[ 326 ] 
XXIII. Observations and Experiments on the daily variation of 
the Horizontal and Dipping Needles under a reduced directive 
power. By Peter Barlow, Esq. F. R. S. of the Royal Military 
Academy. Communicated by Davies Gilbert, Esq. V. P. R. S. 
Read June 12, 1823. 
It is now just a century since Mr. Graham discovered the 
daily change in the variation of the horizontal needle, subse- 
quent to which time numerous observations have been made 
on the same subject by Wargentin, Canton, Gilpin, Colonel 
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 Copenhagen 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 communication 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 di- 
rective power of the needle by means of one or two magnets, 
