1 30 Royal Society. 



Mr. Barlow, the author contends that the phsenomena observed are 

 precisely those which must result from the theory he had himself 

 adopted ; and that they tend in no way to support the hypothesis of 

 their being simply the effects of the magnetic power which the iron 

 receives by induction from the earth. 



The author was also led to suspect the accuracy of another con- 

 clusion which had been drawn by Mr. Barlow, namely, that the 

 length of the needle has no sensible influence upon the extent of its 

 deviations. In orderto determine this point, he began by a.scertain- 

 ing more scrupulously than had yet been done, the values of several 

 of the elements of the calculation, such as the exact positions of the 

 points where the intensity of the magnetism is the greatest, and also 

 of the point of neutrality, or of the magnetic centre: and he next 

 subjected to a more severe scrutiny a law which had been regarded 

 as established by experiment ; namely, that the tangent of the de- 

 viation is proportional to the rectangle of the cosine of the longitude, 

 into the sine of the double latitude of the position of the centre of the 

 needle with relation to the mass of iron as referred to a hollow sphere. 



In the course of his experiments the author ascertained that if any 

 bar of steel uniformly magnetized by the method of double touch 

 have this state of magnetism disturbed by drawing the end of a mag- 

 net from its centre to the end, having the same polarity as that ap- 

 plied to it, then the pole at that end will be shifted towards the cen- 

 tre ; while the opposite pole will be removed further from it : and a 

 corresponding change will occur in the position of the magnetic 

 centre. Changes will also take place in the absolute intensities of 

 the magnetism at each pole. Considerable differences were observed 

 in the extent of the deviations of a needle six inches in length, and of 

 one of two inches long, when successively placed in the same position 

 with relation to the shell of iron. At the distance of 16'8 inches, 

 they amounted to more than two degrees and a half j and the diffe- 

 rence continued to be very sensible, even at a distance of 24 inches 

 from the shell. In general, when the needles were near to the 

 north or south of the centre of the shell, the deviations of the longer 

 needle exceeded those of the shorter ; and the reverse took place 

 when the needles were placed on the east and west sides. Hence 

 be concludes, that the efficacy of a small mass of iron placed near to 

 the needle to serve as a compensation to the effects of more distant 

 masses, will depend upon its being itself at such a distance from the 

 needle as that the difference of its action upon a long and a short 

 needle shall be insensible. 



The author proceeds to deduce from the law which he has proposed, 

 various forms of equations for determining the deviations of a hori- 

 zontal needle due to the action of an iron sphere or shell, applicable 

 to different circum.stances and conditions of the case. In one set of 

 equations, the actions and resulting positions are referred to three 

 rectangular co-ordinates proceeding from the centre of the needle j 

 and in another set, they are referred to polar co-ordinates, relative to 

 the vertical and to the plane of the horizon. He next deduces 

 equations for computing the deviations of a needle in which the 

 magnetism has been disturbed, by applying to one of its poles the 



corresponding 



