—— 
AND THE INTENSITY OF MAGNETICAL FORCE. 315 
Plate 46.—On this plate are delineated, by the usual means of ordinates, the curves of 
the dip and of the corresponding total intensity. ‘The delineation has been made upon 
paper divided into square millimeters; this unit being selected as the least which is 
easily distinguished. It will be seen, by inspection, that the horizontal distances repre- 
sent the progress in latitude or longitude, a millimeter to a mile, and that the ordinates, 
or perpendicular distances, represent the dip, half a millimeter to a minute. ‘The total 
intensity is represented in still another line, Cincinnati being 1000, and a millimeter 
being a unit or a thousandth part of the intensity at Cincinnati. As neither the zero of 
dip nor of intensity come into the scale, the one will be learned at the margin to the right 
and left; and, the other, by the actual numerical value placed at some one or more of the 
points along the line. 
The most striking circumstance developed by these lines is, that they have a contour 
apparently determined by the geology of the region over which the line of observation 
passes. Thus, the line of dip over the horizontally stratified aqueous rocks of the west, 
through Kentucky and Ohio, and along the Mississippi, are mostly in a gradual ascent, 
without inversions, while the same line from Baltimore to New York, over igneous 
rocks, has remarkable fits of ascent and descent, like the contour of primitive or igneous 
mountains. The line along Lake Superior is still more remarkable in its undulations, 
and here too are the strongest evidences of igneous action. ‘The observations require to 
be extended further before a general theory be deduced; but, so far, they go to show that 
the general nature of subsequent rocks may be ascertained by the magnetic elements; 
the instruments serving the purpose of the popular notion of a “divining rod.” Another 
fact conspicuously shown by these lines is, that where the dip takes a sudden and anoma- 
lous increase, the total intensity shows a correspondent decrease; and the reverse; as at 
Bristol, Trenton, and New York; slightly at Lexington, Kentucky, and excessively at 
Porter’s Island. The apparent exception is at Isle Royale, where both curves ascend, 
but this is only apparent, as Isle Royale is quite out of a line between the other places, 
being further to the north. 
Read September 20, 1844. 
Medical College of Ohio, Cincinnati, August 18, 1844. 
To tus AMERICAN PuitosopnicaL Society :— 
Since I had the honour of reading before you my paper on Terrestrial Magnetism, 
I have completed another series of observations, the result of which is herewith com- 
municated to you, in the fascicle for 1844. The paper is hardly a “readable” one, 
being mostly a table of results expressed chiefly in numbers. Some remarks, however, 
on local attractions, and the connexion of magnetism with geology and topography, 
illustrated by a drawing, will, I hope, be found interesting. At any rate, I venture to 
VOL. IX.—83 
