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IX. On the means adopted in the British Colonial Magnetic Observatories for deter- 
mining the Absolute Values, Secular Change, and Annual Variation of the Mag- 
netic Force. By Lieut. -Colonel Edward Sabine, R.A., For. Sec. R.S. 
Received February 6, — Read April 25, 1850. 
So many of the magnetic observatories which professed to adopt and pursue the 
system of observation recommended by the Royal Society have confined themselves, 
apparently even in what they have attempted, to investigations into the diurnal 
fluctuations of the magnetic elements, and into what have been called magnetic 
disturbances, that it may not be inappropriate to recall to recollection the far more 
extensive system of observation which it was the purpose of the Royal Society to in- 
stitute. 
The diurnal variations and the magnetic disturbances form, it is true, a portion, 
and an important portion, of the objects contemplated ; but they can only be regarded 
as the effects of minor forces, superimposed upon the far more powerful and im- 
portant agency of the terrestrial magnetism itself, and from which they are pro- 
bably distinct both in their nature and in their origin. 
In the provision of instruments and in the elaborate instructions contained in the 
Report of the Royal Society for the determination of the absolute values and secular 
changes, as well as of the periodical variations of the three magnetic elements, it was 
obviously the purpose to comprehend as the objects of investigation the whole of the 
phenomena by which the magnetic state of our planet is either permanently or tem- 
porarily affected ; and particularly the permanent and systematic part of the pheno- 
mena which results from that more powerful agency to which the name of ‘^Ter- 
restrial Magnetism” more strictly belongs. 
The determination of the mean numerical values of the elements of ten-estrial 
magnetism in direction and force at different points of the earth’s surface, (the force 
being expressed in absolute measure, intelligible consequently to future generations 
however distant, and conveying to them a knowledge of the present magnetic state 
of the globe,) and the determination of the nature and amount of the secular changes 
whieh the elements are at present undergoing, are the first steps in that great induc- 
tive inquiry, by which it maybe hoped that the inhabitants of the globe may at some 
date, perhaps not very distant, obtain a complete knowledge of the laws of the phe- 
nomena of terrestrial magnetism, and possibly gain an insight into the physical 
causes of one of the most remarkable forces by which our planet is affected. 
It is true that the instruments proposed by the Royal Society, as well as the in- 
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MDCCCL. 
