visible at some distance, and they are white or black, to be used according to circumstances; moreover, the signalmen are furnished both with binoculars and telescopes to enable them to read the signals from remote stations. At night either a bright colza light is made use of, or a spirit flame, into which is blown from to time a mixture of powdered magnesium and resin. A short puff or a long puff constitutes short and long signals, which are displayed, as before, in accordance to the Morse code. Every battalion of infantry and regiment of cavalry in the British army has a proportion of its men trained as signallers, so that these can act at once on taking the field. Their duty is to communicate between outlying pickets and the fighting column, and to do duty where there is no telegraph. For let the field telegraph of an army be ever so well ordered, there is always plenty to do for the army signaller; and he will doubtless find in the heliostat a means of fulfilling_these duties with increased efficiency. H. BADEN PRITCHARD FLOODING THE SAHARA 'HE French scheme of turning part of the Algerian THE Sahara into an inland sea continues to attract considerable attention in France, and scarcely a week passes without some allusion being made to it in the Paris Academy. At a recent sitting M. de Lesseps read a letter from Capt. Roudaire in which the latter gave some details of the results of his sounding of the soil at various points, sands and marls being the beds most commonly met with. At one place, four metres below the surface, plenty of potable water was met with, which will be a great saving in carrying on the work. At the same sitting MM. Ch. Martens and Ed. Desor presented several considerations against carrying out the plan, their opposition to it being shared by several other French men of science. They have themselves examined part of the ground which it is proposed to put under water, so that their opinions ought to have some weight. While giving every credit to M. Roudaire for the accuracy of the survey which he is carrying out, they, however, point out the difficulty of perfect accuracy, which in this case is all important, in the classic country of mirage, where the surface of the ground is constantly altered and deformed by reflection and refraction. Moreover, they point out that to the south of the projected sea is the Wed-Souf, where are ripened the dates known as Tunis dates, the culture of which is a very special one. The least error in surveying, it is shown, might lead to the destruction of this culture, by allowing the waters of the Mediterranean to penetrate the soil where the date-trees are grown, and thus destroy them. The authors do not attempt to touch the argument that even in historical times part of the Sahara now being surveyed was really a great lake; but they point out that there are proofs that in prehistoric times there must have existed an interior sea, at an epoch when the hydrographical conditions of Europe were very different from what they are now. In 1863, when exploring the region between the oases of Guemar and the south extremity of the Shott Mebrir, they found the gypsum beds of the plateaux ended in regular lines like sedimentary beds, and from the soil they collected the débris of shells, truly marine, such as Buccinum giberrulum, Lam., and Balanus miser, L. Above these shells, in the sand, they found Cardium edule, better preserved than they had ever seen it. Thus they found fossils characteristic of salt water, and of those which are a mixture of salt and sweet. The retirement of the waters from the Sahara the authors attribute to the elevation of the land, which is even yet below the level of the Mediterranean, and is to a great extent a network of salt lagoons. It has been said that the creation of an interior sea, of 13,280 square kilometres, would change the pluviometric condition of the country, and even that of the whole of | Algeria. This MM. Martens and Desor regard as a great illusion. Although the laws of the general atmospherical movements are little known, yet it is admitted that the Atlantic is the great reservoir from which come the vapours which are resolved into rain over the European continent. They believe that this is also the case for Africa. The Mediterranean is really only a Gulf of the Atlantic, and they do not believe that an addition of 13,000 kilometres will add anything to its climatic influence. Long calculations have been made as to the quantity of water that would be evaporated by the new sea; but the authors point out that the predominating wind in the region is north, and that if it were rendered either too cold or too moist it would injuriously affect the date-culture carried on in the south. The surroundings of interior seas, like the Caspian and Aral, are steppes noted for their aridity; the shores of the Mediterranean suffer