23 Janll:Lry, 1917. SiI, MAURICEFITZMAURICE, C.M.G., President, in the Chair. (Paper No. 4195.) “ On the Physical Features of ‘Adam’s Bridge ’ and the Currents Across it, considered as affecting the Proposed Construction of a Railway connecting Indiawith Ceylon.” By FRANCISJOHN WARING, C.M.G., M. Inst. C.E. THE subject of this Paper is, it is thought, sufficiently indicated by its title, and in the latter portionof it the Author will refer, in the hope of eliciting discussion, tothe vnrioas methods which have Ileen proposed for c:trrying out the connection in question. Aplrrtfrom its conmerc:i:d and possibly st,rategic import’ance, the work presents unnsnd engineeringdiaculties, due to its n~xgnitnde,tjhe strength of thecurrents at timesprevalent, and other 10c:tl conditions.Nevertheless, in view of the recent exten- sions of the South Indim Rai1w:Ly to Dhanushkodi, theeastern point of Rameswaram Island,and of the Ceylon Government Railway to Tnlai Mannar,the western point of MannarIsland, leaving only “Adam’s Bridge” to be dealt with in order to complete the land connection between India and Ceylon, the commencement of the undertaking may possibly not be long delayed; and thus the light-draughtferry-steamers, by which communicationbetween Dhanushkodi and T;tLi Mannar is now carried on, may be replaced eventually by ;I railway. It shouldbe explained here that the railways *just mentioned bifurcate at Dhanushkodiand Talai Mannar respectively, and terminate in steamer-piers erected on the north and south coasts of the islands of Rameswaram andMannnr. It was intended originally that during the north-east monsoon the steamers would ply between the southern piers, and during the south-westmonsoon Downloaded by [ University College London] on [20/09/16]. Copyright © ICE Publishing, all rights reserved. Proceedings.] ADAM'S FTi1I)CrE AND TtlE CURRENTS ACROSS IT. 28s between the northern piers, Adam's Bridge thus acting its a break- water,and ensuring a smooth sea-passage in bothmonsoons; it was found, however, possibly owing to the oceanic swell in the Gulf of Mannar, that the southern passage, even during the north-east monsoon, was more difficult than was anticipated, while during that monsoon the northern passage was less difficult than was anticipated, and, as a result, since 1914 the steamers have continuously used in both monsoons the passage to the north of Adam's Bridge. In the eleventhedition of the Encycloprerlia Brifannicn (1910) the article upon Ceylon states that:- ('The coast is beset on the north-west with numderless sandbanks, rock3 and shoals, and may be said to be almost connected with India by the islan(l of Rameswaram and Adam'sBridge, a succession of bol(lrocks reaching almost across the gulf at its narrowest point,." ' The impression thus conveyed is erroneous,as Adam's Bridge is a bank superficially of clean, fine, greysand, overlying, it is said, for a portion of its length, rock, which stretches in a fairly directline for about 21 miles between Dhanushkodi, atthe eastern extremity of Rameswaram Island, off the coast of India, and Talai Mannar, the western extremityof Mannar Island, off the coast of Ceylon. With the islands just mentioned and the channels (comparatively narrowand generally shallow) between them and the countries to which they respectively belong, this link between India and Ceylon extends in a fairly straight line from Toniturai orRaman Point in India(north latitude 9" 17', eastlongitude 79"11') for n distance of 55 miles in an east-south-east direction to Taladi, in Ceylon (northlatitude 8" 56' 30", eastlongitude 79" 56' 30"). It not onlysuggests in an emphatic manner the only practicable route for a connectionby land between the twocountries, but also divides the channelbetween Ceylon and India very definitely into two parts of widely different character (Fig. 1, Plate 6). The southern part of this channel, known as the Gulf of Mannar, is an arm of the Indian Ocean of generallyregular outline. Its entrance, facing south-west, has :L width of 180 miles between Cape Comorin and Colombo, taperinggradually to 55 miles between TonituraiPoint and Taladi. The gulf is of considerable depth, nearly1,200 fathoms being found about midwaybetween Cape Uomorin and Colombo ; about 50 miles north of this a depth of 1,247 fathoms occurs about 40 miles from the coast of Ceylon, while even within 17 miles south of Adam's Bridge a sounding of 150 fathoms failed to reach the bottom. Downloaded by [ University College London] on [20/09/16]. Copyright © ICE Publishing, all rights reserved. 