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II. Account of some Additional Experiments on Terrestrial Magnetism made in 

 different parts of Europe in 1837. By JAMES D. FORBES, Esq. F.R.SS.L.fyE., fyc. 

 Professor of Natural Philosophy in the University of Edinburgh. 



Read 6th April 1840. 



51.* IN 1836, I communicated to this Society the results of an extensive 

 series of observations on Terrestrial Magnetic Intensity, made with the HANSTEEN 

 Apparatus, which is the property of the Society. Some results with a small 

 Dipping Needle, belonging to myself, were also given, but without great confi- 

 dence in their accuracy. 



52. I held then, however, the opinion which I still do, and to which the 

 remarkable geometrical researches of Professor GAUSS of Gb'ttingen on Terrestrial 

 Magnetism have given additional weight, that the element of horizontal intensity 

 ought to be determined, and its laws of variation, in the first place, ascertained, 

 independent of any other. Even should the deduction of total intensity be the 

 sole ultimate object, I hold that an observer with only portable, and consequently 

 imperfect instruments at his command, would do well to separate completely his 

 investigations as to horizontal intensity from those upon dip, and then, by the 

 skilful grouping of each set, having obtained a law of variation of each element 

 according to the co-ordinates of Latitude and Longitude, the two partial results 

 may be combined into the general one of total intensity (or hor TO 1 s nt ( ^ Q61ty ), whilst 



either series may be used to check future observations, or be combined with any 

 single future series in which one of the elements should be better determined. 



53. As intensity observations on the HANSTEEN method are generally very 

 superior to those of dip (considering the proportion which a probable error in 

 either would alter the value of the total intensity), it is a pity to render worthless 

 the good part of an observation, which contains an element capable of general 

 and independent determination, by mixing it up with the erroneous results of an 

 inferior observation. 



54. It was on this ground that, in my former paper, read in December 1836, 

 I carefully reduced the horizontal intensity observations by themselves, and it is 



* These numbers are in continuation of those in the former paper on Magnetism, Edinburgh 

 Transactions, vol. xiv. p. 1. The last paragraph of that paper ought to be numbered 50. 

 VOL. XV. PART I. H 



