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.]