in the same way when, as last year, the rains of the north do not extend to the south. For these reasons MM. Martens and Desor think it would be a mistake to insist on the creation of the interior Saharan Sea, Favé endeavoured to show that their fears were groundIn a subsequent sitting, however, it should be said, M. less, especially with regard to the accuracy of the survey; he thinks that the work in connection with the Suez Canal showed that perfect.confidence may be placed in the methods of surveying adopted. THE LONGEST TUNNEL IN THE WORLD SCHEMNITZ, the principal mining city of Hungary, has celebrated the opening of the Joseph II. Mining Adit, the deepest gallery of efflux of that place, and the longest subterranean work of this kind in the world. Its excavation was commenced in the year 1782, during the reign of the Emperor Joseph II., whose name it bears, and has been continued since that time, but with varied energy. The most rapid progress was made within the last five years, so that its completion on September 5th, 1878, was a kind of surprise, and was saluted by guns, which caused a great joy in the city, because it announced a new era for the mining operations of the whole district. Works of such importance deserve to be installed with solemnity, and a festival was arranged for the purpose on October 20-22, 1878. Prof. Szabó, one of the guests from Budapest, delivered a report to the Royal Hungarian Society of Naturalists, as a representative of that body, and we shall not hesitate to communicate an extract of this. As the mining operations were progressing in depth, there was at the same time a well regulated system of sinking shafts and driving tunnels employed. The Joseph II. Adit is the eleventh of that kind; it lies 200 metres deeper than the Francis Adit, which was until now the principal gallery of efflux for the mines of Schemnitz. This was excavated between the years 1494 and 1637 to a length of 1,968 metres; but a greater extension was given to it by continuing the works from 1747 till 1765. After this period the mines of Schemnitz proved to be so lucrative, that the idea of undertaking some greater work for securing the future prosperity of the mines was conceived, and so the plan was fixed of driving a tunnel at the deepest possible level, which could convey the waters to the valley of Gran, the lowest point indeed which could be obtained within a practicable distance. They commenced boring the tunnel west from Schemnitz, near the village of Voznitz, on the left bank of the Gran. The height of it is three metres, the width 16 metres. About the lower third is destined to convey off the waters, while the upper two-thirds are separated from this by a platform, and adapted for transporting the ores. According to the original plan it could have been finished in thirty years at the cost of 1,215,000 florins. The cost per metre would thus have been 87fl. 86kr., and indeed such was the case in the first eleven years; but after the French revolution the value of money was greatly changed, and the prices became so high, that in the next thirty-three years very little was done, the yearly progress not being more than 614 metres, and the cost per metre 371fl. 52kr. From 1826 the works were carried on with greater energy at the cost of 260fl. 40kr. per metre till 1835, when the progress again became slow, and remained so for the next eighteen years, only seventy-two metres being worked yearly, at the cost of 313fl. 45kr. per metre. But after the middle of the present century the sense of the decline of the mines from their former state of prosperity was so prevalent, that the director of the district, M. Russegger, well known on account of his scientific travels in Europe, Asia, and Africa, proposed that they should again devote greater energy to the works in question, as most of the mines were under water, and the raising of this by machines caused an outlay which the mines were not able to bear. For the next twelve years the yearly progress was 293'2 metres, at the cost of 237fl. 63kr. per metre. During the next five years after Russegger's time only 141'1 metres were worked out yearly. The Hungarian government has through the last ten years again developed greater activity in this work, and the parliament has at its request granted the yearly sum of 100,000 florins for the purpose. In the year 1874 there were still 2,326 metres to be worked out, which would under ordinary circumstances have been a task of eleven years; but in 1873 experiments were made in boring with machines, which method was tried for the first time in the Mont Cenis tunnel, with surprising success, then in the St. Gothard railway tunnel, and lastly in the "Sutro" gallery (Nevada). After many trials they succeeded in finding out the most convenient arrangement, and the whole work was done in three and a half years. With