Dec. 1 8, 1879] 



NATURE 



157 



many places, there is a shoal at some distance from the 

 lighthouse, or where a reef of rocks projects seawards 

 from the shore, it sometimes becomes necessary to adopt 

 means for keeping vessels clear of such dangers at night, 

 as, for example, near Souter Point, where Mr. Douglass 

 and Mr. J. T. Chance employed successfully the electric 

 light for guarding a rock near the shore. What is wanted 

 in such cases is to cover not only the danger itself but 

 some area of the surrounding sea by a characteristic 

 which is different from that of the main light. 



If in front of a fixed light apparatus whose optical 

 property is to parallelise the rays in the vertical plane 

 while not interfering with their natural divergence in 

 azimuth, there be placed an arrangement of straight 

 horizontal shades or screens similar to the Venetian 

 blinds which are used for house windows, the means will 

 be supplied for easily producing different distinctions. 

 The breadth of those blinds must be such as to subtend 

 from the central flame the same angle as that over which 

 the necessary distinction has to be shown at sea. By 

 opening and shutting simultaneously and gradually the 

 different leaves of the blind, there will be produced the 

 same characteristics as those of an ordinary- revolving or 

 flashing light, according as the leaves are moved slowly 

 or quickly and kept shut for a certain period, and these 

 distinctions will be accompanied by the necessary gradual 

 waxing and waning of the emergent rays. By simulta- 

 neously opening and shutting the leaves of the blind 

 suddenly, and keeping them open so as to show a fixed 

 light for a certain length of time, and then keeping them 

 shut so as to produce darkness for a certain length of 

 time, the effect of an intermittent light in which there is 

 no waxing or waning of the rays will obviously be 

 produced. 



Should it be considered desirable to vary the appearance 

 over the given arc so as to show a gradually increasing 

 length of light period when a vessel is approaching the 

 danger, the maximum period when it is opposite to it, 

 and a correspondingly shortening period as the vessel 

 leaves it, a single straight opaque mask placed outside of 

 the apparatus, and revolving horizontally and with uniform 

 speed on a vertical spindle will produce the result. For 

 while the periods of change will remain the same over the 

 whole arc the duration of darkness will gradually increase 

 as the danger is approached, and gradually decrease after 

 the danger has been passed. And if this vertical shade 

 be made to rotate at a slow and uniform speed it will 

 produce the effect of a revolving light, and if at a quick 

 speed it will produce the effect of a flashing light, with 

 this difference that the flashes will recur with only an 

 instantaneous interval of darkness, and in both cases 

 there will be a gradual waxing and waning of the rays. 



By these very simple and cheap expedients a fixed light 

 illuminating the whole horizon (by means of a flame of 

 he ordinary size in relation to the focal length) can 

 easily be made to show accurately over any limited angle 

 in azimuth the effects of the different distinctions referred 

 to, and these combinations will therefore supply a deside- 

 ratum which is often much wanted in coast illumination. 

 In some experiments which were made all these charac- 

 teristics were successfully produced by the two modes 

 described. 



Where no light is required in any part of the horizon 

 but in one small arc only, as, for example, in illuminating 

 the middle of a long narrow Sound, all the rays proceeding 

 from the lamp should be spread equally over that arc. A 

 fixed holophote with an opaque disk revolving horizontally 

 in front on a vertical spindle will, if condensing prisms are 

 placed between the disk and the holophote, produce either 

 a revolving or flashing light according to the speed of its 

 revolution, but without any intervening period of dark- 

 ness. If colour distinction be required and a revolving 

 disk of glass be substituted for the opaque mask the 

 characteristic effect produced would be that of a revolving 



red and white light without any intervening dark period 



between the flashes, which would gradually dissoh-e into 



each other from red to white and then from white to red. 



Edinburgh, October 22 Thomas Stevenson 



A FEAT IN TRIANGULATION 

 A NOTEWORTHY advance in geodesy has recently 

 ■**■ been accomplished by the junction of the network 

 of measurements covering a large portion of the surface 

 of Europe, with the African continent. The entire trian- 

 gulation of Algeria was completed by French engineers 

 some time since, and extended to the edge of the Sahara, 

 in lat. 37 . M. Perrier, who had directed in a great 

 measure the triangulation of Algeria, has for the past 

 eleven years been seeking the means of joining the net- 

 work in that country with the perfect trigonometric system 

 covering the surface of Spain, France, and England. The 

 importance of such a junction is easily appreciated when 

 we consider what notable changes in the accurate concep- 

 tion of the shape of the earth and of the length of 

 meridians has been effected by measurements on a much 

 smaller scale. 



For such an undertaking the most careful and pains- 

 taking preparations were requisite. As the result of his 

 reconnaissances between 1868 and J872, M. Perrier found 

 that from all the trigonometric points of the first order 

 between Oran and the frontiers of Morocco, the loftier 

 crests of the Sierra Nevada on the Spanish coast opposite, 

 were visible in exceptionally clear weather. Arrangements 

 were subsequently made with the Spanish Geographical 

 Institute for the mutual and contemporaneous execution 

 of the proposed plan. A corps of Spanish officers, under 

 the direction of the well-known General Ibanez, was de- 

 tailed for this purpose, while the French Minister of War 

 placed a division of officers from the Etat- Major under 

 the command of M. Perrier. The leaders chose for sta- 

 tions in Algeria the summits of Mount Filhaoursen and 

 Mount M'Sabiha, west of Oran, and in Spain the summits 

 of Mount Tetica and Mount Mulhacen, the latter of which 

 is the most elevated point in the kingdom. The direc- 

 tions and distances between these four points were com- 

 puted as carefully as possible, and preparations were then 

 made for the final and determinative observations. At 

 the Algerian stations the nature of the country and its 

 inhabitants necessitated the use of a numerous force of 

 soldiery as well as of means of transport. 



In order to insure the accuracy of the observations, 

 which required the passage of signals over a distance of 

 270 kilometres, it was decided to make use of solar re- 

 flectors and powerful lenses. The efficacy of such appa- 

 ratus for even greater distances had already been tested 

 by M. Perrier ; still for the measurements in question they 

 appear to have utterly failed to answer the expectations 

 based upon them, not a single solar signal beine visible 

 from any station. Fortunately, the success of the observa- 

 tions did not rest entirely upon this one system of signals. 

 Preparations had likewise been made for the employment 

 of the electric light, and on the summit of each mountain 

 one of Gramme's electro-magnetic machines worked by 

 engines of 6-horse power had been placed in position. 



On August 20 last, all the stations were occupied, and 

 the electric lights were displayed throughout each night. 

 Then the patience of the observers was submitted to a 

 lengthy proof. The mists rising from the Mediterranean 

 totally prevented the exchange of signals, until after a 

 delay of twenty days, one after another the electric lights 

 became visible even to the naked eye. Perrier compared 

 the intensity of the light on Tetica nearly 270 kilometres 

 distant, to that of a in Ursa Major, which rose near by. 

 The observations were continued from September 9 to 

 October 18, when this task for which such extensive 

 preparations had been made, was completed in the most 

 satisfactory manner. With its completion we come into 



