DECLINATION. 
the Proce ae io of Mr. Canton. 
viz. from ember 1786, to December 1787, 
During 16 months, 
Mr. Gil- 
ey 
tered in 16 pages of the Philofophical TranfaGions for the 
year 1806, to which we muft refer fuch of our aera as 
are defirous of examining the particulars minutely ; we 
fhall fubjoin his table of the mean monthly variation dif. 
ferent times of the day, which has been deduced from the 
above-mentioned 16 pages 
Mean Monthly Variation of the Magnetie Needle. 
1786. 6A.M.]| 7 A.M,! 8 A.M./10 A.M.]12 A. M. Pp. M.| 2P.M.}4?.M.] 6P.M./ 8 P.M, fio P.M. j11 P.M 
September [23° 0'.0]23° 9/.9)23° 10'.1/23°1 4.5/2 3°22/.2/2 3° 23'.7/23° 23'.9)23°19'.0/23 1863 /23°1 3/.5123°12'.4|23°O'.0 
O&ober 10.4 {1.3 15.2 244 26.1 2641 21.1 174 15.6 14. 13.8 
ovember 12,2 12e5 i523 21. 22.5 22.0 20.3 17,0 15-9 i5.t|. 14.7 
December 14.5 16.1 20.6 22.0 2.2 20.0 17.4 15.8 15.0} 15.0 
1787. 
.|January T4.0 14.2 17.1 22.3 24.1 24.5 21.8 18.4 15.6 14-5) 14.8 
February 14.2 1g. a 23.8 24.8 25.1 23.64 13.8 15.3 15.8; 12.8 
March 12.8 12.8 15-3 26.5 274 27 0 18.4 19.0 15.9 15) 157 
April 9-4 9.9 O:7 13.9 23.6 27.0 2704 22.0 17.8 15.7 15-7, 15.6 
May 7.6 965 Tad 1365 25.2 2.6.6 26.2 21-0 17.4 I7st 16.8} 17.0 
June 8.4 8.2 8.8 16.0 26.6 28.1 28.1 22.6 15.4 17-9 17-8, X77 
July 9.5 9.6 10.3 17.8 27.6 2903 29.4 232 19.4 18.9 19.3] IQ. 
Auguft 11.9 12.0 12.8 19.7 30.3 Bie at. 25.6 19.3 18.7 i8.g} 18.8 
September 15.0 15.1 15.3 20.2 29.8 20.7 30.5 2404 20.1 19.1 IQ.2} 109.2 
ttober 17.5 17.2 214 30.8 31.9 31,6 2764 21.9 20.8 20.2} 19.6 
November 19-4 19.7 20.6 29./ i.1 30.2 27.4 B29 21a 21.4\. 2164 
cem 20.4 24.0 21.8 28.2 29 0 29.0 26.2 22.9 21.9 21.6 
om the whole of the above ftated obfervations it ap- 
ete, that the magnetic needle generally is flationary at 
abo 
n its variation is greate 
monthly diurnal variation of the declination was found, 
March 1787, to be 15’; in June 19! 65 in July 19’.6 ; in 
September 14.8; and in December 7’.6. But ona mean 
of twelve years obfervations, from the year 1793 to 1805, 
Mr. Gilpin as sa the diural ogee of variation in 
o’.63; in Sep- 
mean of the 
nee ies the ie 
faid equinox to the winter "fo Iflice. 
Other obfervers in other parts of the world have remark- 
ed magnetic periods fomewhat different from the above. 
Mr. Cotte’s obfervations, on the diurnal variation, thew that 
comes ftationary fcur times 
hae 
r 
from the meridian, then approaches it 
in June, retires in July, approaches it till O&tober, and re- 
tires from it in November and December ort Marl- 
borough in the ifland of Sumatra, during the year 1794; 
Mr. Macdonald made feveral accurate obiervations on the 
eae variation of the declination ; from which it appears 
the ees at ak t place (where it was little more 
fed from about 7 in the morning, till 5 
the oo and that it decreafed till 7 in the m morning. 
Mr. Macdonald likewife obferved that this: diurnal variation 
of the magnetical declination in time of thunder is greater 
than ufual. Phil. Tranf. for i 
The fame gentleman made fimilar obfervations in the 
ae of St. Helena in the ie ae the years 1795 and 
6. The declination, which a place was 
the morning, 
when : id Sa ae meve and t : sie variation aay till 
it amounts t v t 8 o’clec imini 
ing aaecaaids, “till it P hecome eerie Phil. Tranf, for 
lal 
si 
ifland of Jamaica Mr. Robertfon antl (Phil. 
ak for 7806) that the declination is contft 
aft. But he is led to make this affertion ieee having ob- 
oe oe plans o of eftates, made foon afte the year 1660; 
mo “ 
a have at la a turned to hea ee Befides, it 
muit be remarked, ae farveying inftruraents, as they were 
rade about 140 years ago, were not capable of that Meares 
of accuracy, which is neceary to fhew ae true declination 
of the magnetic needle. 
Having thus far endeavoured to give our readers a fu%- 
cient account of the declination and its variation in the fame 
place, we may now proceed to ttate the moft important 
particulars relative to the declination at teen places 3 . 
which is of the utmoft confequence to navigators. 
On account of the uncertain movements of the magnetic 
needle in different places on the furface of the globe; the 
fafeft expedient which navigators can, and do, adopt, is to 
afcertain the aGual declination at re particular place, where 
it may be deemed neceffary, by ufing azimuth compafs, 
a the manner fhewn under the article Compass. od 
, indeed, a great many eee are continually made 
at ree which might be of ufe to future pmanenOe at no 
