TERRESTRIAL MAGNETISM. 137 



system of examination. The arrangement of the mark and 

 of the cross of wires in the telescope, which was suspended 

 by either a silken or metallic thread, and enclosed in a large 

 glass case, permitted readings to be taken to eight seconds 

 of arc. In this method of observation, the room in which 

 the magnetic telescope was placed might be left dark, thus 

 avoiding the disturbing effects of currents of air, which may 

 be occasioned by the means required for illuminating the 

 scale of otherwise excellent declinometers furnished with 

 microscopes. Entertaining the opinion which I then ex- 

 pressed, that " a continued uninterrupted hourly or half- 

 hourly process of observation (observatio perpetua) for several 

 days and nights was to be preferred to the detached observa- 

 tions of many months," we observed consecutively for five, 

 seven, or eleven days, and as many nights, ( 168 ) at the periods 

 of the solstices and equinoxes, epochs of which all the most 

 recent investigations have confirmed the great importance. 

 We soon recognised, that in order to study the proper phy- 

 sical character of these anomalous disturbances, it was not 

 sufficient to determine the measure or quantity of the devia- 

 tion from the change in the declination, but that there should 

 be added thereto a numerical estimate of the degree of in- 

 quietude of the needle, by noting the elongation of its os. 

 dilatory movements. "We found the ordinary diurnal march 

 of the needle so quiet, that among 1500 results drawn from 

 6000 observations, from the middle of May 1806 to the end 

 of June 1807, the oscillations were mostly only of half a scale 

 division, or 1' 12". The needle was often quite still, 

 or moving only 0'2 or 0'3 of a scale division during 

 very tempestuous and rainy weather; but on the arrival 

 of the magnetic storm, of which the more intense and later 



