The Manitowoc pilot. [volume] (Manitowoc, Wis.) 1859-1932, 1888-12-13
TIDE GAUGES.
An Instructive Talk with Prof. Davidson
of the Coast Survey.

Prof. George Davidson of the coast
and geodetic survey recently tele-
graphed Chief Supt. F. M. Thorne that
the self-registei1ng tide gauge at Sau-
sali.o mad commenced to mark the
traces of an earthquake transmitted
through the ocean. Up to noon the
following day the traces, though very
irregular, continued. A San Francisco
Examiner reporter saw the professor
the same evening and asked him if he
had any means of knowing whether
this earthquake was identical with the
submarine eruptions which occurred in
Gulf Pachili on June 20 last and con-
tinued two months or more.
'.It is not" said the professor. 'The
earthquake waves, as we get them on
the self-registering tide gauge, travel
at the rate of four hundred miles an
hour. They come across the Pacific
ocean from Japan in twelve hours and
from Crakatoain about eighteen hours.
1t takes them from sixteen to eighteen
hours to come from Peru to Chill.
Whenever this earthquake occurred it
must have taken place from six to
eighteen hours before it was registered
here, 50 it could hardly be the one felt
by the steamer Dwarf Maru in Gulf
Pachili."
'.Do you know whether or not this
earthquake is severe one?"
Il should say that it was not" said
the professor. "Ihe waves, as regis
tered by the tide guage, are coming SO
irregularly that feel certain the dis
turbance is but slight. In the case of
large earthquakes the waves are very
regular at first, then break up and be-
come confused"
''Will you kindly explain the work
ings of the tide guage for the benefit
of the Examiners readers?" asked the
reporter.
with great pleasure. A continuous
piece of paper about sixteen inches
wide is moved forward by clock work
at the rate of an inch per hour. Over
this sheet there is a pencil whichs is
connected by chain and wheels to a
float which is lifted by the rise of the
tide and falls with it. The horizontal
movement of the pencil is one-twelfth
that of the change of height of the
water, and as the point of the pencil
rests on the paper continuous curv-
ed line is drawn indicating the rise
and fall of the tide. A scale is then
used to measure the height of the tide
at each half hour of the day and night
above the datum plane. This datum
plane is the average height of all the
lowest waters of each day. When
there is no storm the curve traced by
the pencil is smooth. But if there is a
storm and the waves are breaking Oh
the bar off the Golden Gate then SER
les of sharp serrations is exhibited.
But if an earthquake wave reaches the
harbor the continuous smooth curve is
made somewhat irregular by compara-
lively long rises and depressions frobl
the general line of the curve. These
rises and depressions may, as in the
Crakatoa earthquake, be equal to one
and a half inches from crest to hollow
or eighteen inches in actual change Of
the water-level, and from crest to crest
the time may be thirty-five minutes.
Have many earthquakes been re-
corded on this tide register?n asked
the reporter.
''All the earthquakes have been re-
corded upon it and upon those of San
Diego and Astoria until they were
stopped. In 1854 the three gauges
offered such data that Prof. Bache de-
duced therefrom the average depth of
the Pacific ocean between San Fran
cisco and Japan to be 2,400 fathoms.
This depth has been fully confirmed by
the deep-sea soundings of Commo-
dore Belknap in the Tuscarora in
1874"
''How do local earthquakes affect the
tide gauge?"
Local earthquakes, such as the
sharp one that centered at Haywards
Oct. %, 1868 do not make any record
upon the tide guage at ali. This is
readily understood mechanical de-
duction."