For registering the height of the tide at every instant, hydrographic
services generally adopt quite a simple marigraph. The apparatus
consists in principle of a counterpoised float whose rising and
falling motion, reduced to a tenth, by means of a system of toothed
wheels, is transmitted to a pencil which moves in front of a vertical
cylinder. This cylinder itself moves around its axis by means of a
clockwork mechanism, and accomplishes one entire revolution every
twenty-four hours. By this means is obtained a curve of the tide in
which the times are taken for abscisses and the heights of the sea for
ordinates. However little such marigraphs have had to be used, great
defects have been recognized in them. When we come to change the sheet
on the cylinder (and such change should be made at least once every
fifteen days), there is an interruption in the curve. It is necessary,
besides, to perform office work of the most detailed kind in order to
refer to the same origin all these curves, which are intercrossed and
often superposed in certain parts upon the original sheet. In order to
render such a disentanglement possible, it is indispensable to mark by
hand, at least once every twenty-four hours, upon each curve, the date
of the day corresponding to it. It is equally useful to verify the
exactness of the indications given by the apparatus by making readings
several times a day on a scale of tides placed alongside of the float.
Nine times out of ten the rise of the waves renders such readings very
difficult and the control absolutely illusory.
All these conditions united, as well as others that we neglect in this
brief discussion, necessitate a surveillance at every instant. The
result is that these marigraphs must be installed in a special
structure, very near the bank, so as to be reachable at all times, and
that the indications that they give are always vitiated by error,
since the operation is performed upon a level at which are exerted
disturbing influences that are not found at a kilometer at sea. It
were to be desired that the float could be isolated by placing it a
certain distance from the shore, and transmit its indications, by
meant of a play of currents, to a registering apparatus situated upon
_terra firma_.
In the course of one of his lectures published in the December number
(1883) of the _Elektrotechnische Zeitschrift_, Mr. Von Hefner-Alteneck
tells us that such a desideratum has been supplied by the firm of
Siemens & Halske. This marigraph, constructed on an order of the
German Admiralty, gives the level of the sea every ten minutes with an
approximation of 0.12 per cent., and that too for a difference of 8
meters between the highest and lowest sea. The apparatus consists, as
we said above, of a float and registering device, connected with each
other by means of a cable. This latter is formed of three ordinary
conductors covered with gutta percha and surrounded with a leaden
sheath, which latter is itself protected against accident by means of
a strong covering of iron wire and hemp. The return is effected
through the earth. We shall enter into details concerning each of
these two apparatus in-succession, by beginning with the float, of
which Fig. 1 gives a general view, and Fig. 2 a diagrammatic sketch.
The float moves in a cast iron cylinder, having at its lower part a
large number of apertures of small diameter, so that the motion of the
waves does not perceptibly influence the level of the water in the
interior of the cylinder. It is attached to a copper ribbon, B, whose
other extremity is fixed to the drum, T. The ribbon winds around the
latter in the rising motion of the float, owing to a spiral spring
arranged so as to act upon the drum. The tension of this spring goes
on increasing in measure as the float descends.
[Illustration: FIG. 1.--FLOAT OF SIEMENS AND HALSKE'S MARIGRAPH.]
[Illustration: FIG. 2.]
This difference in tension is utilized for balancing at every instant
the weight of the ribbon unwound, and thus causing the float to
immerse itself in the water to a constant degree. The ribbon, B, is
provided throughout its length with equidistant apertures that exactly
correspond to tappets that project from the circumference of the
wheel, R. When the float moves its position, the wheel, R, begins to
turn and carries along in doing so the pinion, w, which revolves
over the toothed wheels, s1, s2, and s3. The thickness of w
is equal to that of the three wheels, s1, s2, and s3, and a
special spring secures at every instant an intimate contact between
the pinion and the said wheels. These latter are insulated from each
other and from the axle upon which they are keyed, and communicate,
each of them, with conductors, I., II., and III. They are so formed
and mounted that, in each of them, the tooth in one corresponds to the
interspace in the two others. As a result of this, in the motion of
the pinion, w, the latter is never in contact with but one of the
three wheels, s1, s2, and s3.
If we add that the lines, I., II., and III. are united at the shore
station with one of the poles of a pile whose other pole is connected
with the earth, and that w communicates with the earth through the
intermedium of R, and the body of the apparatus, it is easy to see
that in a vertical motion of the float in one direction we shall have
currents succeeding each other in the order I., II., III., I., II.,
etc., while the order will become III., II., I., III., II., etc., if
the direction of the float's motion happen to change.
[Illustration: FIG. 3.]
[Illustration: FIG. 4.]
