372 Intelligence and Miscellaneous Articles. 



special memoir, give the possibility of varying the pressure of the 

 steam in the box and the intensity of a sound of a given pitch. 



I thus prove that, when the pressure of the steam inside the 

 box varies from 0-5 to 1*5 atmosphere, the limit of perceptibility 

 corresponds to sounds the pitches of which are comprised between 

 48,000 and 60,000 simple vibrations. When the siren is furnished 

 with a counterplate, the ratio between the pressure of the steam 

 and the velocity of rotation is constantly too high, and, under the 

 conditions of the experiment, the limit of perceptibility can no 

 longer be attained, not even for the sound of 72,000 vibrations, 

 the shrillest which I have been able to produce. Under these con- 

 ditions the pressure of the steam in the box of the siren reaches 

 2*5 atmospheres, the velocity of rotation of the disks amounts to 

 600 turns, and their circumferential velocity to 113 metres, per 

 second. The supply of steam is considerable, about equivalent to 

 to that of an engine of 8 horse-powers. 



I afterwards caused metal rods fixed at one end to vibrate longi- 

 tudinally, by rubbing them with cloth sprinkled with rosin. By 

 gradually lessening the length of the vibrating rod the extinction 

 of the sharp sound is easily produced. I remark that 



(1) The length of the rod that gives the limiting sound is, for one 

 and the same metal, independent of the diameter. 



(2) For steel, copper, and silver, the lengths are to one another 

 in ratios sensibly equal to those of the velocity of propagation of 

 sound in those metals. 



Thus, taking the ratio belonging to copper as unit, we get : — 



Tor copper 1-000 



For steel 1*002 



For silver 0-995 



These two observations cannot accord with the results furnished 

 by the siren unless, for these three metals and for the length 

 corresponding to the limiting sound, the lengthening due to the 

 vibration is the same. I have not been able to value with sufficient 

 certainty these minute lengthenings ; but, on the other hand, I 

 have ascertained that — 



(1) If a hearing- horn be applied to the ear, the limit of percep- 

 tibility is slightly extended. 



(2) If the rods are excited with different substances — rosin, oil 

 of turpentine, alcohol, ether (which evidently has the effect of 

 modifying the amplitude of the vibration), the limiting length 

 changes, and may vary from a single to a double length : it is the 

 minimum with rosin, which produces the most energetic friction. 



(3) The excessively sharp sound that has ceased to be perceptible 

 to the ear, still acts powerfully on the sensitive flame. — Comptes 

 Bendus de VAcademie des Sciences. April 9, 1883, t. xcvi. pp. 1041- 

 1043. 



