Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/131

Rh ACOUSTICS 115 practice. The stopped extremity of a pipe is always to some extent of a yielding nature, and does not therefore exactly coincide with a nodal surface; nor can the internal air immediately adjoining the open end be perfectly free from variation of density during the vibrations of the whole mass, particularly so at the embouchure, where the blast is introduced by which the tone is originated. It would appear from recent experiments that the pitch of a pipe is somewhat lower than the above theory would indicate. leed pipes. 86. The reed-pipe differs in many respects from the simple pipe which we have been considering. A small elastic strip of metal, fixed at one extremity (the reed), lies over a slit of the same shape, and is set in transverse vibration by a current of air acting underneath. If, as is the case in the accordion and harmonium, the reed is un provided with a pipe, the pitch of its note is regulated altogether by the dimensions of the reed, in conformity with the law of tranversely vibrating plates ; although, it is to be remarked, the note is really due to the vibrations of the air which alternately escapes through the slit of the reed, and is prevented doing so exactly as often as the reed executes a movement to and fro. The proper note of the reed itself is very poor and faint. ifluenceof 87. In the reed-pipe there is added above the reed a pipe ipe on the air in which partakes of the vibratory motion, and im proves the quality of the sound. The pitch is, however, not affected by this pipe, unless it exceed a certain length I, when the pitch begins to fall, and continues to do so as I is increased, till, when the length of pipe is 21, the note is again restored to its original pitch, &c. feber s 88. M. Weber, to whom we are indebted for these and icoiy of other curious facts respecting reed pipes, has explained 3 pipe. them thug. if the ree( j foe exactly at that part of the vibrating air-column where the air-displacements are at their maximum, and where consequently the air suffers no variation of density during the vibratory motion of the column, the oscillations of the reed are not at all affected by the air-vibrations, and consequently the pitch of the reed-pipe is the same as that of the reed itself. But if the reed be situated at any other part of the air-column, and especially at a nodal section, where the air is undergoing alternate condensation and rarefaction, then, when the air- blast from the wind chest pushes in the reed, the air in the pipe is in the act of rarefaction, and consequently tends to accelerate the reed inwards, whereas the elasticity of the reed tends in an opposite direction. When, again, the reed is passing to the other extreme of its vibration, the air in the pipe is in the act of condensation, and tends to accelerate the reed outwards or in the opposite direction to the elasticity of the reed. Hence the reed is affected just as if its elasticity, and therefore the rapidity of its vibra tions, were diminished, and thus the pitch is lowered. PART IX. Singing Flames. as har- 89. The chemical or gas harmonicon, which consists of lonicon. a sma ii flame of hydrogen or of coal gas, burning at the lower part of the interior of a glass tube, and giving out a very distinct note, exhibits considerable analogy with the reed-pipe. For, as Sondhaus seems to have established, the primary cause of the note lies in the oscillations of the gas within the burner and the feeding-pipe, which there fore play exactly the same part as does the reed portion of the reed-pipe. The air in the glass tube being heated by the flame ascends, and the pressure above the flame being thence diminished, the flame is forced upwards by the gas beneath, until an influx of atmospheric air at the top of the tube forces the flame back. Thus a periodic agitation of the flame ensues, accompanied by a corresponding dis turbance of the air-column in the glass tube. The size of the flame and its position within the tube must be so regulated as to bring out the best possible note, which will then be found to be the same as the air in the tube would itself emit, according to the laws of pipes, allowance being made for the high temperature of the air. A series of tubes may thus be arranged of suitable lengths to give the common scale. It sometimes happens, particularly with short tubes, that the note will not come out spontane ously, all that is required, then, is either by blowing gently at the top of the tube, or by singing in unison with the expected note, to give to the air the requisite initial move ment. The flame, which burns steadily with a yellowish light before the tube sounds, will, as soon as the note is heard, be seen to flicker up and down, changing rapidly from yellow to blue and blue to yellow, its intensity also chang ing periodically. These fluctuations are best seen by view ing the image of the flame reflected by a small plane mirror, held in the hand and moved to and fro. Before the note is heard, the image of the then quiescent flame, being im pressed on different points of the retina, appears as a con tinuous luminous strip ; but, when the harmonicon speaks, the various images become quite detached from one another, showing that the portion of the retina over which the reflected light passes is sensibly affected only at certain points of it, which evidently correspond to the instants of time at which the flame, in its periodical fluctuations, is at its brightest. 90. Naked flames, that is, flames iinaccompanied by tubes, Naked may also give out musical notes, and many singular in- flames, stances are mentioned by Tyndall and others of their sensitiveness to external sounds. 91. Koenig of Paris has constructed an apparatus in- Flame tended to indicate the modes of vibration of the different manom parts of vibrating bodies, such as columns of air, &c., by means of flames, and to which he has given the name of the Flame Manometer. We will here describe its applica tion to the case of organ-pipes. An open pipe has three apertures along one side, one at the middle, o (fig. 28), i.e., at a node of the fundamental tone, and the two others, a, 6, half way between o and the extremities of the. .. pipe, and coinciding therefore with the nodes of the first overtone or octave. These openings are closed by thin flexible membranes forming the ends of small boxes or capsules, the spaces within which communicate by caoutchouc tubes with a coal-gas reservoir, and also by separate tubes with small gas burners arranged on a vertical stand. The gas being introduced, and the three flames kindled and adjusted to equal heights of about f of an inch; if the pipe be made now to utter its first over tone, the flame connected with o will remain stationary and of the same brightness as before, but those communi cating with a and b will become longer and thinner, and assume a bluish and faint luminosity. But, if the funda mental be brought out of the pipe, then it is o s flame that is violently affected, while those of a and b are scarcely affected at all. If the flames be originally made less in height (say ^ inch), those of a and b in the former case, and of o in the latter, will be extinguished. These results are due to the condensations and rarefactions of the air in the pipe which are at their maximum at a node, causing the membrane placed there to vibrate outwards and inwards,, and hence to force more or less of the gas into the burner. i In order to compare together the notes of different pipes, four plane reflecting surfaces are connected together in the form of a cube, which is mounted on a vertical axis about which it is capable of being turned round. Each pipe ia Fig. 28.