V C 397 3 
XVIII. Observations of the diurnal Variation of the Magnetic 
Needle , in the Island of St. Helena ; with a Continuation of 
the Observations at Fort Marlborough, in the Island of Su- 
matra. By John Macdonald, Esq. In a Letter to the Right 
Hon. Sir Joseph Banks, Bart. K. B. P. R. S. 
m 
Read May 24, 1798. 
SIR,, Edinburgh, September 30, 1 797: 
O n my arrival in England, I had the honour of observing to 
yon, that I had taken some observations of the diurnal varia- 
tion of the magnetic needle, in the island of St. Helena. I am 
to apologize to you for having, till this period, omitted furnish- 
ing you with these, and with a continuation of those formerly 
taken in the island of Sumatra. The meridian was laid off' by 
means of an apparatus brought from Bencoolen ; and the re- 
quisite allowance made for the alteration of the sun's declina- 
tion during the operation. The meridian-plate remains firmly 
set in a pillar of teak- wood, well fixed, for the use of naviga- 
tors ; who, by applying a compass-card to it, will find the va- 
riation more readily, and correctly, than by amplitude or azi- 
muth. A short residence at St. Helena, arising from the sud- 
den departure of the fleet to which the ship I was in belonged, 
has prevented the observations from being as numerous as I 
eould wish. Their agreement, however, indicates that fifty- 
eight observations are sufficient for affording such conclusions 
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