1874.] Tresidenfs Address. 4i5 



Survey, devised, and which arose from the proposals that emanated from Dr. 

 Oldham, our late President, who pointed out that investigations should be 

 made of " the secular changes in the relative level of land and sea which 

 were believed to be going on at various places on the coast of the Bombay- 

 Presidency, and more particularly at the head of the Gulf of Cutch," are 

 now being carried out and the whole of the detail of the work is given in the 

 very interesting Eeport of the Trigonometrical Survey for the year 1872. 



During the past year, as a contribution of data for the determining the 

 figure of the earth, the Great Trigonometrical Survey Department has been 

 engaged on certain Electro -Telegraphic determinations of differences of lon- 

 gitude on the parallel of 13°. 



This arc of parallel was selected for several reasons. It extends from 

 Madras to Mangalor passing through Bangalor about midway. There are 

 Telegraph stations at each of these places and great facilities for communi- 

 cation. 



The arc is in 13° and is of peculiar interest, in that' it is situated much 

 nearer to the Equator than any similar arc which has yet been measured in 

 any part of the globe. 



The arc is 5° 24i^ 12" or about 364 miles, and it was on this arc that 

 Col. Lambton first endeavoured in the years 1802-5 to determine the length 

 of a degree of Longitude by the method of observing the astronomical lati- 

 tudes and azimuths of a series of reciprocating stations along it, a method 

 which though ill-adapted to low latitudes, was the only one then feasible for 

 him to employ. Thus it will be seen that circumstances have necessitated 

 the selection of the same parallel of latitude for the commencement of the 

 determination of longitudinal arcs by the modern Electro-Telegraphic method 

 that was chosen at the commencement of the present century by Col. Lamb- 

 ton for his corresponding investigations. 



The detailed description will be found in Col. Walker's Report, but I 

 may note that the operations were carried on under certain difficulties 

 for there was the necessity of employing a Telegraph wire which could only 

 be placed at the exclusive disposal of the observers for a few periods and 

 those very brief, and though the operations were invariably performed 

 during the night, when the ordinary traffic on Telegraph lines is compara- 

 tively little, it was found that the unrestricted use of a wire could, as a rule, 

 be only conceded for four periods of 15 minutes each, at intervals of two hours 

 apart ; on two nights, however, the use of the wire was granted for two 

 hours at a time, but the then unfavourable state of the weather at Madras 

 prevented this concession from being taken to account there. 



The preliminary results alone have as yet been obtained and they give a 

 Telegraphic determination of arc less than the Trigonometrical determination 

 by 13.95 seconds of arc. 



