204 ON DETERMINATIONS OF THE ABSOLUTE VALUE, SECULAR CHANGE, 
such modifications in the Portable Magnetometer originally devised by Professor 
Wilhelm Weber*, the magnets of which were only from 3 to 4 inches in length, as 
would be likely to remedy several practical defects which experience had pointed out 
in that instrument ; and to draw up sueh instructions for its use as might make it, 
when thus modified, an efficient instrument for eflfecting those purposes, originally 
prescribed by the Royal Society, which had hitherto failed of accomplishment; viz. 
the determination of the absolute values, secular changes, and annual variations of 
the horizontal component of the magnetic force. The modifications thus introduced 
and the methods of observation recommended, are contained in a work entitled 
“ Magnetical Instructions for the use of Portable Instruments adapted for Magnetical 
Surveys and Portable Observatories, and for the use of a set of small Instruments adapted 
for a fixed Magnetic Observatory ; with Forms for the Registry of Magnetical and 
Meteorological Observations " in which work, various other forms of instruments, suited 
for particular magnetic purposes, are also described, together with the best methods 
of using them, and of determining the several constants required in calculating the 
final results. The Portable Unifilar Magnetometer, with the modifications thus in- 
troduced, was shown to the magneticians assembled at the meeting of the British 
Association at Cambridge in 1845 ; and Captain Riddell’s Manual, printed by the 
authority of Government, has been extensively circulated amongst those persons who 
are engaged in magnetic researches, and has been found extremely useful, I believe, 
by those who have consulted it. A statement was made at the meeting of the British 
Association in 1845 of what had then been accomplished by the Colonial Observatories, 
and also of what still remained to be accomplished, to fulfil the objects for which the 
institution of those establishments had been recommended jointly by the Royal So- 
ciety and British Association ; and the probability was shown of speedily fulfilling all 
those objects by the means which had been adopted. Upon this statement a further 
but limited continuance of the observatories was recommended to Government and 
sanctioned. I am desirous of taking the earliest opportunity in my power of laying 
before the Royal Society a statement of the success which has attended the employ- 
ment of the means thus referred to; because I am in hopes it may be an inducement 
to other observatories, which have either been instituted for the purpose of cooperating 
in the plan proposed by the Royal Society, or who have expressed the intention or de- 
sire of cooperating, to persevere in the fulfilment of all the objects originally contem- 
plated ; either by the adoption of the means which will be now described, or by the 
invention and employment of others which may serve the purpose as well or still 
more effectually. I shall avail myself of the observations which have been made at 
the Colonial Observatory of Toronto in Canada, under the direction of Captain 
* A description of this instrument, translated by Mrs. Sabine, partly from an original account printed by 
M. Weber in the ‘ Resultate des Magnetischen Vereins,’ and partly from manuscript communications kindly 
furnished by M. Weber himself, was presented to the Editor of Taylor’s Scientific Memoirs, and forms art. 
XVI II. in the second volume of that work. 
