i8 4 THE POPULAR SCIENCE MONTHLY. 



discharge a Ley den jar, and observe whether there is any delay in 

 the response of an electroscope a little distance off ; or to observe 

 the needle while a remote electro-magnet is excited. But these 

 experiments, and others like them, have been tried without any 

 interval being determined between the cause and the effect. An 

 upholder of Maxwell's theory understands that such failures are 

 inevitable, and arise from the enormous rapidity of the transmis- 

 sion. "We can only perceive the discharge of a Leyden jar, or the 

 excitation of an electro-magnet, from a moderate distance, say, of 

 ten metres. But light, and electricity as well, according to the 

 theory, pass over such a space in a thirty-millionth of a second. 

 So short an interval of time can be neither perceived nor measured 

 directly. Furthermore, we have no signals by which to define 

 that instant. We do not make a big chalk-mark when we want 

 to tell off a tenth of a millimetre. It would be quite as absurd, in 

 determining a duration of a thousandth of a second, to depend 

 on the sound of a large bell to mark the beginning of the moment. 



The time required for the discharge of a Leyden jar is, accord- 

 ing to our common means of observation, infinitely short. That 

 does not mean that it is not equal to the thirty-millionth of a 

 second ; and, for the present case, it would be more than a thou- 

 sand times too long. But Nature furnishes us another resource. 

 It has been long known that the Leyden discharge is not uniform, 

 but is composed, like the sound of a bell, of a number of vibra- 

 tions of partial discharges, which succeed one another at even in- 

 tervals of time. Electricity is capable, then, of imitating elastic 

 phenomena. The duration of each vibration is much less than 

 that of the whole discharge ; we might, therefore, try a vibration 

 as a standard. Unfortunately, the shortest vibrations that have 

 been observed are of a millionth of a second. While one of these 

 vibrations is going on. its effect is propagated to three hundred 

 metres ; while within the limited space of a laboratory it will ap- 

 pear simultaneous with the vibration. Known phenomena, then, 

 gave no aid, and it was necessary to look for another way. The 

 difficulty was turned by applying the discovery that vibrations 

 are produced in any conductor as well as by the discharge of the 

 Leyden jar, and often much more rapidly. When the conduct- 

 or of an electrical machine is discharged, vibrations are produced, 

 the duration of which varies from the hundred-millionth to the 

 millionth of a second. They are, it is true, only isolated vibra- 

 tions that are extinguished rapidly a condition unfavorable for 

 the experiment. But success would be possible even if we could 

 observe only two or three of the vibrations. In the same way, in 

 acoustics, we substitute, when we want to, brief signals sounded 

 on wood for the lengthened sounds of whistles and cords. 



We now possess signals in comparison with which the thirty- 



