, 1870-05-08
LIGHTNING.
How the Duration OF a Flash il
Measured,
r.

low the Duration OF a Flash is
Measured.
From The Teehnolosts.
Scientific men assert that q flash of 1ipht,
ming does not endure for moro than the mili-
onth part OF second. How did they IND
this out? Obviously, not by any ordinary
meens, for such small portions OF time arc
utterly beyond tie cognlzanco Of our senses,
and even Of our thoughts. Watches which
marx quarter seconds are not uncommon We
have seen men ty to count the beats Of such
watch, and it is but rarely that we have met
any ono who could succeed. Not opIy could
thsy not count them aloud, but they could not
even think Of the numbers 80 as to keep 70
cord Of them in their minds. How, then, was
II possible 70 MEASURE portion Of time SO
Almost every boy has whirled fiery brand in
tho air 60 as to make a t'Tound robin" How
does it happen that such moving point seems
to make fiery circle in the air In this way:
An impression made upon ot1r sense Of sight
cannot be instantaneonsly removed. It Lasts
Tor about one-eighth of second. Hence If
the moving point completes the circuit in less
than the eighth Of a second, it will make a new
impression before the old ono has expired;
the path which it describes will bo constantly
Einb]e, and the circle viii appear complete.
N tho same Way, i wheel with many spokes
be caused to revolye rapidly, the spokes be-
come invisible, and tho wheel appears solid.
But il the wheel were Illuminated by flash
which did not List long enough for one spoke
10 tske the place Of another, the spokes would
be visible, and the wheel would appear at rest
A few years ago we saw such an experiment
TIED in Rochester, n 7. The lecturer had
wheel, which was painted white, and on the
white ground were painted number of red
rays. When whirled with \ very moderate ye-
hue. 4, ~hile TINS revolving, it were iilumi-
mated BY taah from burning gunpowder, the |
spokea would not be visible, the flash of even |
the best gunpowder lasting longer than tho !
time required for one spoke to take the place |
9f another. But when, instead Of gunpow-
der, Illuminating mercury WAS used, the spokes |
we're visible, and the wheel appeared to stand |
stu, 60 much more rapidly tlan gunpowder |
does Illuminating mercury explode.
By diving the wheel little faster, the fash |
from the illuminate failed to show the individ-
nal spokes, showing that even the flash from
Illuminating mercury lasted long enough to al
predecessor before the impression made by |
the latter had faded from the retina.
a Ieyden jar WIS noW charged and dis
charged before the revolving wheel, and then
The Latter could not be driven so fast that the
spokes would not be visible and the wheel ap-
bear to stand still. Now, If ne knew tic
Telocaty OF the wheel and the number Of
'pokee, it would be easy to toll the limo re-
luIed <or one spoke to take the place OF
mother Suppose the wheel had 100 spokes,
Lud 4AoSeu al tue lAl6 OF 4,VVU feVOl0tfOna per
second it is obvious that il the individual
poke be seen. the duration of the fash
must be less than the 100, ,000th part of
second.
Such is a rude outline OF the method of !
which tho duration Of flash of lightning is |
measured. OF course, the apparatus used in
determining this duration is much moro deli
ate than that which We have described, and
Which was used merely for illustration before
! large audience. But the general principle is
he same, and in tlis way it was determined
hat the duration of Q fash Of lightning is less
han the mllhonth part OF a second. Hence,
carnAge wheel in rapid motion, seen at night
y means Of fash OF lightning, appears at
est The HEiftest racehorse would not have
ime to make a Percephble movement Of q
material and even tho swiftest 10
omotive and it's train would appear as if some
y hand had been laid on its energies.
Tint flash Of Lightning enlures for cer-
sin period there can be no doubt, but this
eHod, when compared with the duration Of |
ny mechanical operation, is indefinitely small.
tand behind target at which bullets are be-
1g fred from distance Of say 200 yards, 1nd
rst you will zee the fash; after short in
.rya1 you will hear tho thud Of the bullet, and
nally you VIII hear the report Of the gun In
liz case you might see the flash and afterward
. shot By the bullet. Indeed, so much time
10 bullet, from distance Of $50 yards, that
would be impossible to hit an active man at |
lat distance il there Was q cover to which lie |
,u1d spring when he saw the gun discharged.
How different with lightning! There the
lEh and the stroke arc simultaneous. II We
70 the f1aEh we are safe. He that Is killed by
shtpipE never hears the thunder.