1878.] J.T.Walker — Beduction of Tulal. Observations. 107 



the stud o£ each wheel in succession, it is fastened to the recording pencil. 

 The curve traced by the pencil now represents the aggregate result of all 

 the component tides corresponding to the several wheels, that is to say, it 

 represents the momentarily varying level of the surface of the ocean at 

 the station where the observations were taken. Though the ordinary mo- 

 tion of the pencil is up and down, there is an arrangement by which the 

 pencil is slightly jerked to one side after a certain number of revolutions 

 of the driving axle ; this is done to mark the successive hours on the curves. 

 At spring tides the range between high and low water is considerable and 

 the hour-marks are far apart ; while at neap tides the range is much less 

 and the hour-marks are correspondingly closer. 



One of the great advantages of this instrument is that with its aid 

 the sea-surface curves for an entire year can be drawn in about three hours ; 

 while a skilful computer would probably take a month to obtain the same 

 results by calculation. A new instrument of this kind is now being con- 

 structed in England with all Sir William Thomson's latest improvements, 

 for the use of the Survey Department, and it will be of great value and 

 assistance in the preparation of Tidal Tables for the several Indian ports, 

 a duty which the Government have lately imposed on that Department. 



Mr. H. r. Blaneord asked Colonel Walker whether the discussion 

 of the barometric and anemometric data in conjunction with those of the 

 tidal registers of the Gulf of Cutch would throw any light on the respec- 

 tive influence of pressure and wind-friction in piling up the surface of the 

 ocean. The storm wave that accompanies cyclones is an instance of the 

 combined action of these two agents, and it would be interesting to know 

 in what measure they were severally effective. 



Colonel Walkek replied that at one of the tidal stations, Hanstal, 

 the changes of wind and pressure were so nearly synchronous that it was 

 impossible to separate the effect of the wind from that of the pressure. 

 At another station, Okha, very fairly reliable measures of the same effect 

 of each were obtained. They are given at the end of the analysis of the 

 observations. 



3. Recent Trans-Frontier JExplorations. Communicated ly Colonel 



J. T. Walker, C. B., K. E., Surveyor General of India. 



(Abstract.) 



This communication consists of an extract from the last Annual Report 



of the Great Trigonometrical Survey, giving an account of the explorations 



made by one of the G. T, Survey explorers, called the ' Mullah,' during the 



year 1876, up the course of the Indus from the point where it enters the 



plains above Attock to the point where it is joined by the river of Gilgit, 



which had up to the present time remained a blank on the maps. 



