community to tolerate the company of such, which might be called social selection.

It is often assumed by writers on evolution that permanent differences in the methods in which a life-preserving function is performed are necessarily useful differences. That this is not so may be shown by an illustration drawn from the methods of language. The general usefulness of language is most apparent, and it is certain that some of the laws of linguistic development are determined by a principle which may be called "the survival of the fittest;" but it is equally certain that all the divergences which separate languages are not useful divergences. That one race of men should count by tens and another by twenties is not determined by differences in the environments of the races, or by any advantage derived from the difference in the methods. So easy recognition of other members of the species is of the highest importance for every species; but difference in "recognition marks" in portions of a species separated in different districts of the same environment is no advantage. Under the same condi tions, habits of feeding may become divergent; but, since any new habit that may be found advantageous in one district would be of equal advantage in the other district, the divergence must be attributed to some initial difference in the two portions of the species.

I have recently observed that, of two closely allied species of fat-fish found on the coasts of Japan, one always has its eyes on the right side, and the other always on the left. As either arrangement would be equally useful in the environment of either species, the divergence cannot be considered advantageous. Osaka, Japan. JOHN T. GULICK.

Self-Colonization of the Coco-nut Palm. THE question whether the coco-nut palm is capable of establishing itself on oceanic islands, or other shores for the matter of that, from seed cast ashore, was long doubted; and if the recent evidence collected by Prof. Moseley, Mr. H. O. Forbes, and Dr. Guppy, together with the general distribution of the palm, be not sufficient to convince the most sceptical person on this point, there is now absolutely incontrovertible evidence that it is capable of doing so, even under apparently very unfavourable conditions.

in the current volume of NATURE (p. 276) Captain Wharton describes the newly-raised Falcon Island in the Pacific; and in the last part of the Proceedings of the Royal Geographical Society, Mr. J. J. Lister gives an account of the natural history of the island. From this interesting contribution to the sources of insular floras we learn that he found two young coco-nut palms, not in a very flourishing condition, it is true; but they were there, and had evidently obtained a footing unaided by man. There were also a grass, a leguminous plant, and a young candle-nut (Aleurites), on this new volcanic island-a very good start under the circumstances, and suggestive of what might happen in the course of centuries.

W. BOTTING HEMSLEY.

On Certain Devonian Plants from Scotland. I AM indebted to Mr. James Reid, of Allan House, Blairgowrie, Scotland, for the opportunity to examine a collection of fossil plants obtained by him from the Old Red Sandstone of Murthly and Blairgowrie in Perthshire, some of which have been noticed by Dr. Geikie in his "Text-book of Geology."

The collection is remarkable for the striking resemblance of the matrix and the contained vegetable debris to those of the lower part of the Gaspé sandstones of Logan, and the species of plants are, so far as can be determined, the same.1

Palophyton princeps largely predominates, as in Gaspé, and is represented by a profusion of fragments of stems and branches, and more rarely by specimens of the rhizoma and of the sporocarps. P. robustius is represented by fragments of stems, but is less abundant, and Arthrostigma gracile by some portions of stems. On the whole the assemblage is exactly those of the sandstone beds of the lower division of the Gaspé sandstones. There is nothing distinctively Upper Devonian in the collection. The collection also contains two slabs of dark-coloured sandstone from Caithness, one of which contains what appears to be a fern stipe similar to those of the genus Rhodea. Another shows a remarkable plant having apparently a short stem giving See papers by the author, Journal Geol. Society, London, 1859, and Proseeding's Geol. Society, Edinburgh, 1877.

origin to a quantity of crowded leaves which are long, narrow, and parallel-sided, and show only a very faint linear striation. This plant is identical both in the form and arrangement of the leaves with that found in the Devonian of Canada, and which I have named Cordaites angustifolia. I have, however, already stated in my Reports on the Flora of the Erian of Canada (Geological Survey of Canada, 1871 and 1882), that I do not consider this plant as closely related to the true Cordaites, and that I have not changed the generic name merely because I am still in doubt as to the actual affinities of the plant. Mr. Reid's specimens would rather tend to the belief that it was, as I have already suggested in the reports above cited, a Zostera-like plant growing in tufts at the bottom of water.

