Observations on the Reciprocal Influence which Magnetic Needles Exercise over Each other, When Placed at a Given Distance within their Respective Spheres of Action, at Different Positions on the Earth; with Tables of Numerical Results Obtained at Separate Stations. Also a Method of Discovering Where Certain Local Influences are Acting on the Needle, from which may Be Obtained a Proportional Correction to Be Applied to Magnetic Observations in General.