xlii REPORT — 1864. 



I shall take this request in connexion with the following paragraph from 

 Mr. Airy's last Report to the Board of Visitors of Greenwich Observatory: — 



" The vertical-force magnetometer stUl exhibits sometimes the dislocations 

 in the photographic trace. There is no evidence, I believe, that these dis- 

 locations do not exist in the curves of every vertical-force instrument, for 

 they are always accompanied with vibration ; and no vertical-force instru- 

 ment, I beUcvo, except that of Greenwich, gives a trace strong enough to 

 exliibit vibrations, and the dislocations, therefore, with any other instriunent 

 would appear merely as interruptions of the trace, and would not attract 

 much attention"*. 



Before discussing Mr. Airy's request, I shall endeavour to show that our 

 vertical-force instrument is free from objection. In the first place I am able 

 to state, from having examined our vertical-force ciu'ves in conjunction with 

 my assistant, that when cause of disturbance takes place the vibrations of 

 our needle are impressed upon the photographic paper. Whenever a change 

 takes place in the direction of the forces acting upon a freely suspended 

 magnet, the impulse is followed, and the magnet, after an interval, which 

 may be longer or shorter according to its time of vibration, assumes the 

 new direction. If the changes of force succeed each other more rapidly than 

 will admit of the magnet becoming stationary between their occurrence, it 

 does not cease to vibrate until the intervals between the changes become long 

 enough to permit it to do sof. This state of vibration is quite perceptible 

 in the photographic records at Kew ; but when the time of vibration is so 

 small as in the Kew instrument, where it is seven seconds only, the mean 

 place corresponding to a desired instant is almost always obtainable from the 

 trace. It may suffice that in the six months from July 1 to December 31, 

 1863 (the records of which are now imder reduction), and in which there 

 should be 4416 equidistant hourly positions, there are only five wanting by 

 reason of failures from all causes whatever. In one of these the disturb- 

 ance was so excessive that the trace ran off the recording paper ; in the 

 other four the vibrations corresponding to the fluctuations in the directions 

 of the disturbing force were too rapid to permit the trace to be sufficiently 

 distinct for measurement. Should it be hereafter desii-able to investigate 

 more particularly the phenomena of the changes thus rapidlj" succeeding each 

 other, a shorter, not a longer, magnet than the one in use at Kew woidd be 

 required, having a shorter time of vibration than seven seconds ; but in the 

 mean time, and for the present wants of science, there is, I think, every 

 reason to believe that Mr. Welsh exercised a sound judgment in deter- 

 mining the dimensions, shape, and weight of the Kew vertical-force magnet. 

 The self-recording instruments at Kew are now in the seventh year of their 

 performance, and the curves of each magnetograph, including those of the 

 vertical force, have been carefully examined preparatorj- to reducing them, 

 with the view of eliminating everything of the nature of displacements, 

 whether due to instrumental defects or to the approach of magnetic matter. 

 The curves of the vertical force under this very severe scnitiny have proved 

 themselves as perfect as those of the other magnetometers, that is to say, 

 they are practically faultless as far as one can judge by this means. 



General Sabine has kindly iiudertaken the reduction of the traces afforded 

 by eur magnetographs, and tinds that the vertical-force magnet is capable 



* As far as I am aware, Mr. Airy has not seen any original negative from our vertical- 

 force magnetograph. 



f It has already been recognized by Gauss as a law, that no magnet can correctly record 

 those changes of which the period is not considerably more than that of its own vibration. 