Some of the sandstone slabs from Murthly contain specimens of rounded objects referable to Fachytheca (Hooker), a genus of uncertain affinities but characteristic of Silurian and Lower Devonian beds on both sides of the Atlantic. One of these is perfectly spherical with a shining surface, and 2'75 mm. in diameter, the others have been broken so as to show a central cavity or nucleus about I mm. in diameter, and with a thick carbonaceous wall partly pyritised and showing obscure radiating fibres. Prof. Penhallow, of McGill University, has kindly examined these, and has compared them with slices of Pachytheca from the Wenlock limestone, kindly communicated by Mr. Barber, of Cambridge, and with specimens presented by Prof. Hicks from the Silurian of Corwen and with specimens in the author's collection from the Silurian of Cape Bon Ami; and also with the excellent figures in Mr. Barber's paper in the Annals of Botany. He has not been able, however, to arrive at any conclusions beyond the probable general similarity in structure of the various forms, which may, however, as Mr. Barber suggests, have differed in their nature and origin. The only thing certain at present seems to be that these puzzling organisms had a thicker outer coat of radiating fibres, and of so great density that it was less liable to compression than the other vegetable tissues with which it is associated.

A few small specimens sent more recently by Mr. Reid contain some curious but not very intelligible objects from the same beds. One is a stem coiled at the end very closely in a circinate manner. In form it resembles the circinate vernation of Psilophyton princeps, but is much larger. It may belong to P. robustius, or possibly to a fern, but is too obscure for certain determination. Several others appear to represent flattened fruits or sporangia of obovate form and of large size. One has a stalk attached with what seems a rudiment of a bract, and another shows obscure indications of having contained round or disk-shaped bodies about 2 mm. in diameter. All show minute longitudinal striation. I have not previously met with bodies of this kind in the Devonian, and can only suggest that they may represent the fructification of some unknown plant, possibly that to which Pachytheca belonged. J. WM. DAWSON. Montreal, March 5.

Exact Thermometry.

I AM glad to observe that Prof. Sydney Young and myself are now in substantial agreement as regards the tension theory of the ascent of the zero in thermometers, and approximately in agreement as regards the actual cause of the ascent in the neighbourhood of the ordinary temperature.

Some time ago, in connection with an investigation of meltingpoint, I devoted three years to an examination of the properties of the mercurial thermometer. Among other conclusions which then seemed to me probable, the application of the known plasticity of glass under pressure to account for the enormous ascent (in lead-glass) of the zero at high temperatures appeared of some value. I have never advanced it as a mature theory, and am perfectly open to correction on the subject; but neither Prof. Crafts (with whom I at that time discussed the matter), nor any subsequent experimenter, has submitted the suggestion to a crucial examination.

Prof. Young's experiments (NATURE, March 27, p. 489) are very interesting as far as they go; but the kind of glass of which his thermometers are constructed is not that which brings out the peculiarities of the material in their most striking development. This, indeed, has long been known. It may well be that, in German soda-glass, the plasticity is masked by a preponderating tendency of the harder or more crystalline silicates of the bulb to set. Much could be done towards settling the question as to plasticity, if three thermometers of lead-glass-one vacuous,

one open to the air, and one with air sealed in--were heated together and successively to 100° C., 120°, 150, 200°, 250, 270°, and 3c0°, and the zeros observed. Even then, there still would remain to be explained the strange depression which I noticed in several sealed thermometers of lead-glass in the neighbourhood of 270°. At present, I regard the suggestion as neither proved nor disproved.

