118 SIXTH REPORT 1836. 



tion from some disturbing cause, the discrepancies between the 

 two methods of measuring the force must have been far greater 

 than they are. The error in the deduction of the force by the 

 horizontal method which would be occasioned by an error of 

 observation of 14' of dip, would have greatly exceeded any dis- 

 crepancy existing between the horizontal and statical results in 

 the above table. Considerations of this kind are important in 

 their bearing on the proper method of deducing the lines of 

 total intensity from a series of horizontal vibrations. If discre- 

 pancies in the observations of dip arise chiefly from instrumen- 

 tal errors or errors of observation, the dip resulting from the 

 calculation of least squares should be used in the reduction 

 of the horizontal observations. If, on the other hand, the pro- 

 bability of local disturbance is greater than of instrumental 

 error, the dips actually observed should be employed in the re- 

 duction. Were the dips due to the geographical positions 

 employed in the reduction of the Scottish observations instead 

 of the dips actually observed at the stations, the discrepancies 

 in the resulting intensities would be increased to such an ex- 

 treme amount, as to leave no doubt that, in the case of the Scot- 

 tish observations at least, the observed dips are those which 

 ought to be employed ; and it also follows, that in countries 

 subject to such magnetic irregularities, the dipping-needle should 

 be regarded as an indispensable accompaniment to the horizontal 

 cylinders. 



All the observations made with the statical needle have been 

 included in the calculations, as well as all those with the horizon- 

 tal cylinders, except the vibrations at Loch Scavig, which were 

 evidently affected by some very great cause of irregularity, and 

 their introduction into the calculation could only be productive 

 of eiTor. Loch Scavig, with its hypersthene rocks and its trap 

 dykes, is evidently a very unsuitable place for magnetic observa- 

 tions. 



The general results from the statical and horizontal methods 

 are so nearly the same, that either might be used for the chart 

 with scarcely a perceptible difference. Perhaps the statical re- 

 sults may be considered as entitled to a preference, and they have 

 been employed accordingly. 



Until the relation which the isodynamic lines in Scotland 

 bear to the magnetic intensity in England shall be more 

 thoroughly and satisfactorily ascertained, by a connected series 

 of observations comprehending the whole of Great Britain, it 

 has been deemed preferable to give the Scottish lines no other 

 designation in the chart than that which expresses their relation 

 to each other, thus limiting the conclusion to what is strictly war- 



