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Siile
Opvarmning tiwkker Kviksølvet feig tilbage i linret., og
man bringer med Magneten Indoxen ned i dette. Efter
on saadan Operation gjor man vol i at sammenligne
Instrumentet med Korinaltbermpiiietret.
En anden Ulempe ved Index-Thermoiiietreno er
f’ol-gende. Naar et eller Here saadanne benyttedes, sammen
med Kviksølvpiczometret eller Negretti & Zanibra’s
Ven-detberinometer, til Bestemmelse af Hundtemperaturen, eller
som Lod til Bestemmelsen af en Temperaturrække, fandt
jeg undertiden, efterat have anbragt de behorige
Correctioner ved de aflæste Temperaturer, at et af
Tndexthor-monietrene angav en Temperatur, der var merkelig
forskjellig fra og højere end hvad de øvrige Thermoinetre
angav. Naar jeg strax derpaa sammenlignede med
Nor-maltherinometret, fandt jeg, at vedkommende Thermometer
havde forandret sin Correction Og, naar den sidst fundne
Correction anbragtes, var der i Regelen god
Overensstemmelse med de andre Thernionietre. De saaledes indtraadte
Ændringer kunde beløbe sig til en hel og en halv Grad.
Samtlige vore Iiidexthermometre. ældre og nyere, viste
Saadanne Uregelmæssigheder en eller flere Gange. Det mest
paafaldende var imidlertid, at den pludselig indtraadte
Forandring ikke varede mere end en, højst to Dage; efter
den Tid gjenvandt Thermometrets sine tidligere havte
Correctioner. Det synes, som 0111 Aarsagen til
Fæno-ineiiet mermest skulde ligge deri. at den lille Udvidelse, som
Thormometerrøret har lige under Kuglen, under et storre
Vandtryk i Dybet er bleven sammentrykket og derved har
bevirket en højere Stand (Aflæsning) af Kviksølvtoppen,
men at denne Virkning har tabt sig i Løbet af den
l’ol-gende Dag under almindeligt Lufttryk, idet Glasset har
gjenvundet sin oprindelige Ligewegtsforni. Det har ikke
lykkets mig at paavise ydre Omstændigheder, som i det
enkelte Tilfælde skulde kunne forklare denne Virkning.
Man maa derfor stadig van-e paa sin Post og have Control
ved andre Thernionietre. Det var først i 1877. at jeg blev
opinerksoin paa dette Fantomen. Det tor nok luende, at
flere af de i 1 <S7ü iagttagne Uregelmæssigheder havde en
lignende Aarsag, navnlig ved Thermometer No. VII; men
jeg har, ved at anvende skiftende Correctioner ved dette
Instrument, dog ikke opnaaet nogen gjennemgaaende bedre
Overensstemmelse end med faste Correctioner.
Negretti & Zanibra’s Vendetherniométer, der er
indesluttet i et ydre tilsmeltet Glasrør, som optager
Vandtrykket. saaledes at dette ikke virker paa selve Thermometret,
benyttedes af os i 1878 med den Vendemekanisme, som
Fabrikanterne da havde givet det. nemlig en Trækasse med
en Rende, hvori en Del Blyhagel kunde rulle og saaledes
holde Tyngdepunktet til den ene Side. Under Udfiringen
holdtes ved Lodlinens Fart Tliernionieterkuglen nederst. Ved
Havbunden eller, ved Temperaturradcker, i en bestemt Dybde,
standsede Bevægelsen, Thermometret gaves Tid til at
antage det omgivende Vands Temperatur, og derpaa haledes
a less height and greater diameter, leaving tin* index
sufficiently free to be extracted by the magnet. The subsequent
heating causes the mercury to draw back within the tube,
and the index is then brought down by means ol’ the magnet
into that part. After such an operation, it is advisable to
compare the instrument with the Standard-Thermometer.
Another drawback attaching to the index-thermometers
is the following. When one or more such were used,
either along with the mercury-piezometer or Negretti and
Zanibra’s in verting-thermometer, for measuring Ille
bottom-temperature or as a link for determining serial
temperatures. I sometimes found, after having applied the
necessary corrections to the temperatures read off. that one ol’
the index-thermometers indicated a temperature remarkably
different from, and higher than, that indicated by the other
thermometers. When, immediately after. I made a
comparison with the Standard-Thermometer, 1 found the
thermometer in question to have changed its correction, and.
on applying the new correction, there was. as a rule, very
fair agreement with the other thermometers. The changes
thus occasioned might amount to a whole or half a
degree. All our index-thermometers, both old and new.
gave evidence of the same irregularity on one or several
occasions. The most striking instance however, was that
of this sudden change not having lasted more than one —
at most two days; after that period the thermometers
recovered their original corrections. It would seem as
though the cause of the phenomenon lay principally in the
expansion which the bore of the thermometer has directly
under the bulb giving way when exposed in the
deep to a greater pressure, thus occasioning a higher
reading of the top of the mercury, but in such influence
ceasing to operate during the course of the following
day under ordinary atmospheric pressure, the glass having
recovered its original fonn for equilibrium. I have not
succeeded in finding the outward circumstances which in each
case might explain this influence. Hence the observer must he
constantly on his guard, and control the readings by means
of other thermometers. It was not till 1877 this
phenomenon attracted my attention. Very possibly, several
of the irregularities observed in l<S7li had a similar cause,
jn particular as regards thermometer No. VII; but 1
have nevertheless failed, by applying an interchange of
corrections to this instrument, to attain better general
agreement than with unchanged corrections.
Negretti and Zambia’s inverting-therinomoter, enclosed
in an outer sealed glass tube, which resists the
pressure of the water, in such manner that it does not
act directly on the instrument, we used in 1878. with
the inverting-niechanism which the maker had then
devised, viz., a wooden frame containing a quantity of shot,
free to move from end to end, and which kept the
centre of gravity on one side. When veering out the
instrument, the sounding-line pulls the hull) of the
thermometer downniost. On the sea-bed, or. when taking
serial temperatures, at a given depth, the motion ceased,
the thermometer was given time to accomodate, and then
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