We are, in fact, only beginning to learn what silica and silicates are. I have quite lately, for example, found a critical point in the action of heat upon fire-clays, similar to the 270° point in the zeros (before referred to) of my lead-glass thermometers; and a similar point is known to exist in the relation of the refractive index of quartz to temperature. Results of this kind show clearly that thermometry is by no means an easy subject. Indeed, I might define it as a mixture of very complicated chemistry with very complicated physics. Glasgow, March 28. EDMUND J. MILLS.

The Shuckburgh Scale and Kater Pendulum. By permission of Prof. T. C. Mendenhall, Superintendent of the United States Coast and Geodetic Survey, and of Weights and Measures, I enclose to you for publication, if deemed suitable, a note relating to an abstract of a paper by General J. T. Walker, R. E., F. R. S., published in NATURE of February 20 (p. 381).

As the subject-matter refers to U.S.C. and G.S. Bulletin No. 9, I take the liberty of enclosing it also.

O. H. TITTMANN. United States Coast and Geodetic Survey, Office of Weights and Measures, Washington, D.C., March 13.

Last summer the United States Coast and Geodetic Survey published an investigation, Bulletin No. 9, on the relation of the yard to the metre.

As the result of this investigation, values were deduced for the length of certain historic standards in England which differed very materially from the values previously assigned to them in metric measures.

Thus the length of the Royal Society's platinum metre, certified by Arago to be 17:59 μ too short, was found to be only 7μ too short.

This metre was compared by Captain Kater with a certain space (0-39 4 inches) on the Shuckburgh scale, and this space was in turn compared with his pendulum. It is therefore of interest to know whether the value deduced in the investigation referred to is accurate. It is the object of this note to call attention to a surprising verification of the de luctions contained in Bulletin No. 9. Using the equation for the platinum metre found in that paper, namely―

NATURE of February 20 (p. 381) publishes an abstract of a paper by General J. T. Walker, R. E., F. R.S., "On the Unit of Length of a Standard Scale by Sir George Shuckburgh, appertaining to the Royal Society," in which he states that the Shuckburgh scale was taken to Paris and compared with one of the standard bars of the International Bureau of Weights and Measures, by Commandant Defforges. The result of this comparison reduced to 16 67 C., and as given by General Walker is

the space = I 1'0007619 m.

This agreement is perfect, more so, in fact, than the circumstances allow one to expect.

The agreement implies the correctness of the new values deduced in Bulletin No. 9 for the Ordnance metre and the platinum metre of the Royal Society, and gives the value of the metre as equal to 39 3699 inches as therein computed from Baily's and Sheepshan's comparisons, which established the relation between the Imperial yard and the space on the Shuckburgh scale.

It is to be noted that General Walker, ignoring Baily's and

Sheepshank's comparisons, and adhering to the Clarke value 39'3704+ inches, deduces the (the writer of this thinks) erroneous conclusion, that the space on the Shuckburgh scale equals 39400428 inches, the value according to their comparisons being 39'399896 inches. If to this value be added o‘04090 inch. the amount by which the distance between the knife-edges of the Kater pendulum exceeds the space 0-39 4inches, the resulting length of the Kater pendulum at 16 67 C. is 39'44080 inches, a value practically identical with that published by Kater, which is 39'44085 inches.

The Green Flash at Sunset.

THE explanation of the bluish (?) green flash of light some times seen at sunset given in your note last week (p. 495) does no seem to me to be a sufficient explanation of all the observations If the phenomenon were due simply to refraction it would last for only a fraction of a second, and the colour would be mucà more blue than green. But, so far as my own observations go, the colour may last for several seconds, and is a bright pesgreen, exactly similar to that shown by the sun many degrees above the horizon in South India in September 1883. produce that green, as I have shown elsewhere, all that is required is the absorption due to a great thickness of vapour, combined with a certain amount of dust-water dust or other

Τα

I saw a very pretty example of this last July when off the coast of Vancouver, B.C. The air was very moist and the rainband correspondingly strong, while fine dust was supplied by the land breeze carrying with it particles from the burning forests inland. The sky was cloudless, but the haze was thick enough to allow one to look at the sun while it was still some degrees above the horizon, and the disk appeared of a brilliant golden-red, gradually changing to yellow, and, finally, while part was still above the horizon, it became a bright pea-green The spectrum was similar to that figured in my paper on the green sun (R.S. E. Trans., xxxii. 389).

