C 34° 3 
XIII. Observations of the diurnal Variation of the Magnetic 
Needle at Fort Marlborough, in the Island of Sumatra. By 
John Macdonald, Esq . In a Letter to John Crisp, Esq. 
F. R. S. 
Read April 21, 17 g6. 
DEAR SIR, Fort Marlborough, 15th March, 1795. 
I have now the satisfaction to transmit to you the observa- 
tions of the diurnal variation of the variation of the magnetic 
needle, with which you wished to be furnished. A short ac- 
count of the mode used in laying off the meridian is prefixed. 
A small building, devoid of iron, was erected at some distance 
from my house. This building contained the meridian. By re- 
peated observations, I found the sphere of mutual attraction 
of the needle and iron to be very small ; a common-sized key 
presented to it, did not affect it at a nearer distance than 5 
inches. To exclude every influence of iron, I laid aside such 
as might be about me, previous to the taking of the observa- 
tions, and took care to remove the padlock and key, at the 
same time, to a certain distance from the constructure. The 
hasp and staple made use of for the padlock were of wood ; 
and for the nocturnal observations, I had wooden candlesticks 
made. I may venture to say, that the observations were taken 
with the utmost care ; and that every requisite precaution was 
used to exclude errors that might have arisen from a want of 
minute attention. 
