Balance or Vertical Force Magnetometer. xxxix 



Again vibrated the needle by means of the deflector placed at the side of the 

 needle near its east extremity, so as to displace the needle from the plane at right 

 angles to the magnetic meridian, and observed time of vibration, 



Arc of vibration at commencement 90'0. Time of one vibration lls-29. 



None of these operations seemed to alter the time of vibration to any distinct 

 amount ; the box was accordingly lifted off, and the needle was placed about 3° out 

 of the plane at right angles to the magnetic meridian, the north pole (i.e. west ex- 

 tremity) being moved towards the north, the following observation was then made : 



Arc of vibration at commencement 9'"0. Time of one vibration 10^"58. 



Although the time of vibration in this position differs somewhat from that in 

 the normal position, the previous observations prove that any deviations due to 

 natural changes of force, would be insufficient to cause the differences evident in 

 the Tables for 1844, 1845, and 1846. 



54. ith, It was found in 1844 and 1845, that the time of vibration depended 

 greatly upon the arc of vibration, the time being greatest for large arcs. (See 

 Trans. Roy. Soc. Ed., vol. xvi., p. 70, Table II.) 



55. It is difficult, if not impossible, to offer any explanation of the anomalies 

 in the time of vibration noted above, the knife-edged axle is a fruitful resource in 

 instruments of this class, for the explanation of all difficulties. In this case, the 

 needle is by the best maker (Robinson) ; when examined by a lens, the knife edge 

 appears perfect, and finally, the anomalies disappear at certain times without any 

 apparent difference in the state of the instrument ; thus, in the observations already 

 given, Jan. 14, 1848, the time of vibration for an arc of 3' is exactly the same as 

 for an arc of 100' ; the observations also for the time of vibration with the needle 

 differently inclined to the horizontal (Jan. 18, 1848), speak distinctly in favour of 

 the excellence of the knife edges. It should also be remarked, that previously to 

 the adjustment, Jan. 27, 1844, the curious effect of temperature upon the time of 

 vibration was scarcely exhibited ; and it may be added, that in eight months since the 

 needle has been adjusted with its axis in the magnetic meridian, the time of vibra- 

 tion appears to remain nearly constant and equally independent of temperature and 

 arc of vibration. 



56. It appears certain from these results, obtained from an instrument of the 

 best character, treated with the greatest care, that the time of vibration in a vertical 

 plane cannot be depended on as an element in the reduction of the observations of 

 the balance magnetometer. 



57. The question still remains, to what extent these anomalies in the time of 

 vibration affect the observations for the varying vertical component of the magnetic 

 force ? The following fact appears to render it certain that they have no effect 

 whatever. In determining, by the method of comparisons (see pages xlviii. and xlix.), 

 the effect of a change of 1° Fahr. upon the position of the needle, it has been found 



