LLL NT eee 
An attempt to facilitate Observations of Terrestrial Magnetism. 
By the Rev. Humeurey Lioyp, A. M., M.R.IA. Fellow of Trinity College, and 
Professor of Natural and Experimental Philosophy in the University of Dublin. 
Read October 28, 1838. 
Since the important researches of Baron Humsotpr, Captain Sapine, and Professor 
HanstTeen, much interest has been awakened on the subject of Terrestrial Magnet- 
ism. The intensity of the magnetic force, and the dip, have been determined at 
various parts of the earth’s surface, and are found to follow laws agreeing re- 
markably with the results of theory. But the experimental data necessary for the 
complete solution of this important problem are, as yet, far from adequate; and ac- 
cordingly, the multiplication of observations in remote quarters of the globe, has 
engaged the zeal of many a scientific individual, and even called forth the resources 
of more than one state of Europe. 
The imperfections of our instruments of observation have long presented a for- 
midable obstacle to the advancement of these inquiries. Many of these have indeed 
been guarded against by the skill of the observer, and by the application of suitable 
corrections. ‘There are still, however, some inherent sources of fallacy in the instru- 
ments themselves, which seem almost inseparable from them in their ordinary form. 
The defects of the common dipping needle have been long acknowledged, and still 
continue to embarrass observers. The chief of these defects arises from the nature 
of the suspension. ‘The directive force, it is well known, diminishes with the angular 
distance from the true line of the dip, varying as the sine of that distance. Hence 
when the needle is near the position which it would assume under the influence of the 
terrestrial action, if free to move, the directive tendency may be so small as to be 
more than balanced by the friction of the axle against its supports ; and in this case 
the needle will rest out of the position in which the earth’s force would place it. This 
evil will of course be increased by any irregularity in the form of the axle itself, and 
such irregularities can never be wholly removed. To this is added another source of 
error, arising from the non-coincidence of the axle with the centre of gravity of the 
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