A few days later I had a view of the sunset from the Selkirks, where the air was very dry, the rain-band slight, but the har considerable. The colours of the sun's disk were much less brilliant, and never passed beyond the stage of a reddish-copper tint. C. MICHIE SMITH.

73 George Street, Edinburgh, March 31.

Foreign Substances attached to Crabs.

I MUST of course accept Prof. McIntosh's interpretation of his own statement, and admit that he has found Molgula arenosa frequently in the stomachs of Cod and Hadlock. This Ascidian differs from the majority of its class in having allocryptic habits, but I have not yet made a sufficient number of experiments to be satisfied as to its edibility. It has also been a considerable difficulty to me that the extensive investigations of Brook and Ramsay Smith lend no support at all to the opinion that the Ascidian forms an article of food for ground-feeding fish. In any case the matter, though of much interest, is not one for discussion here, since Molgula arenosa is never one of the foreign substances attached to crabs."

The statement made by Mr. Holt that " Actinia mesembry anthemum is a favourite food of the Cod," was so inconsistent with our knowledge of the habits and distribution of the two species that, as I expected, the grounds for his assertion prove to be entirely fallacious. My statement with regard to the offensiveness of Actinians to fishes was made after prolonged observation of the habits of the living animals and after experiment, while Mr. Holt bases his objection on the ground that the St. Andrews fishermen find A. mesembryanthemum to be a successful bait for Cod. One might as well argue that because bits of red flannel or of tobacco-pipe are highly successful bails in whiffing for Mackerel, therefore these substances form a "favourite food" of this fish. A moment's reflection alo would have shown Mr. Holt tha: an Anemone impaled upon a tishhook is a much less dangerous creature than one under natural conditions and with tentacles expanded.

During the past week an interesting observation of Eisig's has come under my notice which corroborates the view that the association between Crabs and Anemones is of primary importance for the protection of the Crabs. Eisig observed (see Journ R.M. S., iii., 1883, p. 493) that an Octopus in its attacks upon Hermit Crab would instantly retreat upon being touched by the stinging organs of the Actinian associated with it. Plymouth, April 5. WALTER GARSTANG.

AL

THE THAMES ESTUARY. 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 coast 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 nautical miles, and the whole of the space is encumbered 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 increasing trade, necessitating vessels of heavy draught, new channels have to be marked farther from shore, and the demand for additional security to navigation has especially 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 Hydrographer, when, under his instructions, Captain Bullock surveyed the whole estuary between 1835 and 1845. Since then, Calver re-surveyed the whole of the southern part in 1862-63, and examined the northern banks in 1864, and lately the Triton has resurveyed all the important channels and delineated the hanks, 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, describing 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 obstruction in a channel at different epochs, a characteristic permanent charge; 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 Whitstable and at Garrison Point, Sheerness); and in a few places, near 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 hard 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 Goodwin 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 direction: 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-eastward.

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, Queen's 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 distance from the shore has increased so that the marks erected on the land cannot be seen, we have either to erect other marks on the sand-banks and carry out a triangulation, 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 position 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 happens 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 Triton has found the Leman and Ower lightvessel one mile away from her charted position, the

Dudgeon light-vessel about one mile from her supposed position, and the Outer Downing light-vessel nearly two miles from the charted position.

All these light-vessels are either out of sight of land, or can only be seen from an elevated position on the shore on rare occasions.