In order to understand how a variation in currents of this kind can be
applied in general for producing a rotary motion in the two
directions, it will only be necessary to refer to Figs. 3 and 4. The
conductors, L1, L2, and L3 communicate with the bobbins of
three electromagnets, E1, E2, and E3, whose poles are bent at
right angles to the circumference of the wheel, R. There is never but
one pole opposite a tooth. The distance between two consecutive poles
must be equal to a multiple of the pitch increased (Fig. 3) or
diminished (Fig. 4) by one-third thereof. It will be seen upon a
simple inspection of the figures that R will revolve in the direction
of the hands of a watch when the currents follow the order L1,
L2, L3, etc., in the case shown in Fig. 3, while in the case
shown in Fig. 4 the rotary motion will be in the contrary direction
for this same order of currents. But, in both cases, and this is the
important point, the direction of rotation changes when the order in
the succession of currents; is inverted. Fig. 6 gives a perspective
view of the registering apparatus, and Fig. 5 represents it in
diagram. It will be at once seen that, the toothed wheel, r, is
reduced to its simplest expression, since it consists of two teeth
only. The electro-magnets are arranged at an angle of 120°, and for a
change of current the wheel, r, describes an angle of 60°, that is
to say, a sixth of a circumference. The motion of r is transmitted, by
means of the pinion, d, and the wheel, e, to the wheel, T. For a
one-meter variation in level the wheel, T, makes one complete
revolution. It is divided into 100 equal parts, and each arc therefore
corresponds to a difference of one centimeter in the level, and
carries, engraved in projection, the corresponding number. As a
consequence, there is upon the entire circumference a series of
numbers from to 99. The axle upon which the wheel, T, is keyed is
prolonged, on the side opposite e, by a threaded part, a, which
actuates a stylet, g. This latter is held above by a rod, I, which
is connected with a fork movable around a vertical axis, shown in Fig.
6. The rectilinear motion of g is 5 mm. for a variation of one meter
in level. Its total travel is consequently 40 mm. The sheet of paper
upon which the indications are taken, and which is shown of actual
size in Fig. 7, winds around the drum, P, and receives its motion from
the cylinder, W. This sheet is covered throughout its length with fine
prepared paper that permits of taking the imprints by impression.
[Illustration: FIG. 5]
[Illustration: FIG. 6--RECEIVER OF SIEMENS AND HALSKE'S MARIGRAPH.]
[Illustration: FIG. 7]
This stated, the play of the apparatus may be easily understood. Every
ten minutes a regulating clock closes the circuit of the local pile,
B2, and establishes a contact at C. The electro-magnet, E4,
attracts its armature, and thus acts upon the lever, h, which
presses the sheet of paper against the stylet in front that serves to
mark the level of the lowest waters, and against the stylet, g, and
the wheels, T and Z. In falling back, the lever, h, causes the
advance, by one notch, of the ratchet wheel that is mounted at the
extremity of the cylinder W, and thus displaces the sheet of paper a
distance of 5 mm. The wheel, Z, carries engraved in projection upon
its circumference the hours in Roman figures, and moves forward one
division every 60 minutes. The motion of this wheel is likewise
controlled by the cylinder, W.
It will be seen upon referring to Fig. 7, that there is obtained a
very sharp curve marked by points. We have a general view on
considering the curve itself, and the height in meters is read
directly. The fractions of a meter, as well as the times, are in the
margin. Thus, at the point, a, the apparatus gives at 3 o'clock and
20 minutes a height of tide of 4.28 m. above the level of the lowest
water.
This apparatus might possibly operate well, and yet not be in accord
with the real indications of the float, so it has been judged
necessary to add to it the following control.
Every time the float reaches 3 meters above the level of the lowest
tide, the circuit of one of the lines that is open at this moment
(that of line I, for example) closes at C (Fig. 2), into this new
circuit there is interposed a considerable resistance, W, so that the
energy of the current is weakened to such a point that it in nowise
influences the normal travel of the wheel, r. At the shore station,
there is placed in deviation a galvanoscope, K, whose needle is
deflected. It suffices, then, to take datum points upon the
registering apparatus, upon the wheel, T, and the screw, a, in such
a way as to ascertain the moment at which the stylet, g, is going to
mark 3 meters. At this moment the circuit of the galvanoscope, K, is
closed, and we ascertain whether there is a deviation of the needle.
As the sea generally rises to the height of 3 meters twice a day, it
is possible to control the apparatus twice a day, and this is fully
sufficient.
It always belongs to practice to judge of an invention. Mr. Von
Hefner-Alteneck tells us that two of these apparatus have been set
up--one of them a year ago in the port of Kiel, and the other more
recently at the Isle of Wangeroog in the North Sea--and that both have
behaved excellently since the very first day of their installation. We
shall add nothing to this, since it is evidently the best eulogium
that can be accorded them.--_La Lumiere Electrique._
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