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and Methods employed in determining the Direction and Intensity of 

 Terrestrial Magnetism." By Samuel Hunter Christie, Esq. M.A. F.R.S. 



The tedious nature of the observations by which the direction and 

 intensity of the terrestrial magnetic force are determined, and the un- 

 certainty attending the results when obtained, have long been a sub- 

 ject of regret to all who are engaged in the investigation of the phe- 

 nomena of terrestrial magnetism. Sensible of this, the author's at- 

 tention has at different times been turned to the improvement of the 

 instruments employed for these purposes ; and in this communication 

 he proposes methods by which he considers that these instruments 

 might be so improved that the results should be obtained with greater 

 facility and also with greater certainty. The uncertainty attending 

 the results obtained with the dipping needle, as at present construct- 

 ed, arises principally from the two sources, friction upon the axis, and 

 the want of coincidence of the needle's centre of gravity with the axis 

 of motion ; the latter rendering necessary the inversion of its poles. 

 The author suggests a method by which he considers that, probably, 

 the friction may be diminished ; but he has principally directed his 

 attention to obviate the necessity of the inversion of the poles. 



In order to remove the practical difficulty attending the adjustment 

 of the centre of gravity to the axis of motion, an operation in which 

 the artist rarely, if ever, completely succeeds, the author proposes to 

 dispense with this condition ; and shows how the dip may then be 

 determined, without the necessity of inverting the poles of the needle, 

 the position of its centre of gravity having been determined previously 

 to its being magnetized. The advantages attending the method pro- 

 posed by the author are not, however, restricted to the determination 

 of the dip with greater accuracy and greater facility : a further and 

 still greater advantage attending the use of a dipping needle on the 

 principle he proposes, is, that a measure of the terrestrial magnetic 

 intensity will be obtained by the same observations which give the 

 dip j so that, by this means, the observations usually required for that 

 purpose, and which are of the most tedious nature, will be avoided. 

 To effect both these objects in the most convenient manner, he pro- 

 poses that the needle should be so constructed that its centre of gra- 

 vity should be out of the axis of motion, in a line perpendicular to that 

 axis and to the axis of the needle. The requisite formulae for deter- 

 mining the dip and the measure of the terrestrial intensity, in this 

 case and also when the centre of gravity is in any other position, are 

 investigated in the paper. Mayer had previously pointed out that the 

 dip might be determined by means of a needle having its centre of 

 gravity out of the axis of motion, and had given the formulae requisite 

 for that purpose. His object, however, does not appear to have been 

 the same as our author's, — the avoiding in all cases that source of 

 inaccuracy, the inversion of the poles of the needle, — but simply the 

 determination of the dip, whether the centre of gravity of the needle 

 were made to coincide with the centre of motion, or not : the deter- 

 mination of a measure of the terrestrial intensity, by such means, does 

 not appear to have entered into his contemplation. 



As another form in which the same principles might be advan- 

 tageously applied, the author proposes that two needles, similar in all 