this method the entire tunnel could ... ... have been finished in 27 years. The length of the Mont Cenis tunnel is 12,233 metres. St. Gothard is... 14,920 metres. Sutro gallery of mines is 6,147 metres. Joseph II. Adit is 16,538 metres. The total cost amounts to 4,599,000 florins. The importance of this tunnel is very great, firstly as regards geology. The geological and orographical literature of that country is very old; Schemnitz has been repeatedly visited by distinguished men of science from all quarters of Europe, but the difficulties and complications of its geological structure are so great that there is still much to be done. One of the greatest obstacles in the way of investigation is that the surface is very seldom well exposed; dense forests and products of decomposition of the rocks cover many of the slopes. The tunnel furnishes a section more than ten miles in length, and gives not only valuable information as to the downward prolongation of the lodes known in the upper levels, but some new ones have been traversed, and the entire series of rocks, with their mutual limits as well as modifications and occasional transitions is to be seen without interruption. It is important secondly as regards mining. A new region has been made accessible, and the master-lodes can now be worked to their full extent, while in past years all activity was absorbed by the unlucrative Adit itself. Now the works again promise a long continuance. All the machines used in raising the waters are put away, and thence an outlay of more than 100,000 florins is saved yearly. The last and not least advantage consists in enriching the miner with new means of working. The application of mechanical boring may be considered as forming for him a new era, just as did the introduction of gunpowder; he will now much more easily undertake the driving of adit-levels, whenever this is feasible, and so, it is to be hoped, that the neighbouring old mining cities will successively have their galleries of efflux too, which is the essential condition of the restoration of their prosperity in mining. OUR ASTRONOMICAL COLUMN 3IS. BRORSEN'S COMET.-From an observation at Kremsmunster by Prof. Strasser on March 14, it appears that this comet has passed its perihelion several hours later than the time calculated by Dr. Schulze of Dobeln, the corrections to the ephemeris on that date being in right ascension and 3'5 in declination; yet observations at Rome on February 17 and at Arcetri, Florence, on March 10, give different corrections. For the present, as the ephemeris is sufficiently near for finding the comet, the following positions and distances may be extracted from it :— ... 2.59 16 25 44 26 59 ... 3 3 5 28 15 ... 3 658 29 32 ... ... ... 9 ΤΟ ... ... ... 9'9179 On March 10 Dr. Tempel estimated the comet brighter than a star of the eighth magnitude, the theoretical intensity of light at the time being 118; the maximum value attained this year is 3'33 on April 14, and during the latter half of April and the whole of May the comet will no doubt be well observed; from April 14 to June 10 it will be constantly above the horizon of Greenwich. its next return in 1884, its apparent track in the heavens is not likely to be a favourable one for observation, and as long a course of observation as is practicable at the present appearance will be desirable for carrying forward the elements of the orbit to 1890. At MIRA CETI. In 1879 and 1880 the minima of this variable occur at times when the star will be too near the sun to be observable, but the maxima, according to Argelander's formula of sines, take place under very favourable circumstances for accurate determination, in 1879 on September 11, and in 1880 on August 11. From the observations of Dr. Julius Schmidt at Athens, it appears that this formula, which had given the epochs of maximum in 1876 and 1877 (two) earlier than the observed times by 177, 168, and 19'4 days respectively, was only in error in this direction four days in 1878. Among variable stars now favourably situated for observation, may be mentioned Lalande 23617 and 23726, the former has been rated from 6m. to 9m., and the latter from 5m. to 8m. Also Lalande 26211, which has been noted as high as 6m. and as low as 9m.; the variation, however, appears less decided in this case, though Bessel estimated the star 8m.; Lalande's 9m. may perhaps be considered a misprint, as there are known to be similar cases in the "Histoire Céleste." THE MINOR PLANET HILDA.