April z, 1879] 



NATURE 



509 



visible at some distance, and they are white or black, to 

 be used according to circumstances ; moreover, the sig- 

 nalmen 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 in- 

 creased efficiency. H. Badex Pritchard 



FLOODING THE SAHARA 



THE French scheme of turning part of the Algerian 

 Sahara into an inland sea continues to attract con- 

 siderable 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 carr}'ing 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 de- 

 struction 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 debris of shells, truly marine, such as 

 Buccimim gibcrrtiltim, Lam,, and Balatius miser, L. 

 Above these shells, in the sand, they found Carditiin 

 edide, 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 retire- 

 ment 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 litde 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 1 3,cxx) 

 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. 



In a subsequent sitting, however, it should be said, M. 

 Favd endeavoured to show that their fears were ground- 

 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 

 CCHEMNITZ, the principal mining city of Hungary, 

 ^ has celebrated the opening of the Juseph 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. Szabo, 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 Schem- 

 nitz, near the village of \'oznitz, on the left bank of the 

 Gran. The height of it is three metres, the width 1-6 

 metres. About the lower third is destined to convey oft 

 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 



