390 A. M. Mayer—Method of Detecting the Phases of 
at the top and bottom of the pipe. Nothing could be more 
satisfactory, and it was charming to behold how neatly the 
the serrations. I now substituted for the resonator an organ 
pipe, in every respect similar to the one on the bellows, and 
with it I repeated the wave-length measures previously made 
with the resonator; indeed the column of air in the pipe 12 
my hand responded so perfectly to the sounding pipe that 1 
thought it gave more marked results than those produced with 
the resonator. 
The manometric flame-micrometer. 
In'the experiments described above, we examined the appear: 
ances in the mirror with the unaided eye, and with it estimated 
when coincidences and bisections occurred ; but td obtain results 
of precision, a method was devised which determines neatly 
these critical points. For that purpose I have invented the 
following micrometer, founded on a beautiful suggestion of 
Dr. R. Radau, who thus describes in his excellent ‘1’ Acoustque 
(Paris, 1867, p. 272), a method of observing the flames of two 
similar sounding organ pipes. ‘We attach to the two pipes 
two of Kénig’s flames arranged so that the point of one flame 
fides its base, but 
which shows by reflection the base of the other flame. Th 
produces the illusion of a single flame. If now we observe 
this hybrid image in the revolving mirror while we sound the 
two pipes, the point separates from the base, which proves that 
the two flames shine alternately, and the one retracts while the 
other elongates; if the two tubes act on the same flame, t 
effect is null, and the flame remains immovable.” By placimg 
the above “small fixed mirror” on a divided circle; or °Y 
silvering its back and determining its angular displacements 
around a vertical axis by the method of Poggendorff,—that 18, 
by observing through a telescope, the reflections of a fixed scale 
m the back of the mirror,—we have devised a simple aD 
precise micrometer for ascertaining the amount of displacement 
of the resonator's flame. For, having once determined, for # 
given note, the amount of angular motion of the mirror requ 
to move the bases of the flames over the distance between the 
centers of two contiguous serrations we have the angular value 
of a displacement equal to that caused by moving the reson® 
tor through a wave-length, and a fraction of the turn requ I 
to-produce the above movement of the bases of the flames W) 
over a corresponding fraction of a wave-length. Thus cam be 
measured very anal fractions of a i ek Indeed, even 
with the unaided eye and without the use of the micromett? 
