of, say, the heights for any one port for a year, combining the 20 

 constituents included in the machine, would take a practised 

 computer some four to five months' labour. The machine is used 

 now in the predictions of over 30 Indian ports, and Mr. Roberts 

 (who has charge of the instrument and superintends all its opera- 

 tions) also uses it for the predictions of the tides at Singapore and 

 Hong Kong, and expects shortly to include in its predictions the 

 tides of other Bast Indian ports, and also those of South Africa 

 and Canada. 



It may be stated that on the whole the predictions are very accurate 

 and give very general satisfaction. 



Simultaneously with the tidal observations, spirit-levelling opera- 

 tions were carried on, partly with a view to connect the tidal 

 stations and ascertain whether there is any appreciable difference 

 in the mean sea-level at the several places, and partly to connect 

 together and reduce to a common datum the hitherto isolated 

 system of levels which have been executed throughout the country 

 for canals, railways, and other engineering works. During 1877-78 

 Lines were carried from Damaun to Bombay, completing the line 

 wanted to connect the three tidal stations on the Gulf of ditch 

 with the one at Bombay : from Bombay to Callian Junction of the 

 (heat Indian Peninsula Railway, and thence up the Bhore Ghat to 

 Poona, and down to Satara ; from Callian to the Thai Ghat, and 

 thence up the Ghat to Igutpuri. Malegaon, and finally to Chikal- 

 wohol. In addition to these main lines, short branch lines were 

 carried to connect local bench marks of importance, the aggregate 

 length of line completed amounting to 589 miles, fixing 536 bench 

 marks. 



