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instrument of this description. It is founded on the fact that the images of two objects, the rays from which traverse portions of air having different degrees of transparency, will be of different brightness; and that, if their brightness is equalized, the amount of adjustment necessary for the purpose will be a measure of the difference of the transparency of the two portions of air through which the rays from them respectively have passed. To secure accuracy of result, the two objects must be seen with the same eye, in the same general direction, and in the same conditions; and the light from other objects must be excluded. M. De La Rive's instrument consists of two tubes, having each an objective at one end, and their other ends attached to a common eye-glass, of which each objective takes up half the field. The optical axes of the two objectives form an angle which may vary from 0° to 29°, at the will of the observer. The rays passing along the principal axis of each objective are made parallel with the axis of the eye-glass by two rotal and successive reflections—the first from a moveable and the second from a fixed prism. The movement of the moveable prism is so connected with that of the moveable tube, that the angle described by the prism is half that described by the tube. Whatever the points towards which the tubes are directed, the images of these points are in juxtaposition in the focus of the eye-glass. The instrument is proved to be properly adjusted by turning it round through 180°, so that the objects are seen the first and second times through different tubes. The images obtained are equalized by the means used with ordinary photometers. This instrument will measure the comparative brightness of two stars, or of different portions of the heavens.

New GAUGE FOR STEAM BOILERS.—One of the most frequent causes of steam-boiler explosions, is an insufficient water supply. This arises in some instances from neglect on the part of those in charge, but more usually from the difficulty of ascertaining the water level within the boiler. Many modes of automatic indication of a deficiency of water have been employed, and to a greater or less extent with advantage, but none are sufficiently effective to remove any possibility of accident. An American has, however, devised an apparatus for the purpose which is very simple, and appears to be very reliable. It consists in a tube fixed into the boiler a little below the proper water level, and projecting for some distance outwards, the internal extremity being open, and the outward closed. Around this tube is a casing, and the annular space between it and the tube is filled with water. As long as a proper water level is maintained in the boiler, the tube remains full of the fluid, but as soon as the water in the boiler falls below the opening of the tube, the latter becomes filled with steam or foam, and the water in the annular space around it boiling, steam is generated, and passes into a space prepared for it, where it blows a whistle, and even, if desirable, acts on a lever that opens the safety valve of the boiler and allows the escape of steam. Thus, not only is there notice of danger, but it is considerably diminished, until the proper measures are taken.

UNDULATING RAILWAYS.-The idea of an undulating railway is

not a new one: it has long since been proposed to utilise the force generated by descent down one incline for ascent up the next: so that some of the motive power required for propulsion of a train, should be obtained from gravity. An ingenious means of storing up the force of gravity so as to prevent the great variations of velocity which constitute one of the most serious objections to an undulating railway, is being experimented upon in Paris. The engine by which the motive power used for propulsion of the train is furnished, is provided with two heavy fly-wheels, capable of being made to work with or in opposition to the driving wheels. During descent, these fly-wheels, being made to revolve by the driving wheels, cause great retardation, and at the same time store up the power they have thus absorbed. Continuing to revolve with great velocity when it is necessary to ascend, they are so connected with the driving wheels that they cause them to revolve. The train is thus propelled, and at a practically uniform velocity, since from the large amount of matter the fly-wheels contain, they can lose a considerable amount of motion, without their velocity being greatly affected. Some motive power, independent of that obtained from gravity, would, of course, be required to supply the loss of that destroyed by friction, and the resistance of the air; but the amount must be inconsiderable.

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PROCEEDINGS OF LEARNED SOCIETIES.

GEOLOGICAL SOCIETY.— Nov. 6.
Warrington Smyth, Esq., President, in the chair.
A. Tylor, Esq., F.L.S., etc., read a paper on the Amiens Gravel.

