April lo, 1890] 



NATURE 



539' 



THE THAMES ESTUARY. 



A LTHOUGH it is not practicable to say precisely 

 -^*' where the river ends and the estuary commences, 

 it will be sufficient for general purposes if the westward, 

 or inner, boundary of the Thames estuary is assumed to 

 be a line from Southend to Sheerness, the northern 

 boundary as the coast of Essex, and the southern the 

 roast of Kent ; and it may be said to extend eastward to 

 the meridian of the Kentish Knock light-vessel. The 

 area inclosed between these lines is upwards of 800 

 square nautica] miles, and the whole of the space is en- 

 cumbered with banks, between which are the several 

 channels leading to the river. 



As the shores of Essex and Kent are low, and have no 

 natural features by which they may be distinguished at a 

 distance, and as a great part of the estuary is out of sight 

 of land, even in the clear weather so rare in this country, 

 it is evident that artificial marks in considerable number 

 are required to make navigation at all practicable between 

 the banks. In early times, when vessels were small and 

 of light draught, few marks were necessary, but with in- 

 creasing trade, necessitating vessels of heavy draught, 

 new channels have to be marked farther from shore, and 

 the demand for additional security to navigation has espe- 

 cially increased of late years, so that now there are no 

 less than 3 lighthouses, 11 light-vessels, 8 gas buoys, 10 

 beacons, and 117 ordinary buoys marking the channels 

 at present in use ; and the demand for additional marks 

 is likely to increase rather than diminish, for the deepest 

 channels through the estuary have not yet been buoyed, 

 and the changes in progress seem to favour the opinion 

 that before many years some of them will have to be 

 opened up to facilitate traffic. 



In endeavouring to give an account of the changes in 

 the channels of the estuary, it is difficult to obtain any 

 authentic records earlier than the commencement of the 

 present century. If such records exist, they are not at 

 the Admiralty or Trinity House, the earliest surveys 

 worthy of notice being those of Mackenzie, Graeme 

 Spence, and Thomas, between 1790 and 1810; but no 

 thorough investigation appears to have been taken up 

 until Sir Francis Beaufort was Hydrcgrapher, when, under 

 his instruction?, Captain Bullock surveyed the whole estu- 

 ary between 1 835 and 1 845. Since then, Calver re-surveyed 

 the whole of the southern part in 1862-63, and examined 

 the northern banks in 1864, and lately the Tritoii has re- 

 surveyed all ihe important channels and delineated the 

 banks, and from these several surveys some idea can be 

 obtained of the condition of the estuary at different 

 epochs, and of the changes that are taking place. 



These changes seem to be of two kinds ; viz. permanent 

 changes and periodic changes. 



Before, however, des^cribing the changes in progress, it 

 will be well to give a general description of the estuary ; 

 and, to render the description more intelligible, three plans 

 have been constructed, the first showing the whole estuary 

 on a small scale with the tracks followed by vessels ; the 

 second being a diagram showing the state of an obstruc- 

 tion in a channel at different epochs, a characteristic 

 permanent change ; whilst the third plan shows the state 

 of the Duke of Edinburgh Channel from the time of its 

 first opening out to the present date, to illustrate what 

 seems to be a channel opening and closing periodically. 



It is worthy of notice that all the banks of the estuary 

 are of sand intermixed with shells ; even the foreshore 

 consists mostly of sand, between high and low water 

 marks ; in two places only is it of shingle (viz. off Whit- 

 stable and at Garrison Point, Sheerness) ; and in a few 

 places, rear the entrance of the rivers discharging into 

 the estuary, there is a little mud, whilst in the vicinity of 

 Margate there are some ledges of chalk. The sand is 

 very fine, and although, when dry, it possesses a tolerably 

 bard surface,,directly it begins to be covered it is all alive. 



When beacons are erected on any of the banks, or a ship' 

 gets on shore, the tidal streams scour out the sand in the 

 immediate neighbourhood, and cause the wrecks to sink 

 and finally disappear. Although without actual boring it 

 is not possible to give the exact depth of these sands, it 

 is probable that they are upwards of 60 feet thick, for 

 channels of that depth have opened out across the sands 

 and again closed up, so that the bank has been dry at 

 low water where 60 feet formerly existed ; and the Good- 

 win Sands, in the Downs, which have been bored, proved 

 to be 80 feet in thickness. All the banks, and the channels 

 between them, trend in a north-east and south-west direc- 

 tion : this is doubtless due to the fact that the stream 

 outside the estuary is running to the northward whilst 

 the tide is ebbing from the river, and, consequently, the 

 ebb stream in the estuary is deflected to the north-east- 

 ward. 



The channels into the estuary, therefore, must be classed 

 under two headings : {a) those which follow the main line 

 of the flood and ebb streams, and [b) those which do not 

 follow the general stream of the tide. 



In the former category are the Warp, West Swin, 

 Middle Deep, East Swin, Barrow Deep, Oaze Deep, and 

 Black Deep ; in the latter are the Middle Swin, (2ueen'& 

 Channel, Prince's Channel, Alexandra Channel, Duke of 

 Edinburgh Channel, Gore Channel, &c., which are all 

 more or less of the nature of swatchways across the main 

 line of the sand-banks of the estuary. In the Black and 

 Barrow Deeps, which are the deepest and straightest 

 channels through the estuary, the ebb stream runs 7 hours 

 and the flood 5 hours, and the ebb is much stronger than 

 the flood, the stream setting fairly through. In the Duke 

 of Edinburgh Channel, the deepest swatchway of the 

 estuary, the streams at the north and south ends are of 

 a rotatory character, revolving with the hands of the 

 clock. 



I would here explain that in a large space like the 

 Thames estuary the difficulty of buoying the various 

 channels increases very considerably with their distance 

 from the shore. With permanent marks erected on the 

 shore, it is easy to place buoys in selected positions, not 

 far from land, in fairly clear weather. But when the dis- 

 tance from the shore has increased so that the marks 

 erected on the land cannot be seen, we have either ta 

 erect other marks on the sand-banks and carry out a tri- 

 angulation, or we are dependent on floating bodies (fixed 

 by land objects) to fix other floating bodies farther off. 

 That this is an eminently unsatisfactory method will be 

 evident when it is stated that each time the Kentish 

 Knock light-vessel has been satisfactorily fixed, the posi- 

 tion has been very different from that supposed. When 

 fixed by Calver in 1864, she was found to be one mile 

 N.E. ^ N. of her charted position ; and when fixed by the 

 Triton last year, she was found to be one mile and a half 

 S.E. by E. of her supposed position. 



The errors probably creep in somewhat in the following 

 way. Something goes wrong with the light- vessel after she 

 has been satisfactorily fixed : a collision takes place, the 

 fog-siren gets out of order, or one of the many things hap- 

 pens which necessitates the vessel being taken into port. A 

 temporary light-vessel is substituted, and she is anchored 

 in almost precisely the same position as the other, but 

 probably before her mushroom bites the ground it has 

 dragged somewhat. By the time the other vessel is 

 repaired and brought out, the temporary one may be a 

 cable or so away from the original position. As the 

 weather is usually thick, the permanent vessel has to be 

 anchored as nearly as practicable in the position of the 

 temporary craft, and her mushroom may drag somewhat 

 before biting the ground, &c. Thus a series of errors 

 creep in without there being adequate means of checking 

 the position of the light-vessel, and within the last few 

 years the Triiofi has found the Leman and Ower light- 

 vessel one mile away from her charted position, the 



