Evening star., 1938-12-08
By THOMAS r.. HENRY.


BY means of fossil compass nee
dies locked in the earth for more
than a hundred centuries a new
measuring rod of ancient time is
being set up by physicists of the
Carnegie Institution of Washing
ton.
This forms part of the annual
exhibit for the annual meeting OF
the Carnegie trustees tomorrow,
after which it will be open to the
public for three days.
The needles" are the tiny slivers
Of magnetic. or lodestone, which
were included in the mud washed
into lakes by the waters from melt
ing glaciers as the great ice sheet
retreated northward over North
America between 10000 and 10000
years ago.
AS a glacier passed across coun-
tryside it left depressions, which
became the northern lake beds, be-
hind it, Each summer the ice
melted little and the water flowed
into these depressions, carrying
with it considerable amounts of fine
silt and other debris These would
sink to the bottom, forming a layer
With the coming of winter the
melting would stop, to be resumed
the next spring with the consequent
laying down of second layer on
lake floor. This would continue
year after year so long as the glacier
was in the neighborhood. Thus the
number of these layers, or "varves," ,"
indicates the time the process con-
tinued. The thickness of such a
layer also is an acceptable criterion
of the ancient climate, since most
silt naturally would be deposited in
a warm summer with a great deal
of water flowing from the melting
ice.
Alignment Varies.
ANY tiny bits of lodestone would
be free to turn into their natural
position, presumably in line with
the north magnetic pole, while they
were unattached in the water. With
other layers of silt piled on top of
them during future summers they
would be locked in this position.
They would have behaved exactly
as compasses, had there been such
instruments in the closing days OF
the last great ice age.
But, explains DR.. A. G. McNish,
Carnegie physicist, the alignment
Of the compass needle with respect
to true north has varied enormously
during historic times. Thus at Lon
iv,
to the eastward. BY 1900 it had
shifted to "a degrees west of true
north. 1t now has shifted back to
about 10 degrees west. There are
all sorts of local variations to con-
slider before the compass can be
considered an altogether reliable
guide, but, DR.. McNish says, its gen.
eral position with respect to true
north remains fairly constant for
any particular period.
Using an extraordinarily delicate
instrument devised at the Carnegie
laboratory which can measure elec-
trical differences close to one ten
millionth of volt, it is possible to
determine the precise alignment of
of the ancient lake bottoms. The
lakes themselves, for the most part,
have long since been filled up, and
the samples with which the present
experiments are being made were
obtained from the bank of a clay
pit in New Haven, Conn.
Sort of Calendar.
The varves themselves constitute
a sort of calendar. From year to
year, DR.. McNish finds, there is con-
siderable variation in the orienta-
ton of the needles" indicating that |
the compass was acting just as er-
ratically 10000 years ago as today. |
1t is expected to build up calen-
dar of these variations. Then it |
will be possible to use this calendar |
to determine the times when other |
deposits which do not fit into the
established varve pattern were made. |
Glacial silt whose lodeston orienta-
ton was the same as that in some |
layer of an established varve Sc
quence could be assumed to be of |
about the same age The method |
piooooAy WILL NC dPp1lCdo1e, III. MC-
Nish said, to relatively short period |
time determinations, such as spans
of about 200 years. Over longer
periods there might be two or more
cycles of magnetic variation.
While both the fossil compasses
and the historic records show these
great variations in the orientation |
OF needles, DR.. McNish said, physi-
cists are not willing to accept the
explanation that the position of the
north magnetic nnle itself has wan