The author referred first to the prevalent views respecting the gravels of the Valley of the Somme, namely, (1) That there are two deposits of distinct age—the upper and the lower valley gravels; (2) That the former of these is the older; (3) That the Valley of the Somme has been excavated to the depth of forty or fifty feet since its deposition; (4) That both gravels contain bones of extinct, animals, and implements of human manufacture, the lower gravels, however, containing the greater number of species of Mollusca, and the upper the greater number of flint implements; and (5) That the height (seventy feet) of the gravels of St. Acheul above the present level of the Somme is much beyond the limit of floods, and that, therefore, they could only have been deposited before the river channel was cut down to its present level. He then pointed ont that the general effect of these views is to refer back the remains of man found at St. Acheul to an indefinite date separated from the historical period by an interval during which the valley was excavated.

In former papers Mr. Tylor stated his belief that the upper and lower valley-gravels of the Somme are continuous, and of

the same age, which he considered to be close to the historical period. In this paper he stated facts which appeared to him to demonstrate the truth of his views, and described a number of sections near Amiens, in which the levels were laid down from an exhaustive survey by M. Guillom, Chief Engineer of the Northern Railway of France.

The conclusions he had thus been able to arrive at are the following: (1) That the surface of the chalk in the Valley of the Somme had assumed its present form prior to the deposition of any of the gravel or loess now to be seen there; (2) That the whole of the Amiens valley-gravel is of one formation, of similar mineral character, contains nearly similar organic remains, and belongs to a date not much antecedent to the historical period; (3) That the gravel in the Valley of the Somme at Amiens is partly composed of débris brought down by the river Somme, and by the two rivers the Celle and the Arve, and partly of material from the higher grounds washed in by land floods ; (4) That the Quaternary gravels of the Somme are not separated into two divisions by an escarpment of chalk parallel to the river, as has been stated ; (5) That the evidence of river-floods extending to a height of at least eighty feet above the present level of the Somme is perfectly proved by the gradual slope and continuity of the gravels deposited by them; and (6) That many of the Quaternary deposits in all countries, clearly posterior to the formation of the valleys in which they lie, are of such great dimensions and elevation that they indicate a pluvial period just as clearly as the Northern Drift indicates a glacial. This pluvial period must have immediately preceded the true historical period.

ROYAL GEOGRAPHICAL SOCIETY.-Nov. 25.

Sir R. Murchison, President, in the Chair. The following important letter relating to Dr. Livingstone was read, and it strongly encourages the hope that he is yet living, and may be continuing his journey with success.

We present the letter entire, because our readers may have occasion to refer to it on future occasions, when fresh information arrives. In the course of the discussion which took place after the letter was read, additional reasons were suggested for accepting its evidence. If true, many months may elapse before the esteemed traveller, whose fate is an object of such profound interest, can make his way to any locality from which information can be transmitted.

Zanzibar, Sept. 28, 1867. My dear Sir Roderick,—You know that a rumour has been current on the coast to the effect that a white man has been seen near Ujyl. Such a story came to us at a time when it was quite impossible that Livingstone could be the man. Now, however, another narrative has reached us, which, if we believe, it is, I think, difficult to avoid the conclusion that our distinguished traveller

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may even yet succeed, and disprove the story given us of his death by the Johanna men.

“ A Banian trader at Bagamayo told me three days ago that he had heard a rumour that some white man had been seen at Uemba ; of this he seemed to have no doubt. To-day he brought a native whom he introduced and left alone with me. I entered into a conversation with him, and led him in an irregular way to give a general account of his journey, without guiding his imagination by any leading questions, determining to meet him again and fill in the details. When I had dismissed him, after my first conversation, it appeared that a ship would sail for Bombay immediately, and, not to lose a chance, Mr. Churchill, the consul, to whom I gave the notes, at once sent all to Bombay, and a request that the substance might be telegraphed to the Foreign Office-viz., that we had now some grounds for believing that a white man resembling Livingstone has been seen to the south of Ujyl.