It is therefore naturally the object of the Elder Brethren of the Trinity House to utilize the channels closest to the shore, and, as these channels are also the most direct into the Thames, the northern channel following the

general trend of the Essex coast, and the southern that of the Kentish coast, no other channels would require marking if the depth in these was sufficient for the traffic Hitherto the one northern channel has been enough, but this is steadily shoaling, as will be described further on: but the southern channels are mostly shoal, and one after another has had to be opened up as the size of the vessels and their draught of water increased, until there are now five buoyed channels off the Kentish coast, two of which are lit ; but only one can be termed a deep-water channel,

and this would seem to be the very channel which opens and closes periodically, as will be shown subsequently. Should this prove to be the case, there will be intervals during which there will be no deep-water channel into the river on the south side of the estuary.

By a reference to Plan I., showing, on a small scale, the whole estuary, it will be seen that the northernmost channel, viz. that close to the coast of Essex, is named the Wallet, and that this is separated by a series of banks, termed Buxey and Gunfleet, from the channel next it.

These banks, which are collectively 18 miles long, are dr for the most part at low water; there are, however, two narrow passages across them, one separating the Busey from the Gunfleet, called the Spitway, and the other separating the Buxey from the Dengie flat (extending from the Essex coast). The Spitway, which, when sounded in 1800, had a depth of nine feet, has remained at that depth until recently, but now has only a depth of 5 feet at low water; the channel between the Buxey sand and Dengie flat has about 12 feet, and is merely an

outlet for the River Crouch. It will therefore be seen that the Wallet is really only a channel to the Rivers Colne, Blackwater, and Crouch, and is of no importance as a channel towards the Thames. It was last surveyed by Staff-Captain Parsons in 1877, and as its features have not materially changed since 1800, it will probably not be surveyed again for many years, unless the swatchways across the Gunfleet should deepen or others open up of sufficient importance to render the Wallet useful as a traffic channel. There were formerly other swatchways across the Gunfleet, but these are now closed.

The channel next the Wallet is named the King's Channel, or Swin; the eastern part is named East Swin; the central part Middle Swin, and the inner part West Swin. This is the channel through which all the traffic between London and the northern ports of the Kingdom passes. and it is almost always crowded with shipping. The East Swin is bounded at first by the Gunfleet sand to the north-westward and the Sunk sand to the south-eastward, and is 3 miles wide; but 8 miles within its entrance two other banks commence-one, the Barrow, being very extensive, upwards of 13 miles in length and 2 in breadth; and the other, the Middle or Hook sand, a narrow ridge about 6 miles long, extending along the north-west face of the Barrow sand, and leaving a channel nowhere less than of a mile wide between them. It will thus be seen that 8 miles within the entrance of the East Swin it is split up into 3 channels; the northernmost retaining the same name, the channel between the Middle, or Hook sand, and the Barrow being known as the Middle Deep, whilst the channel between the Barrow and Sunk sands is known as the Barrow Deep. The Middle Deep rejoins the Middle Swin, but the Barrow Deep and West Swin both run into what is known as the Warp. The Swin is well buoyed and lighted throughout, but the Middle and Barrow Deeps have not yet been buoyed. In fact, it has hitherto not been necessary to do so, as the least water in the main channel of the Swin has, up to recently, been ample for all that has been required; but a steady shoaling has been taking place in a critical part of this channel since 1800, and it now seems to be only a question of time before the Middle Deep will have to be marked.

To illustrate the changes in progress here, Plan II. has been constructed, showing the condition of the critical part of the navigation in the Swin each time it has been thoroughly surveyed. By this diagram it will be seen that in 1800 the ruling depth in the channel between Foulness sand and the Middle or Hook sand was 35 feet at low water. Forty-three years later, a bar, on which the depth at low water was 28 feet, had formed between the Foulness sand and the Middle. In 1864 the depth had decreased to 24 feet, and, in 1889, to 21 feet, showing a steady decrease since 1800 of about one foot in every six years. The deposit is of sand, shells, and mud. This is the only shallow part of the Swin; and as it is evident that, so far as our knowledge extends, we may expect it to continue to decrease in depth, and as even now, with strong south-west winds prevailing in the North Sea, it is by no means rare for the tide to fall 3 feet below the level of low water ordinary springs, so that the depth would be reduced to 18 feet, it is clear that vessels of heavy draught will either have to wait for tide or use another channel. Already our small armoured vessels of war have to time themselves to reach this obstruction by half-tide. Fortunately, the Middle Deep is an alternative channel with ample depth in it, which only requires to be buoyed, and this can readily be done. This Deep seems to be in a better condition now than it has been for 50 years, for, when surveyed by Bullock, in 1843, there was a bar of 25 feet at its east end. This had disappeared when it was surveyed by Calver in 1864, and there was then a channel of two cables in width between the edges of the 30 feet contour lines of soundings surrounding the Middle

