New-York tribune. [volume] (New York [N.Y.]) 1866-1924, 1872-12-20

propagaton Could You the ether. you would also
ind every individual particle making snai eAourson
10 and fro but here the notion like that OF the water
nartces above referred to- would be across the line OF
propagation TIE vibrations of the air are longitudinal
the vibrations of the ether are trans'erSaL
MOVEMENT OF AIR PARTIcLEs.
rtis my desire that YOU should realize with the utmost
possbe oearness the propagation OF waves, both in
ether and in air. And with this view bring before you
an experment wherein the air particles are represented
nv small spots OF light These spots are parts OF spiras
drawn upon circle OF backened glass, and when the
circle is caused rotate the spots move In successive
pnses over the You have here clearly set before
you how the pulses travel necessarily forward, VINE
fhe parcles that compose them perform any oscllauO
aud fro We have in this case the picture ~ 'o" NV
wave, in which the vibrations are longituua a y y
another arrangement OF our glass wheel, we Prouo ~
image Of transverse wave, ana mere we h n
waves travelling In succession over v an TO
and fro across the line of propagation
q rThere were two sets OF dots Of light successfully
shown in wave motion upon the screen The first were
in straight vertical line and in different states OF ag
gregaton-some being close together and Others IN the
intervals wide apart When the machinery moved, the
pots that were close separated and those that were
separated closed the motion being kept up through the
whole line. The other example gave sinuous line OF
dots exactly representing particles that might be hoat
ing on ocean waves and alternately rising and falngl
The most familiar illustration Of the interference OF
sound-waves is furnished by the beats In music, which
are produced by two musical sounds snghly OUT OF
unson and when they are agitated together the two
sounds flow without roughness as II tne were put one
But Dy aaeng 20 one ~ 20 hrata little more SLOWLY
than its neghbor. Suppose that one OF them performs
1 vibrations the time as the other performs
i0a and LET us assume that at starting the cOndens
tons and rarefactions OF both forks coincide Al we
lust vibration Of the quickest fork they Will again
coincide the quicker fork at this point having gained
vibration, one whole wave upon the other. bsI
10th Vibration the two forks will be opposition here
the one tends produce condensation where the ote
the two forks therefore the sound quenched and
have pause OF silence. This occurs where one 10TH mas
gained half awavelength upon the other AT the iOlst
Vibration we have again coincidence, and, therefore,
augmented sound at the 50h vibration we have again.
SsiSwayes In advance of the other With forks
circumstanced, obtain those nernnttent SHOCKS oi
sound separated by pauses OF silence TO which we give
the name of beats such beats rendered audible to
| all in the lecture)
| [At this point Prof Tyndan brought forward two
large tuning forks tuned in unison and swept bow
across each loud musical note filed the air. He then
attached Cby means OF wax cent to OF the forks
which somewhat retarded its vibrations. He explained
that IT, for instance, the difference between the fork
now were such that while one fork made 100 vibrations
| the other made 10l, 1t would result that al
| every hundredth vibration the waves would combine
TO form the highest wave-i the oudes sound, and
half way between these the crest OF one wave would
meet the furrow Of the other making the least wave
and the lowest sound This effect OF increasing and de
creasing sound was very plainly audible. He then put
another cent on the loaded fork and the differences oil
sound succeeded each other with much more notable
rapidityl
SERIES OF OPTICAL ILLUSTRATIONS
now wish show you what may be called the optical
expression those BEATS and here we have TO fall back
upon the fact that luminous impression persists for
certain interval upon the retina. Attached to large
tunngfork is a small mirror, which shares the yibra
tons of the fork and to the mirror is thrown thin
beam Of light which shares the vibrations OF the mirror
The is still, and the beam reflected from IL is
received upon piece Of looking and thrown back
upon screen where it stamps itself as small lunin-
Gus disk. The agitation of the fork by bow converts
that disk into band OF light, and n you simply shake
your heads and fro will be able TO reduce that
band elements you draw it, In fact ont to snu-
Gus proving the periodic character OF the mo
non which produces 1t. BY sweep OF the lookng-gass
we can also cover the from side to side by this
luminous SCROLL the depth of the sinuosties indicating
| the amplitude of the vibration.