Evening star., 1927-10-09
BY GEORGE PORTER.

/" HAT the tide predicting machine
of the United States Coast and
Geedetic Survey will predict"
Il backward as well as forward
with infallible accuracy was
brought cut recently when the survey
was called upon to settle once and for
all whether the British landed at
Charlestown at the start of the Revolu-
tionary War when the tide was high
or low. 1t was found that on the
night of April IS, 1775, the tide was
lowest at 7:50 pm. and highest at 2
o'clock ON the morning of April 10. AS
history records that the British crossed
between / and ? am, that is, at high
tide, another long standing fallacy has
been discovered.
The delay of the English has tradi-
tionally been explained as having
been caused ty their waiting for low
tide. In view of the surveys verdict,
however, it would seem that another
reason, sometimes advanced, that they
We're waiting for provisions. is prob
ably the more correct. The point is Of
some Importance, for the delay, what
ever it's cause, saved the day for the
Colonists, as it gave Paul Revere
time to make his famous ride, calling
upon the sleeping countryside Ito up
and to arm" Longfellow's poem, his
toriealiy inaccurate in many respects,
was correct in regard to the tide,
which 1t describes as rising:
Fer suddenly all his thoughts were
bent
On shadowy something far away,
Where the river widens to meet the
bay
A lire of black that bends and floats
On the rising tide, like a bridge of
boats"
The state of the tide at the time
of other historic happenings. such as
Ths landing of the Pilgrims on Ply
mouth Rock, has also been deter
nhed, by request, in the office of the
LIFTED States Coast and Geodetic Sur
9 AS tide study l.; purely Il mathemat-
ical process after the data on the
moon and other celestial and terrestrial
objects which influence and determine
the tide have been obtained. it is
quite possible to calculate tides of
years gone by as well as these TO
Ceme.
The only possibility of error in the
case Of former tides arises from
changes in the shore line caused by
river deposits, pounding of the waves
&c. These are very slight in most
plaees never making variations in
harbor's contour of more than few
feet a century, with Il eorrespondingiy
small effect on the tide. so that the
scientist. when he does not know the
extent Of such changes, has no
scruples about leaving them out OF the
calculations altogether.
When the state OF the tide at only
one port at a specific time is desired,
combinations are made by hand, but
when it is necessary to know the sort
of tides which were or Will be at one
or more ports during several days,
months or Years, the hand process is
100 laboriouS. fatiguing and susceptible
to errors. Under such circumstances
a machine-Which doesn't make MIS-
takes and doesn't get tired-ls almost
Indispensable.
The tide-predictor now used by tle
United States Coast and Geodetic
Survey is in a room by Itself in the
surveys building at 205 New Jersey
avenue southeast. 1t measures il
feet in length, feet in width ana
feet in height. 1t is inclosed IN a
glass case for protection against dust,
moisture and temperature changes.
The case, however, is large enough 10
permit the operator to make ADJUST
I.,eAts upon the machine.
To "set up this device, predict and
tabulate tides for one Year at . single