TRANSACTIONS OF THE SECTIONS. 95 
variation the dip is at a minimum about 8 a.m., at a maximum about 11 a.m., after 
which it decreases to a minimum again about 2P.m. Turning now to the intensity, 
the maximum is found to occur about 8 a.M., and the minimum about 11 a.m., after 
which it again increases, reaching a maximum in the afternoon. From these facts, 
then, it would appear that, while the earth exerts a greater attracting power over the 
needle about 11 a.m. than either before that hour or after it, the intensity of the 
force by which this is accomplished is then at its minimum. In other words, we are 
driven to the conclusion that the earth exerts a greater attracting power by a mini- 
mum of force than by a maximum,—a conclusion entirely at variance with all our 
knowledge of the magnetic force. This anomalous result the author traced to the 
assumption lying at the foundation of the present theory of the intensity, viz. that 
the terrestrial force is exerted in the direction of the dip; and from an analysis of 
the phenomena of the dip he arrived at the following laws :— 
1. That the direction in which the earth’s force is exerted, is, at all points upon its 
surface, in the radial line of its centre. 
2. That the vibrations of a horizontal needle being therefore at all points made at 
right angles to the direction of the force, their number at any two or more stations 
in similar times, or at different periods in similar times, indicates exactly the ratio of 
the force at each station and at each period. 
On the Development of a Physical Theory of Terrestrial Magnetism, an out- 
line of which was submitted to the Dublin Meeting. By J. DrummMonp. 
The fundamental principle of this theory was the following :—Assuming the 
prevailing idea regarding the early condition and present state of the globe, viz. 
that it has cooled down from a state of fluidity, and now consists of a solid crust 
enclosing a molten nucleus, the author assumed also that the sun, moon, and 
other planetary bodies must exert the same influence upon the enclosed fluid which 
they exert upon the surface ocean in producing the tides; that, consequently, a 
system of internal tides must be occasioned simultaneously with the external 
tides. Further, accepting the theory of Gauss, that the entire matter of the globe 
is magnetic, he concluded also that the passage of these internal waves must 
occasion corresponding changes in the position of the needle; and reasoning from 
these premises, he arrived at the following conclusions, in regard to the changes 
in position which the needle ought to undergo. A declination needle at any 
station resting on the line of the magnetic meridian ought, upon one of the 
internal waves coming from the eastward, to make an excursion to meet it; as the 
crest of the wave approaches the station of observation, the needle ought to return 
with it; and when it comes immediately beneath the point of observation, the needle 
ought to coincide again with the meridian. As the wave proceeds westward, the 
needle ought to follow it, making a westerly excursion equal to the easterly ; and as 
the wave passes further west, and its influence over the needle thereby declines, the 
latter ought slowly to return again to the meridian. Again, an inclination needle 
ought to begin slowly to dip as the crest of the wave approaches the station of 
observation, reaching its maximum when the wave is immediately beneath it, and 
slowly rising again to its former position as the wave passes westward. And the 
intensity, as indicated by the oscillating needle, ought to increase as the crest of the 
wave approaches the station, reaching its maximum when it is immediately bencath 
it, and decreasing gradually as the wave proceeds to the westward, the maximum of 
intensity thus coinciding with the maximum of inclination. Taking into considera- 
tion the conditions under which a system of internai tidal waves must be produced 
and propagated, the more important of which were pointed out, the author was of 
opinion that the results of observation fully coincided with this theory. 
A Memoir on Electro-Magnetism. By C. L. Draper. 
Contributions on the Submarine Telegraph. By WitpMAn WuirTEnovse. 
