The enquirer. [volume] (Richmond, Va.) 1804-1815, 1809-11-07
sJl@l9reua,,eou8.
for tbe enquirrr.
VARIA ITO Ms
OF TnH
NA G NE TIC WEEDLE
IN VIRGINIA..

%
state, to mark the magnetic variation on their
maps. but owing either to the want of proper in
struInents, Of skill in the use of such might
have been procured, It's judicious Injunction
has been SO irregularly observed, that it has in
duced more confusion than order.
OF the many methods which may serve to as.
certain the absolute variation, perhaps the follow
ing is the most simple, and the most easily redu-
eed to practice, by those who can take an alt-
tude of the sun
In two small holes, drilled exactly through the
middle OF the upright pieces, or sights of tire
common surveyors compass, let thread very
fine wire be fixed as to pass Just above the box
then when the instrument is properly levelled, &
directed to that part of the horizon immediately
snder the sun, the shadow ofthe thread will CO.
incide exactly with the meridian line in the box
Having taken, with quadrant or sextant, equal
attitudes OF the sun, before and after noon, by re-
Election from quicksilver, water or molssses note
his azmuth or bearing at each time by means of
the shadow of the thread half the difference of
those aximuths VIII be- the variation which.
when they are S. of Z. and W., and tho eastern a.
zimuth greatest, will be E but if the western
azimuth be greatest, the variation will be W.
When the bearings are n. ofE and W., the con-
rary. rule must be observed. These observati-
ons should be made when the sun is between two
and four hours distance from noon.
The western azmuth must be corrected on &C.
count OF the suns change ofdeciination between
the observations this correction may be- made
by the following rule, deduced from the tlifferen.
0ai snaiogies Of spherical triangles, Ode la
Lapde's Astronomy, article 5086, first edtion)
VIZ TO the logarithmic secant of the latitude
less radius, add The leg. cosecant less radius of
Che half interval OF time, between the obseryati-
ons turned into degrees, and the common log
Of the total change of declnaton in minutes
1.-The sum will be the common log. of namber
of minutes to be added to the western aIimuth,
from the summer to the winter solstice, but sub
II, - .ly- CT
lime may be SO adopted to the latitude of the
place, that the correction may be 1vays equal 10
twice the suns change of declination in the in
terval. In lat. $50 80 this time is 2h So tror,
aeon, and s later for every degree north of that
line, IN Virginia Or for those who prefer a We
ry near approximation, II may suffice 1t any place
IN Virginia, and within one month Of III equinox
to use lO' as the correction between one and
two months, z; and thence, until within few
days oil solstice, correction of 4, the suns
slow change Of declinatlon then rendering any
farther correction unnecessary
:. To satisfy myself how far this way of s,lbsti.
tuting The common compass for he azimuth com.
pass which IS used for finding tne variation oy e
goki attitudes, &c. at sea, might dc eligibl
saads The expo intent az Norjoke, WILLIAMSBURG,
ana hlch,nona; ..,id found the hne of no variati-
on TO run through, or near to The first of those
places, LIT So 81 long about7o' 90. In VIII
iamsLurg, Ii 37o Is. long. 760 81, found 2S'
variation K aud in Richmond, lat. 3630, long
about TO 81 way. 81 E Hence infer that tue
varistion is L, and increasing AS we advance west
from Norfolk possibly, about minutes tor 80
OF longitude. My compass was of the common
kind, without vernier, and divided only into
degrees.
II the sights are fixed truly perpendicular to
the arms OF the compass, the box furnished with
vernier and levels, and divided into half deg's
am satisfied that the variation might be found
III this manner, true to a minute, by taking the
IN nt fAx ntN- .l,<
expedient of concentric circles, and the shadow
OF bead fixed OF the string of pointed plum
met, VIII enable a very careful observer to take
the azlmuths, so as TO make near approach to
the truth but the circles must be pretty large
and the plane on which they are described level.
led with great care, otherwise the azimuths will
be mcorr, ct
lAmp'itudes can be used only at sea. To take
and to work azimuths by logarithms, requires
some knowledge Of mathematics, and where the
variation is only few minutes, a small error in
the observatory or in the work. will render the re-
sult dubious. The Pointers OF the Bear can be re-
sorted to conveniently, only at particular seasons,
and the method is, in other respects, objectiona-
bic. transit telescope well adjusted, is neces-
sary to etermine the instant when the suns cen
tire passes the meridian, his motion is then ior
some time, apparently parallel to the horizon -
From all these considerations, am induced to
prefer the very simple continuance which have
suggested above, and which has also this advan-
taBe, that 1T neither requires expensive apparatus
nor complex calculation.
The correct determination Of the variation
and the construction of variation chart for this
state, and indeed for the United States is of
more importance than app cars at first view since
Ii is highly probable, that for two or three CEN
turies to come, the circumferentor is the only in
strument which can be used by surveyors in this
country 1t the variation were ence truly ia.
Ken, and marked on stone at certain noted pia-
ces, suppose at every courthouse, as has been
done near the Federal City, the relative variation
might be easily known at any future period. Nor
IS it quite improbable that due attention to this
matter might tend to throw some light upon
subject which has eluded the sagacity ofthe most
acute philosophers An Industrious individual
could, during one summer, find the variation at
every important place in the state, and take the
ATTITUDES, at the same time, without any additi.
onal trouble or delay, for less sum than has of.
Ten been expended On law salt originating in the
mistake of surveyor.
Questions have arisen respecting the manner
Of allowing the relative variation so as to retrace
an old line.
RULE-lf the variation has gone east ward
add tine change Of variation TO the old course, il
Sc or NV, but subtract it, if w or NE When
the needle has moved Westward, inyert the
Rule.
These TRIES are not intended To- claim the at.
tention of mathematicisns, to all of whom the
application of equal attitudes familiar, but they
may be of some use to surveyors, whose oppor.
tuiiites for scientific attainments are too few &
TO their employers, II they serve TO prevent or to
correct errors, and consequent disputes or If
they should induce the suggestion VI any method
more accurate. and as easV IN ia.actice