Worcester daily spy. [volume] (Worcester [Mass.]) 1850-1888, 1854-05-12

I0. r
T.,h;ch 1vas probahly near the Longitude OF
pblade,nbiao. about the year 1900, and then
49,, - rat,,r,, westward. AT the same time,
,,r;versoI change in the variation seems to
- I'm Mace throughout the U. States.-
- y,e tn the present, the line OF no
-.:a+;a-, has been moving 'oest'card, and the
as,sv,x variation fnceasdag, and the easterly
decreasiao. as above stated. :. -,
These facts. evidently Indicate an oschiaA
torv motion of the line of no yariation-but,
in What time this oscillation is described, or
within what limits it is confined, observations
have not been continued suihclent length OF
time in this country to determine.
observations were made, however, in Lon
,l.,, much earlier than in the United States.
According to DIR. Thompson, record of OBJ
ssrvatious were commenced as far back as
I5so. when the variation was IL deg. 10 min.
East-in 1922, deg. East and in 1867, the
needle pointed due North that is, the line of
no variation passed over the City Of London
in that wear. After that time the needle had
a westerly variation which continued to in
crease till 1914, during period Of l57 years.
having attained westerly dechnation of "a
deo. ii min. Since which time it has been
GRADUALLY moving eastward and if it takes as
iong to return, it will reach the point of no
variation in 1871. Should it proceed as far
eastward as it did westward, and require as
long time, it would reach its utmost eastern
limits in the year 2l2S, having described an
arc of As 10 degrees in period of 314 years.
If the line of no variation in the U. States
oscillates in the same time and describes the
same arc, the westerly variation in New Eng
land viii continue to increase till about the
wear 2ll4, when the needle will have attained
a western variation of more than 45 deg, or
viii point nearer west than north. Since it is
at those places only which are within the lin
its Of. the arc of oscillation, and over which
the line Of no variation passes, where the
needle can ever have no variation, or ever
change from an easterly to westerly varia-
ton, and rice versa and since the greatest
eastern limits of this line is not far from the
meridian Of Washington or Philadelphia, it
follows that the needle will aeper point North,
or hare an eastsrly pariation anyscsere in Neso
England. Unless, indeed, the arc of oscilla-
ton itself should have progressive motion,
either East or West-and should this even DO
proved in future ages to be fact, it would re-
quire at least the lapse Of many centuries be-
fore the line of no variation would reach the
Eastern States. The amount of variation in
New England, is different in various places,
but is generally greater as the location is more
easterly, or farther from the line of no yaria-
ton. The rate of the increase Of variation
also differs in different places.
According to Prof Olmsted, the variation at
New Haven in 1850, was deg. 10 min. W.,
with an average annual increase of 46 min,
which would make the variation now about
deg. west.
From recent observations, the variation in
this city, is now deg. west, with probable
increase Of about 10 min. per annum, or at
the rate Of one degree in every six years.
The direction of the polar star being taken
at its greatest eastern elongation, the needle
was found to vary from this i deg. west-de-
Quoting the Azinluth, which in this latitude
is very nearly 2 degrees, gives 9 deg. for the
true variation. This is believed to be close
approximation to the truth, and sufficiently
exact for the practical purposes of the sur
veyor. In running lines and surveying with
the compass, the general variation does not
affect the accuracy Of the ''Field Notes, or
the contents of the survey-but it is evident
from what has been stated, that lines run out
by the compass bearings, could not be re-run
several years after without knowing the vari-
ton Of the needle at the time of the survey,
especially where the bounds are uncertain, as
Is frequently the case. and the given bearings
are the surveyors chief reliance. Hence,
ton Of his compass is at the time and place of
the survey and the compass bearings should
be reduced to the true behring's, and SO ex-
pressed in the field notes, or distinct record
of the variation should be made to serve as
guide in subsequent surveys. Were this pre-
caution generally taken by surveyors, it would
save them much trouble aad perplexity in
tracing old lines and fixing bounds, and would
undoubtedly prevent many disputes and un-
happy ltigations between parties concerned.
Aud it is evident from the various changes to
which the needle is subject at different times
and places, that the only reliable means the
surveyor has for knowing the variation Of his
compass, is frequent observation.
The most simple and expeditions method Of
determining the variation, is by tracing me-
ridian line, from observations OH the polar
star. The manner Of doing this is described
in most Of the standard works on surveying
When meridian is once established, it may
be permanently marked by placing appropriate
bounds at the distance of 10 or 30 rods apart,
and thus remain any length of time for fu-
ture observation. The surveyor has only to
place his compass on one bound and direct
the sights to the other bound, and he has the
variation.
We're surveyors, generally, as they are scat
tered over the country, to establish meridians
at their respective localities, where they could
make frequent and careful observations on the
variation of the needle, it would tend to theil
special advantage, and contribute to the gen
eral cause Of science. ]. B.
Worcester Medical College, April, 1854.
How rrD Tpreu~.r Stvpn uva nnc 49,