286 WARING ON THE PHYSICAL FEATURES OF [Minutes of The northern part of this channel, known as Palk Strait, is, on the contrary, of irregular outline, and is very shallow, the depth over the greater portionof its extent beingbetween 6 and 8 fathoms. A greater depth than 9 fathoms is nowhere found, and even that occurs only in one spot of smallarea. Immediately to the north of the direct line between Toniturai Point and Taladi the width of the strait is increased, mainly by the westerly trend of the Indian coast, from 55 miles to about 84 miles. This width is maintained without much alteration for some 34 miles northwards, when by the westerly trend of the Ceyloncoast and the islands lying off the north-western point of Ceylon, the width of the strait, which then turns to the east, is reduced to about 40 miles, a width approxi- mately maintained to its entrance, which faces north-east and lies between Point Calimere, in India, and Point Pedro, in Ceylon. The physical features of bothparts of the channel,including their entrances, which it will be observed face directly the prevalent monsoon winds, have, in the Author’s opinion, an important bearing onthe currents existing at Adam’s Bridge : hencehis reason for mentioningthem. The whole of the channelbetween India and Ceylon lies inthe region of the south-westa,nd north-east monsoons, the former usually setting in about the latter part of May, or early in June, and the latter in October or November. Both monsoons blow with greatest force at their outset, and strong winds from the respective directions blow steadily for some months ; as each monsoon progresses the wind gradually becomes lighter, and generally towards the end of each monsoon it has but little strength. The effect of these winds blowing steadily from the same direction for considerable periods is to raise the water-level on the windward side of Adam’s Bridge, and to withdraw it from the leeward side. Coupled withthe non-synchronism of the tides, this causes a maximum difference of about 18 inches between the water-levels on thetwo sides, thegreatest effect beingobserved, it is believed, during the north-east monsoon, due possibly to the configuration of Palk Strait, to which reference has just been made. The breadth of the base of Adam’s Bridge reckoning from the 7-fathom line on either side-that being the usual depth of Palk Strait in theneighbourhood-is generally about 10 miles, but owing to a southerly extension of shoal water towards its eastern end it is there somewhat wider. The sand of which it is superficially com- posed extends to a depth of at least 25 to 30 feet below sea-level, as disclosed by borings taken by the Author in 1895, the tackle at his disposalpreventing greater depths from being reached.Borings taken in 1913 by the late Mr. J. T. Lewis, M. Inst. C.E., Chief Downloaded by [ University College London] on [20/09/16]. Copyright © ICE Publishing, all rights reserved. 1’rOCeed;ngs.j AbAbT’S BkIDGE ASD TllE CURRENTS ACROSS IT. 287 Engineer of the extension of the South IndianRailway to Dhanush- kodi, disclosed rock for a distance of 15 miles from Dhanushkodi, at a depth ranging between 29 and 41 feet below mean low-water level, but, eastward of this the rock dips, and none was found at a boring carried to a depth of 36.60 feet below mean low-water level. From Dhanushkodi the general direction of Adam’s Bridge for the first 11 miles of its length is about east-south-east ; it then graduallycurves to the north, and for thelast 5 or 6 miles its direction is somewhat north of east. Along its crest, which gene- rally occupies a central position with regard to its base on either side, as defined above, thus giving, reckoning from sea-level to the 7-fathom line, an average transverse slope of 1 in 628 on each side, R chain of several sandbanks above sea-level occurs, the channels between them being sometimes of considerable depth. The author found 34 feet in one of them in 1895, while Mr. Lewis found in 1913 a depth of upwards of 42 feet in a channel about 4 mile from Dhanushkodi;speaking generally, the narrower the channel the greater is its depth, and through these channels the current runs in the direction of the prevailing monsoon. It has been estimated, not only by Mr. Lewis but also by Mr. H. J. Strscey, M. Inst. C.E., Chief Engineer of the extension of the Ceylon Government Rail- way to Talai Mannar, both of whom had ample opportunities for observing it, that when the monsoons are blowing with their greatest force andthe difference of the water-level on the twosides of Adam’s Bridge is at its maximum, the velocity is about 6 miles per hour; and in fact at such times Adam’s Bridge, with the islands and the channels separating them from the mainland on either side, appears to act as a weir. It must not be inferred from the depths of the channels between the islands forming the crest of Adam’s Bridge that any of them are navigable, except by canoes and boats of very small draught, and even these can pass through only with difficulty and some danger ; for the water, which when the head and velocity are greatest isvery turbid, being heavily charged with sand in suspension scoured out of the channels, begins to deposit it as soon as the head is lost and the currentslackened, in effect forming a bar below each channel.