164 Mr. Barlow on the daily Variation of the [March, 



between three and four degrees, and therefore be such as to 

 admit of distinct and satisfactory observation. 



A course of experiments carried on for a few days, convinced 

 me that my ideas were correct, and that we might, while the 

 needle was kept in its natural meridian, or rather adjusted to that 

 direction, produce a daily variation to almost any amount. I 

 obtained, for instance, the first day, a maximum deviation of 

 3° 40' ; the second, I increased it by bringing up my magnets 

 to 7° ; the third day I reduced it to 2°, and so on. I found also 

 that a very considerable daily change would exhibit itself with 

 the north end held to the south, to the east, west, and, in short, 

 in any position at pleasure, at least within certain limits, which 

 will be pointed out as we proceed. 



For this it is only necessary, first, to deflect (he needle by 

 repulsion into any required position, and then, by means of 

 another magnet, to modify its directive power, in the same way 

 as when in its natural meridian. Or the same may be done by 

 bringing two magnets with their contrary poles pointing inwards, 

 and each opposite to the pole of the same name of the needle 

 placed between them, and by a slight adjustment of the former 

 to produce the deviation in question ; or, which is perhaps still 

 better, the opposing magnets may be brought into the actual 

 direction of the dip, and then adjusted to produce the deflection 

 required. 



Having mentioned my ideas and first experiments to my 

 colleague, Mr. Christie, and having expressed a wish that he 

 would repeat them for the sake of verification, he very readily 

 agreed to undertake a complete set, with the needle in its natu- 

 ral meridian, by means of a very delicate compass, and an appa- 

 ratus he had employed for other experiments, and which admitted 

 of his bringing his neutralizing magnets very exactly into the 

 line of the dip. In the mean time I proposed to undertake the 

 observations on the dipping needle, and on the horizontal needle 

 in different directions ; viz. with its north end pointing to the 

 south, east, west, Sec. Having, however, met with some embar- 

 rassment in the commencement, and having employed, in con- 

 sequence, a longer time in the observations than I had antici- 

 pated, Mr. Christie, after having finished his observation in the 

 meridian, continued them at other points, and has thereby 

 detected several curious and minute peculiarities, which, with his 

 other experiments, will, I hope, accompany this memoir.* 



Account of' the Observations made on the daily Variation of the 

 Horizontal Needle in various Directio)is. 



My first experiments, as I have already stated, were only 

 matters of trial, from which 1 had merely ascertained that the 



• Mr. Christie has detailed his experiments in an extended paper, whieh succeeds 

 Mr. Barlow's present communication in the Phil. Trans. 





