156 Lord Rayleigh's Acoustical Observations. 



discovered by Rijke*, and is perhaps the most interesting 

 of all the cases in which vibrations are maintained by heat. 

 The probable explanation is given in the Royal-Institution 

 lecture. It is surprising that the phenomenon is not more 

 generally known, as on a large scale the effect is extremely 

 striking. I have employed a cast-iron pipe 5 feet long and 

 4| inches in diameter, hung over a table from a beam in the 

 the roof of my laboratory. The gauze (iron wire) is of about 

 32 meshes to the linear inch, and may advantageously be 

 used in two thicknesses. It should be moulded with a hammer 

 on a circular wooden block of somewhat smaller diameter than 

 that of the pipe, and will then retain its position in the pipe by 

 friction. When it is desired to produce the sound, the gauze 

 caps are pushed up the pipe to a distance of about a foot, and 

 a gas-flame from a large rose -burner is adjusted underneath, 

 at such a level as to heat the gauze to a bright red heat. For 

 this purpose the vertical tube of the lamp should be prolonged, 

 if necessary, by an additional length of brass tubing. In 

 making the adjustment a more convenient view of the interior 

 of the pipe is obtained with the aid of a small piece of 

 looking-glass held obliquely underneath. Sometimes a sound 

 is excited by the flame itself independently of the gauze. 

 This should be avoided if possible, as it impedes the due heat- 

 ing of the gauze. "When a good red heat is attained the 

 flame is suddenly removed, either by withdrawing the lamp 

 or by stopping the supply of gas. In about a second the 

 sound begins, and presently rises to such intensity as to shake 

 the room, after which it gradually dies away. The whole 

 duration of the sound may be about 10 seconds. 



Mutual Influence of Organ-Pipes nearly in unison. 



The easiest way of approaching the consideration of this 

 subject is to take the case of an open or stopped pipe, divided 

 into two similar parts by a rigid barrier along its middle plane. 

 In the absence of the barrier, the vibrations of the two halves 

 under the action of the wind are in the same phase ; and at 

 first sight there appears to be no reason why this state of things 

 should be disturbed by the barrier. Nevertheless it is well 

 known to physicists that the two halves do in fact take opposite 

 vibrations, with the result that the sound in the external air 

 at a distance from the compound pipe is a small fraction only 

 of that due to either half acting alone. In the pipe itself the 

 vibration is more, and not less, intense on account of the 

 barrier. It is true that at the very beginning of the sound, 



* Fogg. Ann. cvii. 339, 1859. 



