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XV. — Results of the MaJcerstoun Observations, No. II. On the Relation of the Varia- 

 tions of the Vertical Component of the Eartlis Magnetic Intensity to the Solar and 

 Lunar Periods. With a Plate. By J. Allan Begun, Esq., Director 0/ General 

 Sir T. M. Brisbane's Magnetical and Meteorological Observatory. Communi- 

 cated by General Sir T. M. Brisbane, Bart. 



(Read 20th April 1846.) 



1. The following results ai-e deduced from the observations of the balance or 

 vertical force magnetometer, which consists of a magnetic needle, balanced hori- 

 zontally, and resting, by a knife-edged axle, on agate planes. Much doubt has been 

 entertained as to this instrument's capability of shewing changes of moderate 

 nicety, and it has been considered altogether unavailable for changes of long 

 period :* it has been shewn (Vol. XVI., p. 67), that there are several diificulties 

 in the way of an accurate interpretation of the observations, independent of the 

 instrumental capacity. If it be added, that disturbances seem to affect the daily 

 means of the vertical component,! in a more serious way than they do those of the 

 horizontal component, it will be seen that there are a series of difficulties, which 

 tend to render good and consistent results from the balance magnetometer nearly 

 unattainable. It will be judged afterwards how far these difficulties have been 

 overcome in the present instance. 



2. The changes of the vertical component are, in general, very gradual and 

 regular ; even during considerable disturbances the balance needle moves gi-adu- 

 ally to an extreme position, remaining there, with moderate fluctuations, for a 

 considerable period, and afterwards returning slowly to its mean (or nearly mean) 

 position ; the bifilar magnet, on the contrary, moves with considerable rapidity 

 from one extreme position to another. This difference in the mode of distur- 

 bance, I do not conceive due to a want of sensibility in the balance needle, but 

 rather to a difference in the nature of the distm-bances of the dip, and of the total 

 intensity. 



3. The changes of the vertical component, then, during disturbances, wUl 

 evidently have little or no effect on the regularity of the diurnal curves, although 

 the periods of maxima and minima may be affected ; it will be different for the 

 daily means, for, as the disturbances wiU, in general, be almost wholly negative 



* Revised instructions by a Committee of the Koyal Society of London, p. 37. 

 t I shall generally use the terms vertical or horizontal component, instead of vertical or horizontal 

 force ; the latter are not at all expressive where the changes may be altogether due to variations of dip. 



VOL. XVT., PART II. 2m 



