300 TERRESTRIAL MAGNETISM.—MURPHY. 
iron is at a dark red heat ; however different, therefore, the modifi- 
cations may be which are excited in substances in their molecular 
state, and in the coercive force depending upon that condition in 
experiments of this nature, there will still remain a considerable 
thickness of the terrestrial stratum which might be assumed to 
be the seat of magnetic currents. The old explanation of the 
horary variations of declination by the progressive warming of 
the earth in the apparent revolution of the sun from East to 
West must be limited to the uppermost surface, since thermome- 
ters sunk into the earth, which are now being accurately 
observed at so many different places, show how slowly the solar 
heat penetrates, even to the inconsiderable depth of a few feet. 
Moreover, the thermic condition of the surface of water by 
which two-thirds of our planet is covered is not favorable to 
such modes of explanation, when we have reference to immedi- 
ate action and not to an effect of induction in the aerial and 
aqueous investment of our terrestrial globe. 
“In the present condition of our knowledge (he wrote in 1844) 
it is impossible to afford a satisfactory reply to all questions 
regarding the ultimate physical causes of these phenomena. Tt 
is only with reference to that which presents itself in triple 
manifestations of the terrestrial force, as a measurable relation 
of space and time, and as a stable element in the midst of change 
that science has recently made such brilliant advances by the aid 
of the determination of mean numerical values. From Toronto 
in Upper Canada to the Cape of Good Hope and Van Diemen’s 
Land, from Paris to Pekin, the earth has been covered since 1828 
with magnetic observatories, in which the regular or irregular 
manifestations of the terrestrial force are detected by uninter- 
rupted and simultaneous observations * * * * * * *%” 
and he proceeds to say: “ Terrestrial magnetism and elects 
dynamic forces computed by Ampére stand in simultaneous and 
intimate connection with the terrestrial or polar light, as well as 
with the internal and external heat of our planet whose mag- 
netic poles may be considered as the poles of cold; the bold con- 
jecture, hazarded one hundred and twenty years ago by Halley, 
