1824.] Horizontal and Dipping Needles. 165 



idea I had formed was practicable, and that in certain situations 

 the needle had certain directions of motion, but I had obtained 

 no numerical results. Having', however, provided myself with a 

 needle proper for the purpose, very delicate and light, and eight 

 inches and a half in length, I began, towards the end of March, 

 to register the amount of the daily change at every hour, or half 

 hour, from morning to night ; my son taking the observations 

 during my occasional absence. 



My first observation in the new series was made with the 

 north end of the needle pointing to the west, balanced in that 

 position with two magnets placed to the southward attracting 

 each extremity ; the directive power was considerably reduced, 

 and I obtained a maximum deviation of 3° 15'; which happened 

 at about eleven o'clock in the forenoon, and from which time the 

 variation decreased to a late hour in the evening. The needle 

 was kept in this position for three days, with some change of 

 directive power, but the character of the results, as to the direc- 

 tion of motion, the times of commencement and maximum, Sec. 

 were of precisely the same nature, but the amount was more or 

 less, according to the directive power left upon the needle. 



Having, however, after a few days, removed my apparatus from 

 the room in which the experiments had hitherto been made, into 

 a bower in my garden, and having detected a remarkable differ- 

 ence in the results obtained in these two situations, I determined 

 to commence the experiments de novo in this latter spot, which 

 was at least thirty yards distant from any building ; and after- 

 wards to examine the cause of the difference in question. This 

 examination is reported in the conclusion of this article. 



[Mr. Barlow here gives a series of tables of observed daily 

 variations, with the north end of the needle directed to the fol- 

 lowing points of the compass respectively : — north, south, 

 north-east, south-we&t, east, west, south east, north-west, north 

 north-east, south-south-west, east-north-east, west-south-west, 

 east-south-east, west-north-west, snuth-suuth-east ^ south, (ex- 

 act bearing N. 10° W. and S. 16° E.) and north-north-west.] 



From the above results, although the experiments were not made 

 under such favourable circumstances as 1 could wish, we may 

 draw some very curious, if not important conclusions; such, for in- 

 stance, as the following. That while the north end of the needle 

 is directed to any point from the south to NNW, its motion during 

 the forenoon is towards the left hand (the spectator facing the 

 north end of the needle) ; advancing therefore to some point be- 

 tween the NNW and north ; and while it is directed towards any 

 point between the north and SSE it passes to the right hand, that is 

 still to some point between the north and NNW ; the south end of 

 the needle at the same time passing of course to some point be- 

 tween the south and SSE ; so that it would seem that there ought 

 to be some direction between those limits, viz. between the N and 

 NNW, and the S and SSE, in which the daily motion is zero, or 



