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Published online by Cambridge University Press: 16 March 2015
The chief disturbing causes by which the magnetic intensity, as ascertained by the oscillations of the horizontal needle, are affected, are 1. Variations in the air in which they are performed; 2. The influence of changes in the mechanical conditions incidental to the mode of suspending the needle; 3. Changes in the disposition and intensity of the magnetism of the needle from heat and other causes.