Centre Hall reporter. [volume] (Centre Hall, Pa.) 1868-1871, 1871-10-13

J human race that Time VIII never come
The challenger is sustained. and the
three Jurors named are rejected.
Measuring the Velocity OF Light
We propose giving popular account
of the ingenious yet very simple methods
! by which men Of science have succeeded
in measuring extremely minute portions
of time with such certainty in their results
as that expressed by Professor Rood, whose
confidence ia his billionths OF a second has
probably surprised your readers as much
as his results have interested taem.
/ A wheel painted black and carrying n
distinct white point ON its circumference Is
provided with some means Of giving It .
uniform motion Of rotation Ii the wheel
make one revolution in one,sixth OF = see
ond, the white point will appear AS n con
duecd on the eye remains during one-siNIh
OF y second, therefore during one reyolu-
tion OF the wheel all the successive posi-
tions ON the circumference occupied by the
y
and hence the circle appears unbroken,
Now, il Il ilash Of light in the light in the
place Of the white point should last one
sixth Of second, the circle would appear
complete; bat II il lasted one-twelfth or
one-twonty- fourth Of u second. then would
the point describe one-half or one-quartef
OF tho ci-ele. Thus, by this simple means
remembering that the smaller the arc Of
the circle, the less the duration Of The Hash
we can readily measure, from the length
Of this arc, quite minute portions 81 time.
IT, instead Of having one white point on
the wheel, We have one hundred or more
radial white bands drawn With the space
between them equal 10 their breadth, then
il the wheel makes ten turns in = second,
any radial white band will advance intoI
the position previously occupied by an ad-
joining black band in one thousandsh Of
second, and H. the fash Of light lasted one
thousandlh ola second all the white bands
would, during that interval, have adyanc-
cd into the position cf the black bands. aud
ricc rcrsa, aud the disk would appear with
out bands and covered with uniform
gray tint We can thus readily and aocu-
raiely measure one~thousand Of second.
With the above apparatus Arago. about
the year 1885, hrs. showed that n AAsh Of
lightning lasted less than ono-thousAadth
OF second but did not succeed In fixing
the minimum limit 10 its duration. Pro-
fessor Rood, however. wSs more fortunate
during the well remembered remarkable
display Of lightning in August, lS29, with
an apparatus similar 20 the above lcHtcm-
poriEed from a piece Of pAs.eboard and .
shAwl-pinl, he succeeded In measuring one
fye-hundrcdih Of second As the duration
Of those vivid and extensive flashes.
Il Was soon found that the velocity Of the
revolving disk f,ll Ear behind that Of the
spark OF the Leyden jar. for is hash show
ed the revolving radial bars as absolutely
At rest as when the disk was stationary.
But Prot Nheatstone, 81 London, In 1884,
substituted for the revolving disk mirror
turning on horizontal axis, and instead of
the white point or bars he used the image
Of the spark reflected from the turning mir
ror. II the spark be instantaneous, then
will j, appear in the rotating mirror just as
il is seen when reflected from the mirror at
rest but II the spark las, during even an
extremely minute fraction Of a second, it
will appear drawn into line in the direc-
tion in which the mirror turns Wheat
stone thus measured the enc-million-onc-
hunired-and-hfl hfly-ihousAndth OF second.
and ascertaincd that the electricity from =
Leyden jar goes over copper wire at the
rate Of IS, 600 miles in a second, exceeding
light itself in velocity.
Prof Rood combined the two methods
above given by viewing the appearance Of
stationary parallel and equidistAHt white
and black bands reflected from the rcyolv-
ing mirror while the flash Of tie Leyden
jar Illuminated them. The direction ofro-
iaiion Of the mirror being across the length
Of the bands (which were only sixteen
thousandths Of an inch apart, IL the cash
lasting during the time for the turning
mirror 90 reflect black band into the adj
jaeent white space then the bands would
entirely disappear, aud the plate on which
they were drawn would appear OF n uni-
!orm gray tint BY knowing the number
Of turns the mirroI makes in second. and
the number Of bands in the space Of one
inch, Il is easy TO calculate the time neces-
sary for the oblireration OF the bands. Thus
has he produced, by this simple combina
ton, an instrument surprising In minute
ness and accuracy ofdetormination all that
has gone before-an accomplishment
which cannot but reflect much renown up
on American science. He has succeeded
with mirror making three hundred and
ffty turns In a seeond) in measuring aecu-
rately forty billionths Of second, and
has shown that this is the duration OF the
fash Of n Leyden jar having only eleven
square inches Of surface and one-twenty-
ffth Of an inch striking distanee-an inter-
val Of time just sufficient for a ray Of light
(going Mt the rate Of one hundred and
ninety thousand miles in n second) TO trav-
sl over forty feet The fash from a jar
having one hundred aud fourteen square
inches Of surface lasted four times as long
as the smaller jar Thus, for the range Of
electric flashes we have measures from the
one-fve-hundrSdth 20 the forty-billionth
Of second
Astonishing as is the fact Of the concen-
tration of the power Of lightning flash
into such n minute interval, yet, as won
derful is the extent of the earth's surface
affected by il, as will be seen from the fol-
lowing experiments Of the writer, never
before published A galyanomctcr con-
sists Of delicately suspended magnetic
needle surrounded by coll of copper
wire, through which n current Of electrici-
ty can pass whenever this passage takes
place, the needle rapidly turns around It's
point Of suspension. This being under
stood, connected the wire ofa galvanom-
eter with the water-pipeS Of Baltimore, and
the other end Of the coil was joined to n
gas-pipe ofa house in the southwest part
Of the city Thus n vast metallic system Of
electric nerves stretched away three miles
to the northwest, to the reservoir, and
about as many TO the east and southeast
over the city. A thunderstorm was raging
at the time at so great distance In the