-A new determination of the orbit of this, the most distant member of the minor planet group, by Kühnert, of Vienna, assigns a period of revolution of 2,861 days, or 7.832 years, and an aphelion distance of 4'52; at this point of its orbit 'the planet is distant from the orbit of Jupiter only o'85, so that considerable perturbations are possible. The search for Hilda at the present opposition, so far as we know, has been unsuccessful. which, in this case, is made of straw, so as to secure lightness with absence of flexure under the comparatively heavy weights it sometimes has to bear; cd is a fine torsion fibre drawn from flint glass, to which the beam is suspended; it is cemented at d to a well-ground stopper, so as to admit of adjustment. When in position, cement, made by fusing together eight parts by weight of resin, and three parts of bees' wax, is run round the stopper. At c, the point of junction between the torsion-fibre and the straw beam, is a silvered glass mirror. At the end e of the beam, is a small pan to hold the weights counterpoising the disks, which are suspended to the other end. A flat stirrup of aluminium at f fits stiffly on the straw beam, and carries a flat glass fibre, fg, cemented to it so as to allow of no play, the straw beam, the aluminium hook, and the glass fibre being perfectly rigid. The experimental disks are fixed on the glass fibre by means I NATURE, vol. xv. pp. 224, 299. of a touch of cement at the back. The vertical tube is arranged to hold six disks, the top one, h, being always the same standard lamp-blacked pith; the others, i, j, k, l, and m, being changed each time. A small magnet, n, attached to the central mirror, and controlled by a barmagnet outside, gives the power of bringing the beam to zero, should it happen to get out of adjustment, without having to melt the cement and alter the angle of the torsion fibre by turning the stopper d. Plate glass caps at o and p, cemented to the ground edges of the tubes, give access to the interior; o allows the counterpoises to be adjusted in the pan, and p allows the aluminium stirrup to be unhooked, and the whole of the disks to be lifted out together. The apparatus is connected to the mercury pump by the arm and spiral q. The weights and dimensions of the various parts of the apparatus are as follows: Weight of straw beam, mirror, magnetic needle, aluminium stirrup, and flat glass fibre, &c. ... ... ... ... ... ... Average weight of six plain mica disks Average weight of six plain pith disks Length of straw beam, from centre of counterpoising pan to centre of disks Length of arm from centie of suspension to centre of pan Length of arm from centre of suspension to centre of disks... Glass torsion fibre-Length ... ... ... ... ... Torsion with a glass weight hanging oscillation in from it } 15'75 seconds. Fig. 2 shows the apparatus fitted up for experimentation. The disks are shown in position at a, a brick wall, bc, has holes pierced through it in two places, as shown, one hole, d, being opposite the centre mirror, and the other, e, opposite the disks. The aperture d is lined with card, lampblacked inside, and the interstices between it and the bricks are well plugged with cotton wool. A water cell at d prevents radiant heat from the lamp getting to the apparatus. Through the hole e pass six card tubes, lampblacked internally, 20 millims. diameter, and 23 centims. long. The tubes are firmly cemented to the wall, so that each shall be exactly central with its corresponding disk, and the outer end of each is closed with a cork. The space between the tubes and wall is well stuffed with cotton wool. The apparatus, being once fixed in position, is surrounded on all sides, as well as above and below, with cotton wool. Outside this is a row of glass bottles filled with water, and in front of all is a wooden screen. When protected in this manner, the inside of the apparatus is found to be free from disturbances caused by changes of temperature. When the disks have to be changed, air having been let in through the pump, access is easily obtained to the glass capp (Fig. 1), and the cement being softened by heat, and the cap removed, the disks are lifted out together by seizing the aluminium stirrup with forceps. A fresh set of disks being introduced, the apparatus is again packed up and re-exhausted. A lamp at throws a narrow beam of light on the mirror of the apparatus, through the aperture d. The ray is reflected to the scale g, where its deflection from zero shows the angular movement of the torsion beam when one of the disks is repelled by radiation. The scale is 1 metre from the reflecting mirror. A standard candle (the kind employed in gas photometry, and defined by Act of Parliament as a sperm candle of six to the pound, burning at the rate of 120 grs. per hour") is supported on a heavy stand, h, and can be raised or lowered by means of the sliding piece, i. Another sliding piece, j, carries a pointed wire projecting from it. The upright rod of the stand is graduated and numbered, so that when the sliding piece j is at mark I, the point of the wire is on the prolongation of the axis of tube and disk No. 1, and so on. Then, by sliding the candle up till the most luminous part of the flame