NATURAL PHILOSOPHY. 141 



menter places the latter in communication with the ground, new phe- 

 nomena appear. A portion of the electricity which had penetrated in the 

 wire returns, giving a retrograde current ; and if the head of the wave 

 has had time to reach the other extremity of the line, it gives rise to a 

 contrary current, so that the wire pours out its electricity at both ends, by 

 two inverse currents ! All of these phenomena disappear when the wires 

 are suspended in the air. Mr. Farraday holds, that the velocity of elec- 

 tricity is not absolute, and that it varies with its source : " it varies with 

 the tension of its first urging source." 



ELECTRIC ILLUMINATION. 



There have been recently some attempts made at Paris towar ds illumi- 

 nating the bottom beneath water. At the Lake d'Enghien, M. Duboscq, 

 the successor of Soleil, performed an experiment of this kind before 

 marry competent observers. The electrodes of carbon were placed in a 

 glass globe, being connected with one of Duboscq' s regulators, which 

 communicated with the battery by means of a copper wire covered with 

 gutta-percha. The globe, submerged to a depth of five meters, spread 

 light over a circumference of about ten meters radius, and it remained 

 constant for about ten hours, after which the carbon reqiiired replacing. 

 The idea of this process was suggested to Duboscq by an agent of a 

 company engaged in exploring the bottom of the Mediterranean where 

 the battle of Navarino took place. The diver usually remained beneath 

 the water three-quarters of an hour, after which he came tip to breathe 

 and rest ; his light was an oil lamp, placed on the head of the diver, and 

 fed with air proceeding from his respiration, whence it was in a variable 

 current, and was often extinguished, requiring him to go up and re-light. 

 Duboscq's arrangement was devised to avoid these inconveniences. It is 

 light, so that the diver may carry it in his hand, and at the same time it is 

 strong and well secured hermetically, to resist a pressure of fifty or sixty 

 meters of sea water. It consists of a cylinder of strong glass secured to a 

 brass foot, and surrounded with a gutta-percha sac. The light passes out 

 through a large plano-convex lens, the convexity inward, the focus being 

 so arranged that the rays escapen early parallel. As the lamp is movable, 

 the diver walks about with it, and places it where he wishes to make any 

 search ; and as it is only necessary to tring the electrodes near one another 

 to light it, the diver need only turn a small screw to contiruie the light ten 

 hours, which is more than twice as long as he can remain, at the bottom. 



To illumine the bottom at small depths, Deleuil uses a Fresnel lens, 

 and this is daily in operation in a bathing establishment constructed on 

 the Seine in Paris. The regulator, and also the light, are ten meters above 

 the surface of the water, and the light penetrates sufficiently far to enable 

 us to see the swimmer at a depth of from two to three meters, and follow 

 all his movements. 