- This native, with the rest of the caravan, left Bagamayo, and passed along the usual trade route to Uemba and Marunga, where they remained trading for some time, and again returned to the coast, where, in one of the villages under Marunga, which is a region governed by several chiefs, more or less dependent on one paramount, a white man arrived with a party of thirteen blacks, who spoke Supeli. All had firearms, and six carried double-bar

The white man was of moderate height, not stout, dressed in white, and wore a cloth wrapped round the head. He gave the chief a looking-glass, and was offered ivory, which he declined, stating that he was not a trader. He then went northwards. I do not know that this man can tell much more; he is a simple carrier who formed part of a caravan, but if we can find the head man of the party, it will be possible, no doubt, then to identify this stranger, who seems to our hopeful imagination so like our long lost friend; and then only think of the revelation he will have to make to us.

“It is decided that we go to Bagamayo in two days to make inquiries, but we must do so quietly. The story of a white man having been seen at Ilruwa, to the west of the lake, is a distinct thing from the more definite narrative we now have. But the one adds confirmation to the other, and shows that if it be Livingstone, in whose track we now are, that he has more than half finished his work, and is about to go to the Albert Nyanza. I may mention that there is now no doubt that the white man of whom I wrote formerly, long ago, as having been seen on one of the lakes by an Arab, and who remained on the coast, was a Turk, one of the traders who remained on the coast at Gondokoro, who have been met with in Uganda by Zanzibar merchants. The description fully satisfied me of this, and nothing is more probable. Thus the traders of Egypt and Zanzibar have now met in the interior of Africa. Speke's route has been quickly followed; how far this has been for the immediate benefit of Africa others may judge. In the end Africa will be overrun with traders in all directions, and then the vast resources of the continent will be shown.

“P.S.-Since writing the above I have again seen my informant, and placed before him my books of photographic portraits. In the first book he did not recognize the likeness of the man he saw in the interior, although it contained a very fine side view of Dr. Livingstone. In the second he at once pointed out a staring likeness of Livingstone, which I kept as a caricature, and said, • That is the man. But,' he added, "come to Bagamayo and see my master and the other men; they have seen him also, and will tell you all they know.' Suspend your opinion for a little, Mr. Churchill, and I

go

in two days to Bagamayo to make inquiries. Please communicate this view to Mr. Webb, Miss Livingstone, and other friends, but until my next maintain some caution.

“JOHN Kirs."

ROYAL MICROSCOPICAL SOCIETY-Nov. 13.

James Glaisher, Esq., President, in the chair. J. Gorham, Esq., read an interesting paper on a peculiar venation chiefly traceable in the leaves of certain compositæ, of which the marginal veins found in Eryngo offered an interesting specimen.

At the close of the regular business the meeting was made special to alter the bye-laws. In future, the entrance-fee will be £22s., and the annual subscription £2 2s. The composition fee for new fellows, £21.

NOTES AND MEMORANDA.

Ross's New FOUR-INCH OBJECTIVE.- Mr. Ross has very judiciously decided on meeting the demand of microscopists for a low-power object-glass, adapted to viewing large live objects, polyzoa, etc., and has produced a four-inch combination of great merit and utility. Many highly interesting objects—including ana. tomical preparations, entire insects, small star-fishes, sponges, corals, etc.—can be shown better with this glass than with any other we have seen. With the A eye-piece of Mr. Ross' series it takes in an object 7-16" in diameter, and the field is beautifully clear and flat. With deeper eye-pieces a higher magnification may be obtained, accompanied by much greater penetration than deeper objectives and lower eye-pieces will give, which is an immense advantage in many investigations. Mr. Ross has not hitherto, like Messrs. Beck, given his instruments enough rack-room for such low powers. He can, however, by a simple arrangement, accommodate the new glass in a perfectly satisfactory way. Every microscopist who sees the working of this four-inch glass will infallibly desire to be its possessor.

CHEAP COMPRESSORIUM AND SLIDE-CELLS. - At the suggestion of the Editor, Mr. Curteis (Mr. Baker's) has turned his attention to the manufacture of Compressoriums, which, while not pretending to all the convenience and accuracy of the most expensive kinds, will meet the average requirements of students at s

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