AND ANNUAL VARIATION OF THE TERRESTRIAL MAGNETIC FORCE. 217 
of the isodynamic lemniscates (as they have been usually called), and are the points 
of greatest intensity of the force (on the surface of the globe) of apparently two 
magnetic systems, distinguished from each other by the very remarkable difference in 
the rate of secular change to which the phenomena in each system appear to be sub- 
ject. In the present state of the terrestrial magnetic phenomena, the principal of 
these two points, or the centre of the larger loop of the lemniscates, is situated within 
the British territories in North America; and by the magnetic survey of those terri- 
tories, undertaken by the British Government on the recommendation of the Royal 
Society, and executed in 1842 and 1843 by Captain Lefroy, its geographical position 
was approximately ascertained, and the difference between the magnetic force at this 
central point and at the Toronto Observatory was very carefully measured, and is 
recorded in the Philosophical Transactions (Part III. for 1846). In this point of view 
therefore the accurate determination of the Force at the observatory at Toronto has a 
peculiar value, both for the present and for after times. It will, I think, be clear to 
those who have followed the details of this communication, that by the skill, assiduity 
and perseverance of the Director of the Toronto Observatory and his assistants, (non- 
commissioned officers of the Royal Artillery,) this object has been accomplished within 
very small limits of uncertainty as dependent on observation or accidental error; and 
that when the small corrections which have been noticed, as requiring to be in- 
vestigated on the return of the instruments to England at the close of the series, 
have been ascertained and applied, the value of the total force in absolute measure at 
Toronto, and by its means the value at the central point, will be assigned with a 
degree of accuracy which we may believe will be regarded as satisfactory, not only at 
the present day, but at those distant periods, when the determination may be referred 
to as presenting the earliest record of the value of the terrestrial magnetic force at 
its point of maximum in the northern hemisphere. 
The determination of this value at this particular time may derive an additional 
importance from the present relative situation of the two magnetic centres* which 
are not yet far removed from their greatest distance apart, viz. 180° in geographical 
longitude ; a state of the phenomena constituting possibly an epoch in the cycle of 
secular change, characterized by that portion of the force at each centre which is 
derived from mutual influence being a minimum. The analogy of the southern hemi- 
sphere, where the two centres are nearer to each other in respect to geographical 
longitude than in the northern hemisphere, and where the force at each is higher 
than at the corresponding northern centre, may justify this supposition. The geo- 
graphical longitude of the principal northern maximum was ascertained by Captain 
Lefroy, in the years J842 and 1843, to be about 270° East ; that of the minor maxi- 
* It will of course be understood that by the employment of the word “ centres ” it is not intended to con- 
vey that the points of maximum are themselves centres of the magnetic force of the systems to which they may 
respectively belong. The expression is merely used to designate central points of certain phenomena observed 
on the earth’s surface, where alone it is in our power to observe them. 
2 F 
PJDCCCL. 
