I. TERKESTRIAL MAGNETISM. 19 



more or less vertical. Any limited portion of the earth resolving itself thus into a 

 local magnet, may have its opposite poles accessible ; while a corresponding column 

 of the earth, having only its normal portion of terrestrial magnetism, will present, 

 so far as our examination of heights and depths extends, only one pole ; and that, 

 scarcely affected by the small distance by which we are able to approach towards 

 the neutral centre, may be considered the pole of a magnet of infinite length. 

 Now, if we consider a trappean hill to be itself a magnet, presenting one pole at 

 the base and the other at the summit, it follows that the indicated magnetism at 

 the summit will be the normal teri-estrial magnetism of that place plus the local 

 polarity of the magnetical column of the trappean hill ; while the indicated mag- 

 netism at the base will be the same normal terrestrial magnetism minus the local 

 polarity of the same magnetical column. 



It is not, however, the intention of this paper to speculate, or advance and advo- 

 cate theories; and the previous remarks may be regarded rather as inviting the 

 attention of the scientific reader to the very curious facts which I have ascertained 

 at Snake Hill, the Palisades, Weasel Mountain, near Patterson, Smith's Quarry, 

 and the Brandywine, near Wilmington, &c., than as with any desire to advocate 

 pertinaciously a tlieory to account for the phenomena. (See " Trans. Am. Phil. 

 See," 1846, p. 322.) 



Pole of Greatest Intensity of Magnetic Force. — That the pole of greatest magnetic 

 force is far distant from the pole of convergence, or of dip, was first suggested by 

 Col. Sabine, and has now been fully confirmed by him. To determine precisely 

 the point of that pole is a far more difficult problem than to determine the pole of 

 convergence. This difficulty arises from the complication of the force of local 

 attractions, with what may be considered the force of the normal magnetism of the 

 earth, and the impossibility, so far, of separating, by any reliable standard, the one 

 from the other. As a figurative illustration of this subject, we may suppose the 

 force of terrestrial magnetism to be represented by a prolate spheroid, and that the 

 distance from the centre to any point of the surface is proportionate to the intensity. 

 By means of local attractions, the surface of this spheroid would be rendered uneven 

 and hilly, having table lands and low swamps, sharp pinnacles and deep pits, as it 

 is with the surface of the earth. Now the magnetical surveyor comes upon this 

 surface as if he descended from a balloon in the night, and ascertained the altitude 

 of his point by a barometer, without any means of knowing the height of that point 

 in reference to any standard, as " the level of the sea," and without knowing whether 

 he has alighted upon a mountain or in a valley. His measures are, therefore, merely 

 comparative amongst themselves. From a great many observations at different 

 points, we may approximate to the law of curvature which shall represent the normal 

 magnetism of the earth, if it be first assumed that this normal magnetism be repre- 

 sented by any regular curve. By means of this kind of calculation. Col. Sabine 

 has inferred that the point or pole of maximum intensity of magnetic force is at 

 latitude 52° 19' N., and longitude 91° 59' W., although no actual observations have 

 been made within about three degrees of that point. It is not my purpose to attempt 

 to show that Col. Sabine is in error in his calculation ; on the other hand, I believe 

 he is right, so far as observations have been made on which to found the calculations 

 made. Nor have I made it at all a study to compare the general curvature which 



