Oscillations of the Horizontal Needle. 7 



brations in a rare medium, and confining them within very small 

 arcs, — as small as possible, so that the arc be consistent with accu- 

 racy of observation, (as shewn in Table III, observations on the 

 Shade, p. 22) ; with a magnet about 5 inches in length, and about 

 ^th of an inch square, suspended from one of its edges, I have 

 succeeded in obtaining between 200 and 300 vibrations, before 

 the arc became reduced from 5° to 3° ; the time of 100 vibrations 

 deduced from 200 vibrations, and estimated from the 0th to the 

 100th vibration, from the 10th to the 110th, and so on, up to 

 the 200th vibration, according to the method pursued by Profes- 

 sor Hansteen, being upon each 100 vibrations exactly the same. 

 Indeed, when such irregularities as those already stated (a) (b) 

 are considered, and which almost always happen to a greater or 

 less extent, when the magnetic oscillations are observed in air, 

 and taken in large arcs, it is not unreasonable to infer that ano- 

 malies and slight differences of time may appear on a great num- 

 ber of vibrations, notwithstanding the theoretical corrections pro- 

 posed to neutralize them. It may not, therefore, be safe to attri- 

 bute the cause of minute differences in time to variations in the 

 terrestrial intensity, before the instruments of research are made 

 so perfect as to be altogether without the limits of the above 

 mentioned causes of error. 



12. The mode of suspension and other mechanical conditions 

 connected with an oscillating bar, are not less worthy of conside- 

 ration than the disturbing influence of the air. 



In all experiments with the horizontal needle, it is evident 

 that we do not measure actually the magnetic intensity of a given 

 place, but only one of its resolved portions, except the place 

 should be directly in the magnetic equator. It is therefore of 

 consequence to the experiment, to preserve the bar in the same 

 relative position. Thi s we suppose to be horizontal, butsince 

 every freely suspended magnet inclines more or less in various 

 places, it becomes necessary either to measure accurately the angle 

 of inclination, or otherwise correct it in some way. The latter 



