The Indianapolis journal. [volume] (Indianapolis [Ind.]) 1867-1904, 1888-10-28
TIDE GAUGES.
An Instructive Talk with Prof Davidson, OF
the Coast Survey.

An Instructive Talk with Prof Davidsop, of
the Coast Survey.
san Francisco Examiner.
Prof. George Davidson, Of the Coast and Geo
detic Survey. recently telegraphed Chief Super
intendsnt +. n. Thorne that the self-reeistering
the traces Of 40 earthquake transmitted through
the ocean. Up to noon the following day the
traces. though very irregular, continued. An
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 Paehili, on June 20 last, and contip ued for
two months or more.
sit 10 not" said the Professor. 1.-THE earth
quake waves, az We get them on the seli-regis-
terine tide gauge, travel at the rate Of 400 miles
an hour. They come across the Pacific Ocean
from japan in twelve hours, and from Crakatoa
Ia about eighteen hours. Ii takes them from
sixteen to eighteen hours to come from Peru
and Chill. Wherever this earthquake occurred
IL must have taken place from six to eighteen
hours before it was registered here, 20 that 1t
could hardly be the one felt BY the steamer
Dwarf ksru In Gulf Pachili"
dc you know whether or not this earthquake
is q severe onea"
II should say that it was not" said the Pro
lessor. The waves, as registered By the tide
gauge, are coming so irreRularly that / feel GER
tain the disturbance 10 but slight. In the case
Of large earthquakes the waves are very regular
st first. then break MP and become confused"
Will you kindly explain the workings of the
tiIe gauge for the benefit Of tho Examiners
readersp asked the reportsr.
with great pleasure. A continuous piece OF
paper about sixteen inches wide 10 moved for
ward by ~clockwork at the rate of an inch per
hour. Over this sheet there is pencil which
10 connected by chain and wheels TO q float
which 10 lifted by the rise Of the tide and falls
with it The horizontal movement OF the pen
ell Is oue-twelfth that Of the change Of height Of
the water, and a. the point of the pencil rests on
the paper, q continuous curved line 10 drawn,
IA then used to measure the height of tho tide az
each half hour of the DAY and night above the
datum plane. This datum plane 10 the average
height OF all the lowest waters Of each day.
When there 10 no storm the curve traced by the
pencil 10 smooth. But II there 10 q storm and
the waves are breaking on the bar of the Gold
en- Gate, then o series 0f sharp serrations IL ex-
hibited. But IL an earthquake wave reaches
the haroor the continuous smooth curve Is made
somewhat irregular by comparatively long
rises and depressions from the general line of
the curve. These rises and depressiops may, as
in the Crakatos earthquake, Re equal to one and
. half inch from crest to hollow, or eighteen
inches in actual change OF the water-level, and
from cres: to crest the time may be ~thirty-fve
minutes"
"HAve many earthquakes been recorded on
this tide -registerp asked the reporter.
A the great earthquakes have been record
Cd upon it and upon those of San Diego and As
toria, until they were stopped. In 1854 the three
gauges offered such data that Prof. Bachs de-
duced therefrom the average depth of the Pacific
ocean, between San Francisco and Japan, to be
2,400 fathoms. This depth has been fully. con-
firmed by the deep-sea soundinRs Of Commodore
Belknap, in the Tuscarora in 1874"
'How do local earthquakes affect the tide
gaugeP
Local earthquakes, such az the sharp one
that centerd As Hsywards, Oct %, 1868, do not
make any record upon the tide-gauge ct all.
This is . readily understood mechanical deduc.
tion.n