is level with the point of the wire, it is known that the light will shine full on the disk under experiment. A half cylinder, k, covered with black velvet, protects the candle from draughts. The candle-stand slides along a straight edge, Im, screwed to the bench, so graduated that by bringing a mark on the sliding stand to one of the divisions, it indicates the number of millimetres separating the surface of the experimental disk from the centre of the candle flame. The experimental powders are laid on one surface of mica or pith disks as a water paint, no cement being used to promote adhesion. Disks of mica or thin metal were punched, while other materials were cut or filed into the shape of disks 17:25 mm. in diameter. The exhaustion, which had to be effected after each change of the experimental disks, was carefully brought to the same degree both by actual measurement on a McLeod gauge, and by getting the same repulsion on the standard black disk. In this way all the different results were fairly comparable one with the other. The presence of aqueous vapour was specially guarded against by means of tubes containing phosphoric anhydride. To show the effect of residual gas intending to equalise the amount of repulsion on variously coloured surfaces, I devised an experiment with pith disks, one being lampblacked and the other retaining its natural white surface, the standard candle being at the same distance in each case. When the exhaustion is good enough to cause a fair repulsion, the ratio between the amplitude of swing when the black is exposed, and that when the white is exposed, is as 100: 55'5; at a little higher exhaustion the ratio is, Black: White: 100: 42:5; at a still better exhaustion the ratio is, Black: White:: 100:35. The results of the quantitative examination of the repulsion resulting from radiation when falling on about 100 different substances I have arranged in fourteen tables, for details of which I must refer to the Bakerian Lecture for 1878. The repulsion is measured, first when no screen is interposed, and secondly, when a cell of water is inserted in the path of the rays. In comparing the two results it must be remembered that the actual amount of repulsion on the standard lampblacked disk, when the water screen is interposed, is only one-twelfth of the amount obtained when no screen is in the way, the distance of candle and other things being equal. one with the other the divided by 12. TABLE I.-Results of the Examination of Black Powders Compared with lampblack = 100 these have an average value of 92'2, which becomes 99'1 by the interposition of water. In order therefore to compare result behind water must be TABLE II.-White Powders These have an average value of 33'5, which is reduced to 83 behind water, The powerful absorption for the invisible heat rays which white powders exercise is somewhat remarkable. Assuming that the ultra red rays from a candle are almost entirely cut off by a water screen, the comparatively strong action (335) produced by the naked flame must be mainly due to the absorption of the invisible heat-rays; and when these are cut off by water, the action is diminished nearly fifty times. With black powders the water only diminishes the action about eleven times. TABLE III.-Red Powders screen is interposed, the action becomes 69'5, in comparison with standard lampblack = 100. Omitting selenium, the mean action on red powders without a water screen is 32°2, and with a water screen, 24'9. TABLE IV.-Brown Powders Amongst these, peroxide of thallium is remarkable as being repelled under the influence of radiation to a greater extent than any other body hitherto examined, its value being 1217, in comparison to lampblack = 100. Brown powders behave most like black, the averages of the columns without and with a water screen being 92'7 and 94'5. TABLE V.-Yellow Powders Among these, anhydrous tungstic acid resembles scarlet selenium in its anomalous action, the figures being, without water, 508, and with water, 72°2. The averages of the other yellow powders are 35'7, and behind water, 13.8. TABLE VI.-Green Powders These show some discrepancies, which will be referred Amongst these precipitated selenium is noteworthy. To the naked flame its value is 35'8, but when a water to farther on. TABLE VII.-Blue Powders These show a much stronger proportionate action behind a water screen than with no screen, the averages being 558 and 65.2. having no selective action on any visible ray of light, it can be used in conjunction with any coloured powder without complicating the results. Alum acts in a similar manner to water; coloured solutions act as water with a super-added action due to their colour. Very thick than a thin layer of water. Sulphate of copper, in a solution so weak as to appear only slightly green, has a very strong action when artificial light is used, as it cuts off the lowest visible red rays as well as the ultra red. TABLE VIII. Dyes and Colouring Matters of Organic plates of glass have less action on the invisible heat rays Origin Among these may be noticed saffranin, and a product of the decomposition of chlorophyll, which show an increased ratio or action when the heat-rays are cut off by water. Leaving out these, the mean actions of the other substances are, with no screen, 44'5, with a water-screen interposed, 28.1. TABLE IX.