80 Address. [Feb. 



briefer intervals. To the noi'th and east the long wave can be traced for 

 about 450 miles. To the west it travelled over great distances, reaching 

 Cape Horn and possibly the English Channel. To the south and east the 

 propagation of the disturbance was limited. The shorter waves reached 

 Ceylon and Mauritius. The velocities of the first great wave in mid-ocean, 

 as observed at different points round the Coast of India, were ; !N"ega- 

 patam 357 miles an hour. Madras 338. Vizagapatam 338. False Point 

 308. Dublat351. Beypore 326. Bombay 331. Karachi 340. At Aden, 

 347 miles an hour. 



Electro-telegraphic Longitude Observations. — The services of two 

 astronomical parties, in charge, respectively, of Lieutenant-Colonels 

 George Strahan and W. J. Heaviside, R. E., beiDg available, the opera- 

 tions for the electro-telegraphic determination of arcs of longtitu.de 

 were resumed in Southern India and seven arcs were measured, viz. — 



From Nagarkoil to Bangalore, Madras and Mangalore and from 

 Mangalore to Bellary (new). From Madras to Bangalore and Mangalore 

 (revised) ; and from Mangalore to Bombay (completed) ; thu3 completing 

 the South Indian work to Cape Comorin. 



In the course of these operations Colonel Strahan has made experi- 

 ments with the object of throwing light upon the causes of the errors 

 which occur in the observations ; and especially towards ascertaining 

 whether the mere elevating or depressing of the telescope affected in 

 any way its position in the meridian ; or in other words, whether there 

 is unequal flexure in the two halves of the transit axes acting at 

 different altitudes in an uncertain and valuable way, for which no 

 law has yet been traced. The method employed was by comparison 

 of observations on a star's transit by direct vision with those obtained 

 from it by reflection from the surface of mercury. It is satisfactory to 

 note that Colonel Strahan found from his experiments that the times of 

 transit by direct and reflected vision differed by only "05 of a second, 

 and that with the least trustworthy of the two telescopes employed. 



This season's measurements afford additional evidence towards 

 establishing a curious geodetic fact which now rests upon so strong a 

 basis as to be practically proved, viz. : — the deviation of the plunibline 

 round the coasts of India in the direction of the sea. This fact has 

 already been brought to notice in the preface to vol. IX of the " Opera- 

 tions of the Great Trigonometrical Survey of India." The conclusion is 

 based on the fact that, in every case of longitudinal arcs running east 

 and west with one or both stations on the coast, the astronomical arc 

 is less than the geodetic one, thus showing a displacement of the zenith 

 in the opposite direction from the sea. The numerical results cannot 

 be given at present, but the tendency is obvious and confirms the 



