The San Francisco call. [volume] (San Francisco [Calif.]) 1895-1913, 1895-04-28
What Distances at Sea the
Lights on the Coast Are
Visible.
HOW THE AIR HAS DECEIVED.
Professor Davidson Tells OF Some
Strange Phenomena Due to
Refraction.


ment that 1.-THE highest order Of light in
operation in the United States lighthouse
service can be seen only twenty-one miles
IN clear weather" When asked about it
yesterday Mr.. Davidson, whose work in
the Government Coast and Geodetic Sur
vey has made him informed of facts ad
infnitum, said:
"The statement is wholly erroneous.
The old lighthouse on Point Iona, San
Diego Bay, 462 feet above the sea, was
visible at sea from height thirty feet
above ships deck little over thirty six
statute miles distant. The present south
east Farallon light, off the Golden Gate,
which is $50 feet above sea-level, is visible
under similar conditions nearly thirty
three miles away. Moreover, you will find
in my Coast Pilot on page 200 of the last
edition, that this light is clearly visible
from Sulphur Peak, near the Geysers, at a
distance of seventy five miles, or GA
nautical miles. Sulphur Peak is BAIL feet
above sea-level
Cali visitors to Mount Diablo know that
the light on the southeast Farallon can be
easily seen from there-a distance of sixty
two miles. The Cape Mendocino light can
be seen thlrty-fiye miles. And had the
lighthouse on Point Reyes been built on
the highest point of that headland the
focal plane would have been $40 feet above
the sea and the light could be seen over
forty miles away.
1.-The fact is we do not know how far one
of our tlrst-class sea coast lights NAV be
visible; it is mere question of elevation,
and thereby overcoming the curvature of
the earth. There would be no difficulty in
seeing such light as the southeast FaraI-
lon from Mount Shasta to Mount Diablo
at 3 A. NJ, when the refraction is greatest
and overcomes the curvature of the earth's
surface by lifting up, as it were, objects.
That distance is pa miles.
cit has been asked why, then the coast
lights were not located upon the highest
attitudes possible. can answer that with
one word-fog. When, in the sos, we I'M
dertook the location of these lights, we
naturally started out to do that very thing.
We placed the original light at Point Iona
at an altitude of 462 feet; that at Point
Conception, near the west entrance of the
Santa Barbara Channel, 258 feet; that at
Point Bonita, BOG feet; that at Point Reyes,
$40 feet, and several others likewise. But
the experience of navigators, both mer-
chant-marine and naval satisfied US that
the lights should be lowered whenever
practical, for the reason that the most
usual fogS are generally from $50 to 200
feet high-that is, their under sides, / may
say, are that height, leaving clear stratum
of air beneath. So we lowered the Point
Iona light to 80 feet, that at Point Con-
ception to 135, Point Bonita 126, made
Point Reyes over 800 feet lower, and Cape
Meares which is now 223 feet, was largely
cut down to that height.
''Some lights have had to be built high
to avoid the sea swells and waves, like that
on st. Georges reef, which is Las feet, and
the Tillamook rock light, off the Columbia,
4 E. Il l., L.
not been able to get any lower, or we
would. These are: The southeast Faral-
lon, $50 feet: Point Sur, 272; Point Arena,
156; Cape Mendocino, 43, and Cape Blanco,
256. Among the very low lights are:
Point Pinos, 81 feet; Cape Arago Coos
Bay, rs, and Point Adams on the Column
bia, which is SO feet, and is frequently WIS.,
ible when Cape Disappointment, across the
river, 232 feet high, is enveloped In fog.
81 incidentally mentioned aerial refrac-
ton little while ago. Well, great many
peculiar phenomena are due to it. Owing
to it light may be actually repeated in
the air vertically above the real light.
"This refraction has been responsible for
the discoveries by famous navigators of
many islands which never had any ex-
istence. A celebrated instance of this was
the reported discovery of the island of San
Juan (calied also st. Johns) in the Santa
Barbara Channel. Martinez made that
discovery in 1889 during passage from
Monterey to San Bias, and in Vancouver's
narrative you can find a very matter-of-
fact reference to it. Vancouver said he saw
it from the San Diego preside during
very clear evening. The truth is, it was
simply the island of San Clemente lifted
up, apparently, by refraction. Captain
Cook reported the discovery of some such
islands in Boring Sea.
"Refraction has fooled the Aleuts fire.
quently. Once upon remarkably clear
day with northerly winds they saw islands,
as they report, to the southeast and north
east and many trips were made to reach
them, the Russian Commercial Company
sending out vessels on that mission. In
1867 | investigated the matter, and found
that each direction was on a line with one
of the highest peaks of the Aleutian
Islands, and that what they mistook for
islands were these peaks, which had been
so lifted up as to make them easily visible.
"Why, up in the Arctic, navigators,
owing to this refraction, sometimes plainly
see vessels which they know to be actually
between 80 and 80 miles away and yet they
become so plainly visible as to be distin-
guishable.
II have had some amusing experiences
on land. Once while locating the San
Pedro base line on the Los Angeles plains
/ was most laughably deceived. You know
we use white flags and the Morse system
for signaling. Well, my man was some
distance away and it appeared to me that
he was signaling all sorts of things which
neither | nor Mephistopheles himself
could interpret, and what [ wanted was
that flag to be kept still. The more it sig
nailed the madden | became. felt like
communicating back few phrases that
would best appear in print In plain dashes,
but the Morse system is little too slow for
that sort OF thing. Finally | went over,
furiously, to have that fellows scalp.
When I got up to him saw his flag. lying
on the ground. He told me it had been
there all the time, so there was no scalp
raised and | felt little cheap.
81 have observed most extraordinary in
stances OF refraction from Mount Diablo.
Three years ago | saw distinctly from that
emlnence across the Sacramento plains a
distance of 142 miles, the crest line of the
Sierra Nevadas reflected in the sky. The
mountain tops, except for being inverted,
were an exact reproduction. To make the
matter conclusive there was my heliotrope
signal on the top of Mount CONFESS
ha