ARTICLE XI. 
Observations made in the years 1838, ’39, ’40, ’41, ’42, and ’43, to determine the Magnete- 
cal Dip and the Intensity of Magnetical Force, in several parts of the United States. 
By John Locke, M.D., Professor of Chemistry and Pharmacy in the Medical College 
of Ohio. Read April 19, 1844. 
POPULAR EXPLANATION OF THE ELEMENTS OF TERRESTRIAL MAGNETISM. 
In endeavouring to ascertain, by experiment, the elements of terrestrial magnetism, the 
operator proceeds, without reference to any theory, to determine them as the values of a 
simple force acting in a certain direction, regardless of what may be the nature or cause 
of that force. The nature of the research will be understood, by referring to the investi- 
gation of a similar force, namely, gravitation, which has likewise been examined as a 
simple force. The points of inquiry lie in the quantity of that force, and the direction in 
which it acts. By this investigation of gravitation, the following facts have been ascer- 
tained : 
1. That gravitation is not uniform over all parts of the earth, but is less towards the 
equator than towards the poles. 
2. ‘That it is less in high than in low situations. 
3. That it acts perpendicularly to the surface of the sea, and in general perpendicularly 
to the surface of the earth; but as the earth is not spherical, but spheroidal, and a section 
of it is an ellipse, gravitation does not tend towards the centre, except at the equator and 
at.the poles; the point of convergence for regions near the equator being nearer than the 
centre, and for regions near the poles, farther than the centre. 
4, That a mountain will, at a certain rate, in proportion to its mass and proximity, deflect 
the course of gravitation, and cause the lower end of a plumb line to lean towards it. 
5. That the moon, sun, and planets, have an influence on the direction and force of 
gravitation, that influence depending on the mass, direction, and distance of the said 
bodies. 
And, finally, there has been deduced the admirable Newtonian law: That gravitation 
as @ mutual force, acting, between all of the particles of matter in the universe, increasing 
directly as the quantity of matter, but diminishing, like other radiants, as the squares of 
VOL, IX.-—-75 
