1S7S.] Ohsrrvatiom in tie Qulf of Cufch. 27 



secular changes in the relations of land and sea, and that no conclusive re- 

 sults could be obtained unless the observations were carried over a period of 

 rather more than a year at the commencement, and a corresponding period 

 at the close, of the investigation. He further saw that if this were 

 done, the value of the operations would be greatly increased, because the 

 results would not only serve the purpose for which they were originally 

 contemplated, but would materially contribute towards the attainment of the 

 better knowledge of the law of the tides, which is considered by the British 

 Association to be so important a desideratum, and which is expected to lead 

 to an evaluation of the mass of the moon, to definite information regarding 

 the rigidity of the earth, to an approximation of the depth of the sea from 

 the observed velocities of tide-waves, to the determination of the retar- 

 dation of the earth's rotation due to tidal friction, and also to the various 

 practical benefits which necessarily accrue from accurate predictions of the 

 height of the tide at any given time. 



Preliminary Preparations. — With the sanction of the Secretary of 

 State for India, Lieut, (now Captain) A. W. Baird, R. E., Assistant Super- 

 intendent Gr. T. Survey, who was then in England on furlough, was deputed 

 to study the j^i'actical details of the mode of tidal registration and of the 

 harmonic analysis of the observations, which were recommended and prac- 

 tised by the Tidal Committee of the British Association. 



Lieut. Baird also tested at Chatham a new self-registering tide-gauge 

 constructed by Adie, the well-known optician and mathematical instrument- 

 maker, on the same principle as those he had previously sent out to India, 

 which were provided with barrels of unusual length (five feet) in order 

 that the tidal curves might be drawn on the largest scales practicable. The 

 new tide-gauge was on the same pattern, but with a few modifications, the 

 most important of which was the substitution of a chronometer escape- 

 ment instead of a pendulum or gravity escapement for the driving clock, in 

 order to permit of the instrument being erected on positions where the con- 

 cussions of the sea waves would interfere with and perhaps stop the action 

 of a pendulum clock. On trial it was found to work very satisfactorily. 



No tidal registrations can be deemed complete without simultaneous 

 registrations of the condition of the atmosphere, because it is well known 

 that the rise and fall of the tides on a line of coast is materially influenced 

 by the direction and force of the winds, and that it also varies inversely 

 with changes in the barometric pressure. Arrangements were therefore 

 made for supplying each tidal station with an anemometer and a barometer, 

 both self-recording. The anemometers registered both direction and 

 velocity and were similar to Beckley's, but smaller, in order to be light 

 and portable. The barometers were aneroids, because safely portable self- 

 registering mercurial barometers could not be obtained. 



