Prof. W. A. Norton on Terrestrial Magnetism. 5 
direction P’n’. This amounts to saying that the magnetic force 
of A in its action upon the north end of the needle is directed 
tangentially in the circle from right to left, as shown by the 
arrow, and in its action upon the south end of the needle is 
directed from left to right. 
Upon the undulatory theory of magnetism these differences of 
action are attributable to ethereal waves whose transversal forces 
of vibration lie in opposite directions, and to certain differences 
in the magnetic states of the two ends of the needle. 
3. The intensity of the magnetic force of a particle of the 
earth, at a given distance, is assumed to be approximately propor- 
tional to its temperature, or amount of sensible heat. This as- 
sumption was made under the idea that the sun was the source, at 
the same time of waves of heat, light and magnetism, and that 
the molecular forces of vibration due to the different kinds of 
he points where the annual mean magnetic intensity of the par- 
ticles near the surface is the same, will, according to the present 
View, coincide with the isogeothermal lines, and very nearly there- 
Let 
ore with the isothermal lines. Fig. 2. 
then, AB, CD, EF, fig. 2, represent 
portions of three isogeothermal lines, ; 2 a 
regarded as parallel to each other, * : 
and GPH an are of a great circle ~ we 
g 
2 
5 
09 
o 
a 
6 
re 
3 
: 
E 
td 
o 
D 
If we take four points m, n, 7, s, sim- “ft j ; 
‘ 
the north end of a magnetic needle Hae ee 
will be perpendicular to mP and directed obliquely downward. 
he magnetic : 
cles r, s, will be respectively perpendicular to 7P and sP and di- 
tected obliquely upward. Now it is evident that while one effect 
of the action of m will be to urge the north end of the needle to- 
ward ©, the particle » will have an equal tendency to urge it to- 
ward D. In like manner, the components of the forces of r and 
8, which solicit the north end of the needle in the directions PC 
