Evening star. [volume] (Washington, D.C.) 1854-1972, 1890-07-26
ACCURATE MEASUREMENT.
Instruments Required as Near Perfec
ton AS Possible.

Instruments Required as Near Perfec-
ton AS Possible.
| From the Philadelphia Press.
since the time when man reckoned a long
distance as . days journey" and q shorter
Instance AZ so many PAGES" and the height
VI morse as so many 'hands," the needs of
'he arts and sciences for the accurate deter
ihinason OF lengths and distances have become
liore ana more exacting, until now tho art
VI measuring has almost become in itself fae
art
Ia an address delivered recently before the
society OF American Mechanics Mr.. ]. A Rra-
smear OF Allegheny City gave some interesting
ILLUSTRATIONS OF this point at has been very
appropriately remarked" says Mr.. Brashesr.
mat the arm Of King Henry 1, or the barley
corn, though possibly they furnished standards
soou enough for that time would hardly sat
isiy the demands Of our modern mechanics ar
700, ,ooi makers who work very often within the
mn ,rrni. Vi one-thousandth of an inch and even
with surprising unconcerp and no less accnracv
r.oi Wm A. Rogers has also shown that MANY
vi our modern mechanics can calliper To one
thirty-thousandth of an inch"
ueee, however, are coarse, rough measures
When compared with those demanded In max
| ing some OF the latest appliances in use among
aou ohomers The science of astronomy in
ueeu, mas put upon the instrument maker the
heaviest r tax imposed by any of the sciences
;1 is can DO easily understood when we con-
sluer the sort OF work which the astronomer.
IN MS turn, Is expected to do for ma or which
ne imposes upon himself in the interest of
sciehce. lime measures to take an illustra-
t ion; nave reached wonderful accuracy. When
.n1 Dig pei strikes the hours of the dial we
pun out our watches and are satisfied II thew
are correct within q quarter of a minnte .For.
eIy our astronomer at the Allegheny OBJ
servatory, says Mr. Brashear. lis not 40 eosiIv
satisnea. II the stars will but shine he is not
U ,a.. A the error be sixty times less j. e.,
tirnres 81. vi a second, and L recently saw the
hsures Tor several days time work where the
s s were not greater than three one-hun-
ureutns OF second. We all know the great
oeheHt OF this time transmitted to our railway
.ehters, and II human ingenuity could but have
W. uains Keep time with the stars we should
I've. hoter nave the paradoxical phenomenon of two
IRA Trans endeavoring to occupy the same track at
the same time" ~
~ attain THIS degree Of accuracy the astron-
oher must be provided with a transit instru
ment winch is all but perfect in its construction
anu auJustment Its journals must be turned
;r o. ana II must be placed SO exactly
III the right position that it will sweep the sky
rom zenith to horizon without swerying by
80 much as fraction Of hairs breadth from
tne true direction of the meridian. and so
nicely must it be adjusted that its supports
n,rot ho' De touched by the warm hand and
mus. DC carefully protected from currents of
ar, lest the instrument be thrown out of level
| ny ,ne unequal expansion of its parts.
lane another illustration: One of the diff
cut problems upon which astronomers have
peen AZ work Tor more than a century is the
| accurate determination of the distance Of tbo
sun. Not to go into the details of the several
metnoos employed for this purpose it NAV be
SAID that they ali resolve themselves in the end
INTO determination of the apparent semi
uiameter OF the earth as viewed by an imagin-
",y observer al the center OF the sun. The re-
suits thus FAR obtained by the different meth
ods, or by different astronomers using the
| same methods, are fairly but not exactIy CON-
cordant: The angular measure sought is now
agreed to be less than g.. at is almost GER
talnly, says Prof. Young, "between .75v and
200 Inns tenth Of a second Of error which
the ingenuity OF astronomers, backed by the
sKili OF instrument makers, is taxed to elimi-
nate, rs no more than the angle subteuded by a
single hair at the distance of nearly 800 feet I"
i no" "ouId Ming Henrys arm or even bar
leycorn Inquire in measuring q quantity SO
minute as this? ~
Among the parts Of an optical instrument the
most difficult to make with even approximate
accuracy-and at the same time among the
most important parts-are the divided circles
ior measuring angles and the micrometer
screws Tor the finer measurements A screw
80 nne and SO accurate that a slight fraction of
turn will move its point invariably to the
same minute distance is a very different affair
from boit used In building Corliss engine
10 illustrate the difficulty Of making such a
screw Mr.. Brashear cites an instance in which
Prof Rogers, who has disting uished himself
among Americans in such work attempted to
carry success still further and failed. Prof.
Rogers employed upon the work of making a
screw two OF the most skillful workmen of the
Waitham Watch Manufacturing Company. sat a
dollar an hour, =" says Mr. Brashear. with the
instinct OF an employer, and when 14O hours
TABOR had been expended on the screw and it
was pronounced done, it proved on trial to
have periodical drunkenness" and was.
therefore, useless for fractions Of a turn.
although accurate for complete turns Such a
labor 1t Is to make simple screw that shall be
perfect.
But even such accuracy as this does not
brlng US to the and Of the wonders Of mechan-
lcal skill In spectroscopic work the best
spectra Tor some purposes are obtained not
WIN prlsms IN the ordinary way but with what
are known as "diffraction gratings," which
consist OF fine lines ruled with a diamond upon
glass; the finer the lines the better the grating
MIR. L. NJ. Rutherfurd of New York succeeded
some years ago in ruling gratings which con-
tanned II, $50 lines to the inch. For a number
OF years MIR. Rutherfurd was without rival in
this work either in this country or in Europe:
but his achievement has been recently sur,
passed by Prof. Rowland of Baltimore, who has
constructed q machine by the aid of which he
can rule gratings OF six inches in breadth, con-
tanning 2S.s76 lines to the inch. Some of the
celebrated "Nobert lines" now used an test
objects for microscopes, are still finer, being
on the scale Of &c, 600 to the inch,
1t would be easy to multiply instances like
this in which the requirements Of scientific in
vestigation have called upon the skill of the
artisan and in which he has shown himself
equal to the demand. But science itself il con-
tinually dealing with quantities more and more
minute and is becoming more and more exact
ing In its requirements. There is, therefore.
always a border land where there Ia failure for
a time, yet in nowise discreditable to the me-
chanic, because it 10 the invariable precursor
of success. q
As an instance illustrative Of how exactin0
these men Of science are, some years AGO prof
Pickering undertook to determine the size of
the two minute satellites of Mars. They are too
small to appear as anything but tiny stars, even
In the largest telescopes. The only way of es-
timating their size was, assuming that chew
were intrnsically as bright as the planet itself.
to compare their apparent briehtness with
that Of the planet. Theoretically this was a
simple operation. It was only Necessary 70
cover the bright disk of the planet, as IL an
peared in the eye-piece Of the telescope. with a
thin metal screen, in which was a minute hole
that could be varied in size experimentally nm
til the light received through 1t from the planet
just equaled that Of the satellite. The size of
the hole being then ascertained and COMPARED
with that Of the entire disk gave the
relation required. But who should make
the hole? pin hole would not answer the
purpose, for the planets entire disk was hardly
larger than that prof D;oLo-;,-