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lished by MM. Gauss and Weber, in the ‘Atlas des Erdmagnetismus ’ (Leipsic, 1840);and 2°, in the Tables and Maps of the present paper. For the values of the magneticForce, which in the Atlas of MM. Gauss and Weber are expressed in the ArbitraryScale, of which the fundamental value is T372, or (as written by M. Gauss) 1372=theForce in London in 1836,1 have substituted the Absolute Values, corresponding to 10'28as the Absolute Force in London at the same Epoch, in the scale which was originallyadopted in conformity with the Report of the Committee of Physics of the Royal Society,1840, page 21 *. In all the three Elements there are some blanks in the columns derivedfrom the data in the present paper, owing to observations being either wanting or insuffi-cient in those localities. Some of these blanks, viz. those in the vicinity of the Pole ofthe Earth , it will, probably, never be possible to fill up; but many of those in Lats. 40°and 45° may probably be supplied, when the evidence which this paper affords is sup-plemented by results South of 40° of N. Latitude, which are now in hand.
* The Section of the Report in which the Scale is premised, in which the values of the magnetic Porceshould thenceforward -be expressed, generally known as the “ Scale of British Units,” was from the pen of itsChairman, the late Sir John IIep.schet,, Bart.; the Scale is thus defined by him :—“ The number thus obtained,for the Porce of the Earth ’s Magnetism, expresses the Ratio which that Porce hears to the Unit of Force; theUnit of Porce being that which acting on the Unit of Mass, through the Unit of Time, generates in it the unitof Velocity. For the unit of Mass we take, a grain; for the unit of Time , a second ; and if a Foot he taken asthe unit of Space, the unit of Velocity will he that of one foot per second.”