Full resolution (JPEG) - On this page / på denna sida - Sidor ...
<< prev. page << föreg. sida << >> nästa sida >> next page >>
Below is the raw OCR text
from the above scanned image.
Do you see an error? Proofread the page now!
Här nedan syns maskintolkade texten från faksimilbilden ovan.
Ser du något fel? Korrekturläs sidan nu!
This page has never been proofread. / Denna sida har aldrig korrekturlästs.
belyse Magnetens Skala, maatte en særskilt Theodolit
anvendes til Bestemmelse af Azimut. Theodoliten opstilledes
i 1876 paa sit eget Stativ med sin Kikkert i samme Højde
som Magneten og saa nær denne som muligt. Da dette
voldte meget Bryderi, lod jeg forfærdige paa Hortens
mekaniske Verksted et Underlag af Messing, der kunde lægges
paa Magnetometret og fæstes til dette ved Hjælp af de
samme Indretninger, som anvendtes ved Deflektionsstängens
Befæstigelse. Paa den ene Side af dette Underlag
anbragtes Theodoliten, med Fodskruerne i smaa dertil
afpassede Huller, og paa den anden Side en Modvægt af
Bly. Det var med denne Indretning altid let at faa se
og kunne indstille Magnetskalaens Midtstreg paa
Theodo-litens Vertikalfilament. Observationerne udførtes i Regelen
paa følgende Maade:
Ophængningstraadens Torsion ophævedes. Theodoliten
nivelleredes, og indstilledes med Filamentet paa Skalaens
Midtstreg i Magneten, hvorefter Theodolitens Nonier
aflæstes (Magn. I).
Theodoliten drejedes, saaledes at dens Kikkert pegede
lidt vestenfor Solen, i Solcentrets Højde, og fastklemtes.
Tidspunkterne for Overgangen af forangaaende og
efterfølgende Rand af Solen over Vertikalfilamentet noteredes
efter Kronometer. Denne Iagttagelse gjordes enten med
Blændglas foran Okularet eller ved at projicere Solens og
Filamentets Billede paa en hvid Skjærm. I mange
Tilfælde var. der to Iagttagere, af hvilke den ene observerede
Solrandenes Passage og raabte "Nu" i det Øjeblik, de
tangerede Filamentet, medens den anden observerede og
noterede tilsvarende Øjeblikke efter Kronometret. Nonierne
aflæstes.
Kikkerten lagdes om gjennem Zenit (Nadir), drejedes
180° om Vertikalaxen og begge Solrandes Passage
observeredes i denne Stilling efter Kronometret, ligesom
Nonierne aflæstes.
Magneten omlagdes, idet den drejedes 180° om sin
Længdeaxe, og Filamentet i Theodolitkikkerten indstilledes
atter paa Magnetskalaens Midtstreg, og Nonierne aflæstes.
(Magn. II).
Naar Omstændighederne tillod det, observeredes atter
Magneten i Stillingen I. Stundom blev den omlagt flere
Gange.
Ligesaa flyttedes, ved saadanne Lejligheder,
Theodoliten ved den følgende Sats saaledes, at Fodskruerne kom
hver i andre Huller, hvorved dens Cirkels Nulpunkt
forandredes 120°. Det viste sig imidlertid, at dens
Delings-fejl ikke var saa betydelige, at denne Vexling af Stilling
var nødvendig for at opnaa den forlangte Nøjagtighed af
c. 1 Minut.
I flere Tilfælde bestemtes Azimut af en Mire, til
hvilken Observationen af Magneten i begge Stillinger blev
knyttet.
Kronometrets Stand blev i de fleste Tilfælde
bestemt ved Solhøjder tagne meel Sextant paa Observations-
having only a concave mirror by which to illuminate the
scale of the magnet, the azimuth had to be determined
with a separate theodolite. In 1876, the theodolite was
mounted on its own stand, with the telescope lacing the magnet,
and as near it as possible. This, however, proving
excessively troublesome, I procured from the Horten Mechanical
Works a brass support, which, when placed upon the
magnetometer, could be attached in the manner adopted for
fixing the deflection-rod. On one side of this brass support
was mounted the theodolite, with the foot-screws fitting
into holes made for the purpose, the other side being given
a counterpoise of lead. With this arrangement, the scale
of the magnet could be easily sighted, and the middle
division bisected by the vertical, wire of the theodolite.
The observations were generally taken as follows: —
The torsion of the suspension thread having been
first removed, the theodolite was levelled, and the wire
made to bisect the middle division of the scale of the
magnet, after which the verniers of the theodolite were
read oft’ (Magn. I). The theodolite was then moved in
azimuth and altitude till its teloscope pointed a little to
the west of the sun. at the altitude of the sun’s centre,
and clamped in that position. The times for the transit
of the preceding and following limbs of the sun across the
vertical wire, were noted by a chronometer. This
observation was taken either with a coloured glass before the
eye-piece or by projecting the image of the sun and the
wire on a white screen. Frequently, there were two
observers, in which case one observed the transit of the solar
limbs, calling out at the moment they were tangent to, the
wire, while the other observed and noted the corresponding
readings of the chronometer. Then, after reading off’ the
verniers, the telescope was turned through the zenith
(nadir), moved 180° on its vertical axis, and the transits
of both solar limbs observed in that position by the
chronometer. the verniers being also read off.
The magnet was now inverted, being turned 180°
about its longitudinal axis, and the wire of the telescope
of the theodolite again made to bisect the middle division
of the scale of the magnet, after which the verniers were
read off (Magn. II).
Circumstances permitting, the magnet was again
observed in position I. Occasionally, it was inverted several
times.
Moreover, on such occasions the theodolite was so
moved previous to the following set of observations, that
the foot-screws should correspond with different holes, and
change the zero of its circle by 12U°. Its errors of
division, however, did not prove so considerable as to render
imperative such change in position for attaining the desired
accuracy of about one minute.
In several cases, the azimuth of a mark was
determined from the observations of the sun, and the
observations of the magnet, in both positions, connected with the
direct observations of the mark.
As a rule, the error of the chronometer was found
from solar altitudes, taken with the sextant at the place
<< prev. page << föreg. sida << >> nästa sida >> next page >>
