— 
C 
1871.] 69 [Chase. 
An observer, therefore, near the centre of the land hemisphere, would 
find, at the four cardinal points of his true horizon, magnetic, thermal 
and geographic positions of peculiar importance, and indicative of inter- 
esting mutual relations. The recognition of such relations gives a new 
interest to the often noticed resemblance between the isoclinal and iso- 
thermal lines, the analogy which I have myself pointed out between the iso- 
gonic and cotidal lines, the parallelism of the boundary lines and of the 
axis of the westerly isogonic belt with the boundaries of the correspond- 
ing annual isabnormal belt, the isogonic curvatures in or about regions 
of isabnormal heat or cold, ‘se different angular relations of the isogonic 
lines to the customary paths of hurricanes and storms, and the approxi- 
mate perpendicularity of direction and ~— of curvature between 
the westerly wind belt and the isogons. - All of these features, which 
may be satisfactorily explained by the general indie on which storm 
laws are based, furnish cumulative, if not irresistible, evidence of the de- 
pendence of magnetic currents upon the same laws of gravitation, which 
tend to restore the equilibrium of air and sea, after tid al or thermal dis- 
turbances. The evidenceis sustained not only in the general distribution 
of the magnetic lines, but also in their particular details, the course of 
the isogonic lines, at every point, being an evident resultant of the com- 
bined equilibrating tendencies between land and water, and between 
centres of normal and isabnormal heat and cold. 
The ocean currents corroborate the gravitation theory of magnet- 
ism, perhaps even more strongly than the wind belt. A physical atlas 
like Petermann’s, which marks the most rapid portions of the several cur- 
rents with the deepest tints, shows their relation to the magnetic and 
coast lines very satisfactorily. A comparison of the more minute details 
exhibits additional interesting evidence that the original impulse of all 
terrestial currents, atmospheric, magnetic and oceanic, is given by lumin- 
ous, thermal or tidal disturbances, that the currents are maintained by 
gravity in its continual tendencies to restore the continually dis- 
turbed equilibrium, that the magnetic currents are least, while the ocean 
currents are most interrupted and modified by land contours, that each 
of the more sluggish currents exerts a secondary modifying influence on the 
more rapid, that extraordinary variations in thermal or luminous undu- 
lations, whether originating at the sun or at the earth, produce ‘‘mag- 
netie storms,’’ and that, whatever theory may be adopted as to the mode 
in which the solar undulations are transmitted, there is no philosophical 
necessity for the hypothesis of any cosmical origin or disturbance of ter- 
restrial magnetism other than variations in the amount of light and heat 
received and in the directions of gravitating tidal and equilibrating 
lines. * 
* It is so difficult to make the necessary allowances for the distortions of the ordinary magnetic 
the it | would recommend any one, who may desire to make the comparisons which | have 
g , to trace the lines on aglobe. A slate globe is especially satisfactory. The data for my 
own comparisons were taken from the lines of equal magnetic tion and of equal horizon 
force for 186 24 edition of the ** Admir by Manual for ascertaining and applying the de 
tions of a compass, caused by the iron in a ship.” Walker’s ‘* Terrestial and Cozmi¢al Magnetis 
Cottin’s “ Winds of the Northern Hemisphe¢ Dove’s “Isothermal and Isabnormal chart 
“ Petermann’s and Johnston’s Physical Atlase In order to judge of the resultant influences of 
the normal and isabnormal thermal disturbances, it will be well to mark the « es of isabnormal 
heat and cold,as well as the points of greatest average heat and cold. 
