The enquirer. [volume] (Richmond, Va.) 1804-1815, 1810-10-23
TO THE EDITOR OF THE ENQUIRER. -.
BY inserting the following, you will much
oblige,
Yours, with much respect,
GEO: BLACKBURN.
Variation cfshe Magnetic Needle, at
CAPE HENRY.


The various opinions which prevail respecting
the real and the relative quantity the Magnetic
Variation, and the many disputes which originate
IN mistakes on This subject, should induce those
who are interested in the cause Of science TO
seize every opportunity Of making such obscr.
vatons as may serve to ascertain a matter so very
important to Surveyors and Navigators, & which
is now deemed of sufficient moment TO engage
the attertion of the Legislature.
To determine the variation at Cape Henry,
which IS by some Mariners supposed half point
west lately visited that place, accompanied
by Mr., KIDD, Surveyor Of Norfolk, and gen.
ueman of The vicinity; the method which used
IS that OF equal attitudes, which, for its great
simphcity and superior accuracy, ought probably
TO be preferred to any other. We observed with
an Azimuth Compass, and two Surveying Com
passes converted into a3muth compasses, BY
means OF a thread stretched between the sights
directly over the meridian line in the box /
give the observations in full in order that those
who wish to repeat such experiments, or to test
their accuracy, may have correct idea of the
method.
Gafe henry Light House. lat SD 55m. 80,
Saturday, Sept. 2eth, 1910.
The forenoon observations were. at 81 543m
Ner. Ait of the Suu's lower limb Sid I'm
aHd the bearings of his centre BY the several
comPasses were Gid som SOD ION and God
ON. the mean Is 60c. 2Om. s. w At Oh o.
lin. Alt $50 SO l2m bearings SD Gu SD
ON Sid Som The mean IS sgd ION At On 81
3m. Alt Aid 2m bearings AID Som 44d som and
Add ON mean 44d 20m
ihe corresponding observations in the evening
were, Alt Aid 2m. bearings AID Ism 42d ON (
42d Som tue mean is 42d SIN. Alt SD so1m.
bearing 49d SON sod SIN 49d 35m; mean 4ed
Son AN. Va 1Im bearing SID. 45m. SD. 2Om
q 3Td. som 4 SD 49m. Half the difference of the
eastern ana western azimuths is the variation.
And is L. because the eastern aSimuth il great,
est; hence the variation from each of the three
sets respectively, is Id. Am. Id 10 and gam !:
Out these results must be corrected on account
OF the alteration in the Suns declnation between
si,e equa, attitudes, which correction is, in this
KAT. nearly equal to tine whole change of declina
tion III the interval of time in this case. slm
.Sm and 3m which corrections are subtractive
from the summer 10 the winter solstice hence
the above variations corrected, are Id. ION Id
On: and Sym the mean of which is Id SIN r
%nicn conclude to be the true variation now at
the Cape:-Amplitudes were also taken. which
Enough they do not in general give very aeon.
rate results, did not differ materially from the
above. ' 'ie
"A mistake OF half point in the variation 10
DO great an error, that it certainly ought to be
noiced, as II Will take a ship ten or twelve
miles out of her true course in run of about one
hundred 'u' o"e
surveyors either omit to mark the variation Of
eheir maps, or, what is still worse mark il erro.
neouaiy, and thus lay the basis of numerous Iii.
$atons, particularly in towns where fen feet
OF land are often worth as many hundred collars
from the above and former paper of mine
{ See The Enquirer of Nov 7th, 1909 } it is eyi.
deht that, with COCOA Instruments and careful oh
servat'on, the variation Mght be accurately as
certhined, and. " marked on stone At certain
noted places, is rate of change and other phsno
mena would AT length become known
have already stated, that neither amplit,ides
asmuths, the pointers Of the Bear, nor the SUM-
M rdian altitude are to be relied on Equal al
titudes, end the Suns bearing, when on the
meridian, the instant beng noted b. a well repu
lated watch or transit teliescope, are the
only accurate methods with which IN acouain-
Red at present; the mean results of several com
passes ought TO be taken, until we can find mar
netic needles so well constructed as perfectly 10
agree many of those, now in use, differ a whole
degree, and almost all of them one from IS TO 40
minutes
Itook the same day the Mer Alt of the Suns
iower limb, 500 22m 15s whence the lat of the
light is Sod 55m 32s and, allowing the shin
channel to be Am 2Ss or about miles more nor.
tneriy, which 1t is nearly, the lat. of the ship
Channel is SD OmN.
GEORGE BLACKBURN,
Professor Of Mathematics in the ColLeer OF
William and Mary.
CT Most of the above Communication has
appeared in the Norfolk Hersld-The 9Th & 9TH
Paragraphs are entirely new
- Printers who choose to give this place
IN their papers may oblige some of their rea-
deis
See american Coast PILOT