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, 
Are of vibration at commencement 900. Time of one vibration 115-29. 
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 (¢.e. west ex- 
tremity) beg moved towards the north, the following observation was then made : 
Are of vibration at commencement 9"0. Time of one vibration 10558. 
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. 4th, It was found in 1844 and 1845, that the time of vibration depended 
greatly upon the are of vibration, the time being greatest for large arcs. (See 
Trans. Roy. Soc. Ed., vol. xvi., p. 70, Table IT.) 
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 are of 100’; the observations also for the time of vibration with the needle 
differently imelined 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 
are 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 achange of 1° Fahr. upon the position of the needle, it has been found 
