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(ler forøvrigt er lukket, kommer omtrent O.’"5 (20 Tommer)
uncler Skibshunden og visende ret forefter. Denne Afstand
fandt vi var den hensigtsmæssigste. En mindre Afstand
bragte Hullet for nær det af Skibshunden medslæbté Vand.
en større Afstand rønnede for meget paa det fritstaaende
Rør under større Fart. Pakningsskruen c tilskrues.
Ovenfor c bør Røret a have en mindre Kran d. Fra a’ s
øvre-Ende gaar et tyndt Biv rør til Stigerøret g, der har omtrent
10™ (4 Tommers) Diameter. Dette maa placeres
midtskibs, vertikalt og saa lavt. at dets nedre Ende er godt
under laveste (nederste) Vandlinies Plan. Det er forsynet
med Blænde, for at ikke Vandet under Skibets Bevægelse
skal pumpe i Røret. I dette Stigerør er anbragt en
Flyder, fra hvilken der gaar en Snor over en Metalrulle paa
Toppen af Røret, derfra over en anden Rulle paa Toppen
af Skalaen, og Tampen er stukket gjennem et lidet Blylod,
der vandrer mellem to tynde Messingstænger langs Skalaen,
og som fæstes til Snoren ved at man trykker ind en liden
Trækile nedenfra i det Hul, gjennem hvilket Snoren er
trukket.
Under Skibets Fart forover trykkes Vandet op i
Stigerøret. og eftersom Flyderen kommer højere, synker Loddet
langs Skalaen. Saasnart Farten er bleven jevn. staar
Loddet uforandret paa samme Højde paa Skalaen, hvor Fartens
Størrelse da kan aflæses i et Øjeblik.
Vandloggen er. som man ser, en speciel Anvendelse
af Pitofs Rør. Kaldes den Højde, hvortil Vandet stiger i
Stigerøret over det Niveau, det indtager, naar Fartøjet
ligger stille, li, Skibets Hastighed v, Tyngdens Acceleration
g og er M en Coefficient, saa har man1
Coefficienten M har efter Dubuats Forsøg en Værdi,
der er større end 1. men bliver mindre, naar Hastighederne
bliver større, uden dog at naa Enheden. Ved en Hastighed
af l.m8 pr. Sekund fandtes M = 1.08.
Ved Vandloggen, saaledes som den er indrettet af
Lieutenant Petersen, er Coefficienten M sat lig 1.
Rigtigheden eller Tilstrækkeligheden af denne Antagelse til ethvert
praktisk Brug tilsøs, selv paa lange Rejser, er godtgjort
ved den Anvendelse, Apparatet har havt paa -’Vøringen"
under 1877 og 1878 Aars Expeditioner og under dens Gang
i Fragtfart paa Østersøen og Spanien, hvor lange
Strækninger er udsejlede uden Afbrydelse. Endvidere har det
samme Resultat vist sig af Vandloggens Anvendelse paa
Oplodningsdampskibet "Hansteen" hvert Aar siden 1875
og paa Korvetten -Nornen" paa et Togt til Vestindien.
bottom, till the small aperture at its lower end, which for
the rest was closed up, had been made to project about
20 inches beneath the bottom of the vessel, while pointing
straight forward. This Ave found, by repeated experiment,
to be the right distance. If diminished, it would bring the
aperture too near the water carried along by adhesion to
the ship’s bottom: and if increased, it would, with greater
speed, expose the projecting tube to a serious strain. The
gland c has now to be screwed on. A little above the
gland, the tube a should have a smaller stopcock d. From
the top of a a slender leaden pipe led to the upper tube
g. which had a diameter of nearly 4 inches. This tube
must be given a vertical position amidships, and far enough
down to bring its lower extremity well below the level of
the lowest load water-line. It was provided with a blind,
to prevent the water from pumping in the tube by reason
of the motion of the vessel. In this upper tube there was
a float, from which a line passed first over a brass roller
at the top of the tube and then over another at the top
of a graduated scale, the end of the line being rove through
a small leaden weight that played against the scale between
two slender brass rods, and was made fast to the line by
inserting from below a small wooden wedge into the
opening through which the former passed.
Now, on the ship moving ahead, the water will be
forced into the upper tube and the weight descend along
the scale as the float rises. So soon as the speed has
become uniform, the weight will keep stationary, at the
same point on the scale, and the rate may then be read
off at a glance.
The water-log is obviously a special adaptation of
Pitof s tube. Let h be the height to which the water in
the upper tube rises above its level when the ship is
stationary; v the speed of the vessel; g the acceleration of
gravity; and M a coefficient, — we shall then have the
formula1
The coefficient 21 has. according to Dubuat, a value
greater than 1. which decreases however with the
increasing velocities, though without reaching unity. For a velocity
of 1.8m pr. second, the value of M was found to be 1.08.
For the water-log on Lieutenant Petersen’s
construction, the coefficient M is put equal to 1. And this
is practically correct, even for comparatively long voyages,
as shown from experience derived on the cruises of the
"Vøringen" in 1877 and 1878, and on her voyages in the
freight-trade to the Baltic and to Spain, very considerable
distances having then been run at a stretch. Equally
favourable results have been obtained, too. by the use of
the water-log (since 1875) on board the Coast Survey
steamer "Hansteen," and on a cruise of the steam-corvette
"Nornen" to the West Indies. Now, supposing the coef-
1 Bresse, Cours de mécanique appliquée. Seconde partie, p. 335.
Bresse, Cours ile mécanique appliquée. Seconde partie. p. 335.
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