-Metals prepared in different Ways and coated with Lampblack, Mica, &c. Curious results are shown with iron and with gold, the former metal chiefly absorbing the invisible heat rays, whilst the latter metal is principally acted on by the luminous rays. I TABLES X. AND XA.-Various Silver Salts The chloride, bromide, and iodide of silver in their different states were exposed to the standard candle after being submitted to the action of magnesium light, sunlight, and daylight. The results show how readily a change in the state of the surface is detected by an increased amount of repulsion under the influence of radiation. TABLE XI.-Selenium-Crystalline and Vitreous The former is in the state most sensitive to light action. With the crystalline disk results have been obtained which seem to show that the impact of light on its surface produces a superficial disturbance there and in the adjacent gaseous molecules, which takes some time to subside. This is connected with the change in electric conducting power of crystalline selenium-a change which, when the element is transferred from light to darkness, also takes some time to subside. TABLE XII.-Miscellaneous Substances-Pith, Mica, Charcoal, Glass The complicated nature of these actions was well shown in the results I obtained with three pith disks, the first being plain white, the second lampblacked on the front, and the third lamblacked on the back. The first was repelled with a power of 177, the second, which was the standard, with a power of 100, whilst the third was not moved at all. The repulsion exerted on the white surface must have been the same in each case, but the pressure behind the pith caused a radiation of heat from the back surface, which produced molecular pressure just sufficient to neutralise the pressure in front. To show that physical condition has more effect in causing repulsion than chemical composition, I experimented with various kinds of charcoal. I found that the repulsion suffered by cocoa-nut shell charcoal is much less than that of white pith, being only 116 against 17'7. At the same time a radiometer made of cocoa-nut shell charcoal, lampblacked on one side, was only moderately sensitive, instead of being superior to one made of pith lampblacked on one side. The low figure shown by the charcoal was caused by its density enabling it to conduct heat from one surface to the other. Molecular pressure is therefore generated on both the back and front surfaces, and the figure I obtained is simply the difference between the two opposing actions. I used other screens, besides water, to filter the radiation of the candle before it fell on the disk. I, however, preferred water. It is almost perfectly opaque to the invisible heat rays, and therefore its employment allows easy discrimination between actions due to heat and to heat and light combined; secondly, it is colourless, and I found that the substances I had experimented on might be divided into two classes. 1. Negative, those in which the repulsion behind water is greater in proportion to the standard than when no screen is present. 2. Positive, those in which the repulsion in proportion to the standard is less behind water than when no screen is present. Amongst Class I may be mentioned copper tungstate, saffranin, scarlet selenium, and copper oxalate; these are more affected by light than by invisible heat. Amongst Class 2 I may mention pale green chromic oxide, persulphocyanogen, hydrated zinc oxide, barium sulphate, and calcium carbonate; these substances are more acted on by the ultra-red rays than by the luminous rays. To render these differences of action more comparable, I divided the averages obtained by the water An examination of this table shows that the results can be proved by balancing one powder against another in a radiometer. A bulb was therefore blown on the end of a wide tube, as shown at Fig. 3. The top of the bulb was opened and turned over to form a lip; this was ground smooth and polished, so as to be readily closed by cementing on it a piece of plate glass. A glass stem supports a fine needle in the centre of the bulb, and on this rests a glass cap, to which is attached four radial arms of aluminium. these arms disks of mica or pith can be fastened so as to form the movable fly of a radiometer. The disks can be changed by uncementing the glass top, and lifting the fly To |