342 



SCIENTIFIC NEWS. 



[April 13, 



view will extend 37 miles, and from the summit of Mont 

 Blanc more than 150 miles. 



But this geometrical extent of horizon extending 

 around us rarely coincides with the scope of distinct 

 vision. Visibility at such great distances is exceptional 

 and occurs only in a clear and pure atmosphere. 



One of the most remarkable cases is that of the 

 Pyrenees seen from Marseilles which presents a peculiar 

 interest on account of the singular conditions under which 

 it is produced. 



Yearly, about February loth and October 31st, Mount 

 Canigou, situated in the Eastern Pyrenees, and of the 

 elevation of 9,050 feet above the sea level, may be seen 

 from Notre Dame de la Garde, projected against the disc 

 of the sun at the moment of his setting. 



The peak of Treize Vents, the height of which is below 

 9,000 feet, though more remote, is likewise visible to the 

 left of Canigou. 



From the summit of Marseille-Vyre, at nearly five 

 miles to the south of Marseilles, the same observations 

 may be made on February 13th and October 28th. 



A straight line joining Notre Dame de la Garde to the 

 apex of Canigou measures 253 kilometres, and passes 

 about 390 feet below the surface of the Mediterranean, 

 but the atmospheric refraction in consequence of which 

 light is propagated in the lower strata of the air with a 

 slight curve enables us nevertheless to perceive the 

 Pyrenees. 



Baron Zach first determined the chief conditions under 

 which this phenomena is produced. On February Sth, 

 1808, from the eminences of Notre Dame de la Garde, 

 the height of which is i6i metres, he saw for some 

 minutes with a telescope Canigou projected in black 

 against the clear ground of the sky when the sun had 

 already disappeared below the horizon. M. L. Fabry, of 

 the observatories of Paris and Nice, undertook some 

 years ago the necessary observations, verified the 

 observations of Zach, and indicated the dates at which 

 the two Pyrenean summits may be seen projected on the 

 disc of the setting sun. On October 31st, 1882, he 

 observed the passage of Canigou before the solar disc 

 from Notre Damede la Garde, and on October 27th, 1886, 

 from the summit of Marseille-Vyre, an elevation of 440 

 metres. 



M. Marius Codd has likewise observed the visibility of 

 Canigou from Notre Dame de la Garde, on October 30th, 

 1886. This year more complete observations have been 

 made by M. Leotard, the secretary of the Scientific 

 Society of Marseilles. On the loth and nth of February, 

 Canigou and Treize Vents were seen as black masses 

 on the brilliant ground of the western heavens. They 

 were visible during about 25 minutes, and could be 

 recognised, not merely with telescopes and opera-glasses, 

 but even with the naked eye. In clear weather they 

 would probably be visible for several days before and 

 after the dates given. 



Purification of Water. — According to Professor 

 Leeds, of New York, water swarming with bacteria may 

 be rendered almost entirely free from them by the 

 addition of half a grain of alum to the gallon. This 

 perfectly agrees with the experiments of Dr. Brautlecht, 

 and is in harmony with the use of alum for the purifi- 

 cation of suspicious waters in China, and the employ- 

 ment of ?alts of alumina in the treatment of sewage. 



MINOR PHENOMENA OF THE ELEC- 

 TRIC GLOW LAMP. 



THE electric glow lamp has already been cited 

 as giving an illustration of irradiation. When 

 in full action, it presents no longer to the distant eye a 

 simple loop of glowing carbon. By irradiation the out- 

 line is lost, and it resembles a gas flame. The same 

 object illustrates very perfectly some of the phenomena 

 of persistence of vision. The old example of the whirl- 

 ing ember, the thaumatrope and many other scientific 

 toys, could be cited that are based upon this principle. 

 It is also well known that if a spot of specific colour is 

 looked at intently for some time and the eye is then 

 turned upon a white surface, the complementarily 

 coloured spectral image of the spot will appear. The 

 nerves of the eye, it is assumed, become fatigued for the 

 original tint, and hence, receiving white light, are 

 affected by the residual colours. This is the reverse 

 effect of persistence of vision. By the latter, properly 

 speaking, the true image retains its effect. But the two 

 are intimately related, as opposite effects so often are in 

 the world of nature. 



If an incandescent burner is gazed at while near the 

 eye, the filament can be distinctly seen. Now, if the eye 

 is closed, the image of the filamicnt remains, and appears 

 in clear outline in a purplish or violet colour. The 

 characteristic shape of the particular filament is especially 

 distinct. If, in looking at it, particular care is taken to 

 avoid remembering its appearance (and this disciplining 

 of the mnemonic faculty is not difficult), the spectral 

 image seen with the closed eye will vividly portray the 

 filament and its peculiarities. Sometimes it seems as if 

 the effects of irradiation could be partly overcome ; as if 

 the image of a distant lamp could be reproduced free 

 from the glow of irradiation. To a degree this may be 

 possible, but a fully ignited and distant lamp always 

 gives a confused spectral image to the closed eye. 



The filament of the high resistance lamps illustrates 

 elasticity very well. Surrounded by a vacuum, so as 

 to be free from the damping effects of the air, no object 

 more sensitive to vibrations or shocks can be found. 

 The least tremor of the wall to which the lamps arc 

 attached makes the loop vibrate for a long period. This 

 only takes place when they are cold. When the current 

 is passing and they become red or white hot, they are no 

 longer so elastic, and cannot be made to vibrate as before. 



By the same vibrations sound is often produced. The 

 vacuum of the globe prevents the sound from being 

 heard by atmospheric transmission. But the filament 

 is in solid connection with the globe and socket. If it is 

 set in strong vibration, and the lamp is held pressed 

 against the ear, a ringing metallic sound will be heard. 

 This cannot be done with all burners. A certain size is 

 naturally essential, as the sound is at best a weak one. 



It would seem possible that a visual seismograph of 

 extreme sensitiveness could be made on the general 

 lines of an incandescent burner. It is probable that a 

 filament could be obtained in this way more affected by 

 external vibrations than is the most sensitive device now 

 used. — Scientific American. 



Electric Conductivity of Sulphur. — At common 

 temperatures sulphur is a very bad conductor of elec- 

 tricity, but M. E. Duter {Comptes Rendiis) has observed 

 that at its melting point it conducts electricity fairly 

 well. 



