1824.] Variation of the Horizontal and Dipping Needles . J 63 



Article II. 



Observations and Experiments on the daily Variation of the 

 Horizontal and Dipping Needles under a reduced directive 

 Power. By Peter Barlow, Esq. FRS. of the Royal Military 

 Academy.* 



It is now just a century since Mr. Graham discovered the 

 daily change in the variation of the horizontal needle, subsequent 

 to which time numerous observations have been made on the 

 same subject by Wargentin, Canton, Gilpin, Col. Beaufoy, and 

 others, which have all confirmed, with certain shades of variety, 

 the general fact as first described by the ingenious philosopher 

 above named. 



The actual daily change, however, is so small, even in the 

 horizontal needle, that it can only be detected with the most 

 careful observations and with the most delicate instruments ; 

 and in the dipping needle that change, if any, is so extremely 

 minute, as hitherto to have escaped observation ; for it was only 

 in the year 1820, that the Royal Academy of Sciences of Copen- 

 hagen proposed the determination of this motion, on satisfactory 

 experiments, as the prize subject for that year ; but the prize, I 

 understand, has never been adjudged, no satisfactory communi- 

 cation having been received. 



Under this difficulty of observation, it occurred to me, that it 

 would be possible to increase this deviation on both needles, so 

 as to render it distinctly observable, by reducing the directive 

 power of the needle by means of one or two magnets, properly 

 disposed to mask, at least in part, the terrestrial influence ; a 

 method which has been long practised by mineralogists and 

 others, when the object has been to detect minute attractions 

 I expected by this means that the cause, whatever it might be, 

 that produces the daily variation, would exhibit itself in an 

 increased degree, and thereby render the results more perspicu- 

 ous, and fix with more precision than has hitherto been done, the 

 time of change and moment of maximum effect. 



Suppose, for example, that a finely suspended horizontal 

 needle, under the natural influence of the earth, makes one 

 vibration in 2", and that by masking the terrestrial influence by 

 magnets properly adjusted, that time of vibration is increased to 

 8" ; then it would follow that the directive power was reduced to 

 one-sixteenth of the former, and consequently, that any lateral 

 magnetic force acting upon the needle would produce an effect 

 sixteen times greater than before ; so that if the former were 

 12', the new effect or deviation might be expected to amount to 



« Abstracted from the Phil. Trans, for 1823, Tart II. 



m2 



