197 



Observations and Experiments on the Daily Variation of the Hori- 

 zontal and Dipping Needles under a reduced Directive Power, By 

 Peter Barlow, Esq. F.R.S. of the Royal Military Academy. Com- 

 municated by Davies Gilbert, Esq. V.P.R.S. Read June 12, 1823. 

 [Phil. Trans. 1823, p. 326.] 



By disposing magnets so as partly to counteract the influence of 

 the earth's polarity on a magnetic needle, the author suspected that 

 its daily variation might possibly exhibit itself in a very increased 

 degree ; and in experimentally prosecuting the idea, he found it to 

 be the case to a very considerable extent in regard to the horizontal 

 needle ; and to take place also, though less satisfactorily, with the 

 dipping-needle. 



In the former experiment a finely suspended horizontal needle was 

 used, the directive power of which was reduced by two magnets, 

 properly placed for the purpose; a deviation of 3 15' was thus ob- 

 tained at 11 A.M., which decreased to a late hour in the evening. 

 The needle was kept in the same position for three days, with some 

 change of directive power, with similar general results. After ad- 

 verting to a difference in the direction of the variation in and out of 

 doors, Mr. Barlow details the results of several experiments, which 

 lead him to the following conclusions : that while the north end of 

 the needle is directed to any point from the S. to N.N.W., its mo- 

 tion during the forenoon advances towards the N. ; and while di- 

 rected towards any point between the N. and S.S.E. its motion i& 

 still to some point between the N. and N.N.W., so that there ought 

 to be some direction between those limits in which the daily motion 

 is a minimum : but whether this is a fixed direction during the year, 

 or whether it has any vibratory motion as the sun changes its decli- 

 nation, is a question requiring further experiments to determine. 

 Another conclusion which the author draws is, that the daily change 

 is not produced by a general deflection of the directive power of the 

 earth, but by an increase and decrease of attraction of some point 

 between the N. and N.N.E., or between the S. and S.S.E. 



The variation of the adjusted dipping-needle did not indicate the 

 gradually increasing and decreasing power manifested by the hori- 

 zontal needle ; and although it evidently is subject to diurnal change, 

 the law of that change remains to be determined. 



On the Diurnal Deviations of the Horizontal Needle when under the 

 influence of Magnets. By Samuel Hunter Christie, Esq. M.A. 

 Fellow of the Cambridge Philosophical Society : of the Royal Mili- 

 tary Academy. Communicated by Sir Humphry Davy, Bart. P.R.S. 

 Read June 19, 1823. [Phil. Trans. 1823, p. 342.] 



This paper contains a detailed account of a series of observations 

 on the diurnal magnetic variation made upon the principle pointed 

 out by Mr. Barlow, in his communication already before the Society. 

 In regard, however, to the arrangement adopted by the author, it 



