MAGNETICAL OBSERVATIONS IN SCOTLAND. 117 



the case in the statical method. This accords with the antici- 

 pations of the inventor of the statical method. And when it is con- 

 sidered that an error of a single minute in the ohservation of the 

 dip will occasion, in so high a magnetic latitude as Scotland, a cor- 

 responding error of -0009 in the deduction of the total intensity 

 from the horizontal vibrations, it must be acknowledged that the 

 statical method possesses a great advantage in such latitudes, in 

 being free from this source of liability to error. I must add, 

 however, that I do not attribute the discrepancies observable in 

 the results of the horizontal method, beyond those of the statical, 

 altogether to defects inherent in the horizontal method. The ne- 

 cessity of economizing time obliged me frequently to keep the 

 dipping needle and the horizontal cylinders employed at the same 

 time, when it was of course necessary to place them several yards 

 apart. In a country so subject to local magnetic disturbance 

 as Scotland, it is not too much to say, that it might not always 

 follow that the dip should have been precisely the same at the 

 two spots of observation, I have already noticed that at the 

 two sides of the harbour at Loch Scavig I observed a difference 

 of above 5° of dip ; and although that was no doubt an extreme 

 case and one of very rare occurrence, it can scarcely be sup- 

 posed but that irregularities do occur in a minor degree not 

 unfrequently. In strict justice to the horizontal method, the 

 cylinders, in countries liable to such local influences, should 

 always be vibrated precisely in the same spot where the dip 

 is observed. The place of observation of the two methods 

 not being the same, may also subject the instruments to actual 

 differences of intensity apart from the magnetic direction. In 

 such cases the results may be true measures, though differing 

 from each other; but discrepancies of this nature should 

 not exceed the limit of the local irregularities of intensity, 

 the existence of which we may infer from the statical residts. 

 When they are considerably greater, a more probable mode of 

 accounting for them is that the horizontal needle was affected 

 by a different dip from that acting on the dipping needle and 

 used in the reduction. In all such cases the total intensities 

 derived from the horizontal vibrations are of course in error. 

 It has been shown by the corresponding table of the dip obser- 

 vations that there were seven stations at which the observed 

 dip differed from the dip computed by a combination of the ob- 

 servations at all the stations to an amount varying from 14'-5 to 

 25'* 1. At six of these stations there were also horizontal and 

 statical observations of the force. Had the differences between 

 the observed and computed dips in these cases been errors of 

 observation, and not actual irregularities in the magnetic dircc- 



