C. GENERAL PHYSICS. 151 



required that such light should be secured as would pene- 

 trate through any slight fog or haze, it having been general- 

 ly acknowledged that not even the sunlight itself can pene- 

 trate an ordinary dense fog. The electric light established 

 at Souter Point after condensation is equal in power to 800,000 

 standard candles, being eight times as powerful as the best 

 American fixed lights. The electric spark passes between 

 slender pencils of carbon, which are themselves consumed at 

 the rate of about one inch per hour. The electric current is 

 generated by two of Professor Holmes's patent rotary mag- 

 neto-electric machines, driven by steam-engines of six horse- 

 power. The number of revolutions made by each machine 

 is 400 per minute, and 12,800 sparks pass per minute when 

 both machines are at work. These sparks are, of course, 

 formed so rapidly that the eye does not separate them, and 

 the result is an intense beam of light, so dazzling that the 

 eye of a person within the lantern can not rest upon them 

 for an instant without intense pain. As observed from a 

 distance of several miles, this light is so bright as to cast a 

 well-defined shadow upon the deck of a vessel. Elliots 

 European Light-house System, p. 120. 



ELECTRIC LIGHT FOR LOCOMOTIVES. 



A series of satisfactory experiments has lately been made 

 in Russia in regard to lio-htin^ railway tracks from the loco- 

 motives by means of the electric light. The track on one 

 occasion, with a battery of forty-eight cells, was brilliantly 

 illuminated 492 yards ahead. 23 A, April 9, 1875, 467. 



THE BLACK-BULB-IN-VACUUM THERMOMETER. 



As is well known, the black-bulb-vacuum thermometers 

 employed for observing the solar radiation give very dis- 

 cordant results, even in the hands of the best observers, and 

 the origin of this has recently been studied by Mr. Hicks, 

 of London, who states that in his opinion the discordances 

 are in a great measure due to the imperfect vacuum that 

 exists within the inclosing bulb. Having made a large num- 

 ber of thermometers with special care, in which the vacuum 

 has been reduced to the lowest attainable limit, Mr. Hicks 

 finds that it is possible with proper care to always construct 

 instruments that shall be perfectly comparable with each 



