242 
MAJOR-GENERAL SABINE ON THE DISTURBANCES OF THE 
mind of every reflecting magnetician the possibility, almost amounting to probability, 
that the second system of the terrestrial magnetism, which by the change in its rela- 
tions to geographical space seemed to be distinct and dissevered from the magnetism of 
the earth properly so called (i. e. the collective action , of all the permanent magnetic 
particles of the earth’s mass, having its seat in the earth itself), might, like the decen- 
nial variation, be in truth assignable to a cosmical origin. The movement of translation 
on the earth’s surface of the second system, and with it the whole phenomena of the 
secular change, would thus be regarded as belonging to, or being part of, a cosmical 
variation. It has, indeed, all the characters befitting such a relation, besides appearing 
inexplicable on any other hypothesis : we do not, indeed, yet know the duration of this 
far longer period, nor are we able to trace its course by visible signs on any of the 
heavenly bodies, as we trace the decennial period by the changes in the magnitude 
and frequency of the sun-spots. We infer its existence only from the terrestrial mani- 
festation afforded by the secular change in the magnetic elements. 
The “ Terrella,” by which Halley figured to himself a cause capable of producing 
phenomena of the order and regularity of those which his laborious and extensive 
generalization had disclosed to him, has never, I imagine, found favour as a probable 
physical reality. Viewed simply as an illustration of the systematic arrangement, sym- 
metrical progression, and exceeding regularity of the effects , and the consequent neces- 
sity for the admission of qualities of the same order in the causes , of the terrestrial 
magnetism and its secular changes, Halley’s Terrella had its proper value ; and it 
would have been well if the lesson which it inculcated had received more considera- 
tion than it has done from those who, more than a century after his publications, have 
attempted to explain the phenomena of the progressive magnetic change by accidental 
or adventitious variations in the superficial temperature of the globe or of its atmo- 
sphere, or in the occasional development or protrusion of magnetically attractive or 
repulsive rocks beneath its surface. The order and harmony of the facts manifested by 
the researches of a much earlier date had already effectually removed them from the 
category of partial or accidental occurrences. The symmetry of their general distribu- 
tion, the counterpart to each other presented by the phenomena of the northern and 
southern terrestrial hemispheres, and the regularity with which the periodical changes 
take place, indicated a systematic causation which, obscure as it might be, was obviously 
anything but fortuitous. And when to the increased knowledge of the general pheno- 
mena acquired in the last and present centuries, confirming and extending the previous 
conclusions, was added the evidence obtained by the observations of the British Colonial 
Observatories, that the secular change is progressive in the extremest sense , that each 
week shows (and that if the means of observation were sufficiently refined it is more 
than probable that each day would show) an exact aliquot part of the annual change, 
the conviction became almost irresistible, that the causes which produce such remark- 
able effects can only have a cosmical origin. 
The objections that might have impeded the reception of such an hypothesis before 
