158 
POPULAR SCIENCE RETIEW. 
It is very wonderful to reflect, how almost infinitely small is 
the amount of vibratory motion here rendered evident by our 
sensitive flame. The molecular motion of the air excited by a 
sound does not diminish merely in proportion as the distance 
increases, but far more rapidly : it is inversely proportional to 
the square of the distance. Remembering this, and that a de- 
cided action upon the flame was produced by no very loud sound, 
made at a distance of thirty or forty feet away, we are startled at 
the smallness of the motion capable of producing so sensible an 
effect. Further, when we add to the diminution by distance, 
the great enfeeblement of the motion uf the sound waves in 
overcoming the solid obstacles that intervened between them 
and the flame, the result is still more amazing, and, were it not 
attended wflth the certainty of a natural fact, one would be in- 
clined to doubt its truth. 
A sensitive flame, such as here described, can be applied to 
many purposes of experimental illustration. For instance, it 
may be used to indicate the fact that, a body, when sounding, 
divides itself into vibrating segments, separated by intervals of 
rest. This is usually shown by the unequal disturbance of 
some solid, placed on the vibrating body ; but, for an audience, 
who cannot look down on the lecture table, this method is open 
to objection. A gas flame, which need not be very sensitive, can 
then be used with advantage. If, for example, a large brass 
plate, fixed at its centre, be thrown into vibration by means of 
a fiddle-bow, a strained and intense divergence of the flame 
is obtained, when the higher notes of the plate are sounded. 
Holding the plate thus sounding, close to and parallel with the 
flame, and moving it slowly, so as to bring different parts in 
succession opposite the flame,* the principal nodal lines can be 
traced as easily as with sand. The intervals of rest in the vibra- 
ting plate allow the flame to raise itself up, and, in its sluggish 
combustion, to stand, as it were, at ease, whilst the ventral seg- 
ments drag it down to active burning and soldierly attention. 
Again, the so-called beats , produced by the interference of 
nearly unisant sounds, can be rendered more apparent by the 
movement of the flame. Striking, with a fiddle-bow, a large 
bell, or the brass plate above mentioned, these beats, though but 
faintly audible, cause evident changes in the flame. At every 
beat, the momentary silence allows the flame partially to regain 
its original height, from which, however, it is almost imme- 
diately thrown down by the sound which follows, erecting itself 
at the next beat, only to be thrust back as the sound again 
wells up. Thus, a sort of breathing flame is produced, the 
aspirations of which are strictly timed to the sighing of the 
bell. 
But it is not to every sound that the flame is equally sensi- 
