NATURAL PHILOSOPHY. 109 



tlieque de Geneve. When the discharge of a Leyden jar takes place 

 the duration of the spark is considered to be of so short a space of 

 time as to be inappreciable. Professor Wheatstone found that the 

 sparks which he obtained by employing a copper wire the fifteenth 

 of an inch in thickness and half a mile in length had a duration of 

 about the 24,000th part of a second. He also showed that to pass 

 over that space the electricity travelled at the rate of the 1,152,000th 

 part of a second. To explain this he had recourse to a hypothesis, 

 and proposed to admit that the diameter of the wire was not large 

 enough to permit the charge of the Leyden jar to traverse it otherwise 

 than in a successive manner. After mature reflection and experi- 

 ment, Professor Eijhe believes that he has found a more simple ex- 

 planation, which he puts in the form of the following proposition : 

 the space of time which electricity requires to traverse a conductor 

 is much less than that which the discharge of the same conductor re- 

 quires. 



An Electric Spark of Induction produced by Ruhmkorff's great 

 machine at Paris has pierced through a plate of crown glass nearly 

 two inches thick, and another about one inch and a quarter thick. 

 These plates were recently laid before the Academy of Sciences by 

 M. Faye, who stated that such thick plates had never before been 

 pierced by the spark of induction. The holes were fine, and of a some- 

 what spiral form. There was no trace of fusion or of metallic deposit ; 

 and M. Ruhinkorff added that an energetic compression of the sub- 

 stance of the glass appeared to have accompanied the passage of the 

 spark. 



Origin of Electricity. In a memoir translated in the Bibliotheque 

 de Geneve M. H. Buff considers the analogy of the sources of the elec- 

 tricity of friction and contact, and gives as the results of numerous 

 experiments his opinion that the contact of heterogeneous substances 

 is the cause of the former. This conclusion is cornbatted by the ed- 

 itor, ]VI. A. De la Bive, who does not consider it to be fairly derived 

 from the facts related. He considers that the experiments on the 

 generation of electricity by pressure, from which M. Buff' derives 

 one of his principal arguments in favor of the contact theory, seem 

 rather to prove that the origin of the electricity is much more related 

 to the molecular movements, which arise as much from pressure as 

 from rubbing, since, according to the nature of these movements, 

 it is sometimes the positive, sometimes the negative fluid with which 

 the two rubbed or pressed substances are charged. 



ELECTRO-MOTIVE MACHINES. 



Electro-motive machines depend on the power which soft iron pos- 

 sesses of acquiring, under the influence of the electric current, an 

 enormous magnetic power, and of losing it instantly that the current 

 ceases to circulate, whereby a rotary motion of immense rapidity is 

 easily produced. Unfortunately, this attraction diminishes rapidly 

 with the distance. To overcome this inconvenience, a distinguished 

 engineer, M. Froment, has devoted much time, and believes that 

 within certain limits these machines may be made useful. We learn 

 from a memoir by M. De la Rive that a machine constructed of eighty 



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