lOG J. Waterliouse — Tidal Observations in the Gulf of Cutch. [April, 



(Abstract.) 



This paper contains an account of the operations connected with, and 

 the final results of, the first series of Tidal observations made, in seasons 

 1873-74 and 1874-75, by a party of the Great Trigonometrical Survey 

 under Capt. A. W. Baird, R. E., with the primary object of deter- 

 mining the existing relations between the level of the land and the 

 sea at certain points on the coasts of the Gulf of Cutch, as a first step 

 towards investigating the question whether progressive changes are taking 

 place in the level of the land at the head of the Gulf, as has long been sup- 

 posed to be the case. 



The paper will be published in full in the Journal, Part II. 



Colonel Walkek observed that the reduction of tidal observations 

 is a very laborious matter, but that when once the values of the two con- 

 stants — the amplitude and the epoch — have been determined for each of the 

 several hypothetical tides and their sub-tides, the varying height of the sur- 

 face of the ocean, from hour to hour and from day to day, may be graphi- 

 cally represented, with great facility and rapidity, by an instrument recent- 

 ly invented by Sir William Thomson, which is at present in the collection 

 of scientific instruments at South Kensington. 



He showed that any one of the constituent tides might be graphically 

 represented by the action on a rotating cylinder — such as the barrel of an 

 ordinary self-registering instrument — of a pencil connected with a point in 

 the circumference of a revolving wheel. When the wheel is suspended 

 vertically and the pencil is held in a vertical groove, so as to be free to 

 move up and down against the barrel, the pencil is made by its connection 

 with the point on the wheel to travel backwards and forwards through a 

 distance equal to the diameter of the wheel, and it thus describes, on the 

 surface of the rotating barrel, a curve of which the ordinates are equal to 

 the height of the point above the centre of the wheel at any moment, while 

 the ahscissoe denote the times corresponding to the heights. Just as a 

 single tide can thus be graphically delineated, so may the resultant of a large 

 number of tides be represented. In Sir Wm. Thomson's machine, as many 

 wheels as there are tides are constructed, the radius of each wheel being made 

 to correspond with the amplitude of the tide it has to represent ; the wheels are 

 centered to a vertical frame, half of them above and the other half below an 

 axle by which each wheel is driven and caused to revolve on its axis with a 

 velocity corresponding to that of its tide. To the circumference of each wheel 

 a stud is attached at a point corresponding to the epoch of the tide. A silk- 

 en thread is fastened to the stud of the wheel most distant from the pencil 

 in the upper row, it is then brought down and passed round the stud of 

 the wheel immediately below, then carried up to and over that of the next 

 wheel above, and so on until, eventually, after having been passed round 