sand and Barrow. There is now a channel four cables in width between those contour lines in the narrowest part of the Deep.

The Barrow Deep, referred to as the third channel branching away from the East Swin, is deep throughout, and without obstruction. It varies somewhat, as shown by the different surveys, but is an excellent highway, which only requires buoying to be available for traffic. At present the London County Council are allowed to empty rubbish in this Deep, which seems rather a pity, as there is no knowing what may be the result eventually, more especially as we have at present no observations to show to what depth the tidal scour is of service. Any interference with the channels, likely to cause an obstruction, should be avoided.

The Sunk sand, which is the south-eastern boundary of the Barrow Deep and the north-western boundary of the Black Deep, has undergone great alterations since originally surveyed in 1800. In that year it is shown as a long sand which really extended from the present northeast end in one continuous line of shallow water to the inner end of the Oaze sand, a distance of 26 miles. On it were many dry patches, named Great Sunk, Little Sunk, Middle Sunk, Knock John, &c., and the only passage across was a three-fathoms channel at low water at the eastern end of the Oaze. When surveyed by Bullock, 1835-45, this chain of sands had altered very considerably, and had several channels or swatchways across it -a swatchway of 22 feet at low water between the Great and Little Sunk sands: a swatchway of 60 feet at low water between the South-West Sunk and the Knock John sands; a 35-feet channel 1 mile wide between the Knock John and North Knob sands; and a swatchway of 26 feet between the North Knob and the Oaze. When surveyed by Calver, 1862-64, this series of banks had again altered: the swatchway between the Great and Little Sunk sands had only 12 feet in it at low water; the swatchway between the South-West Sunk and the Knock John had shoaled to 40 feet; but the channel between the Knock John and North Knob had deepened to 45 feet, and a narrow channel of 40 feet at low water had opened out between the Oaze and North Knob.

In 1888-89, when surveyed by the Triton, the swatchway between the Great and Little Sunk sands had entirely disappeared; the swatchway between the SouthWest Sunk and the Knock John sands had narrowed and shoaled to 29 feet; the channel between the Knock John and North Knob shoals had decreased to 24 feet, whilst the channel between the North Knob and the Oaze had increased its width to one mile, with about the same depth (viz. 40 feet) at low water. In fact, the chain of sands known as the Sunk, Knock John, Knob, and Oaze, which were, in 1800, one continuous bank, after breaking up into separate patches, again show signs of resuming the form they possessed when originaliy surveyed, the only deep channel across them now being between the Oaze and North Knob.

The Black Deep is the channel bounded to the northwestward by the chain of sands just described, and to the south-eastward by another chain of sands named Long Sand: Shingles, Girdler, and the flats extending from the Kentish shore. It is a deep-water channel, the inner part of which has been buoyed since 1882, and lighted since December last, as it communicates by a deep-water swatchway, named the Duke of Edinburgh Channel, with the deep water off the North Foreland, and so forms a convenient outlet for the heavy-draught vessels bound southward from the Thames. There seems to be some tendency to shoal in the north-east end of the Black Deep, but it has only once been sounded--viz. by Bullock, in 1843; and we have not yet quite completed our examination of it throughout, so that no thorough comparison is yet practicable.

The chain of sands which bound the south-east side of