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the normal position of the declination magnet, and to study the 

 laws of each taken separately, as well as in their combination, be- 

 came therefore a preliminary work to the right understanding of 

 either. That some such mode of examination would be required, 

 had indeed been anticipated in the instructions drawn up with so 

 much sagacity by the Committee of Physics of the Royal Society, 

 for the guidance of those who should engage in the direction of the 

 Colonial Magnetic Observatories then in contemplation. In the 

 preface to those instructions, it is expressly stated that " the pro- 

 gressive and periodical magnetic variations are so mixed up with the 

 transitory changes, that it will be impossible to separate them so as 

 to obtain a correct knowledge and analysis of the progressive and 

 periodical, without taking express account of and eliminating the 

 transitory or casual.'* The difficulties which impeded, and which 

 still impede an entire compliance with this instruction, viz. the per- 

 fect elimination of the transitory and casual changes, were found to 

 be very great. The direction which the magnet assumes when it is 

 under the influence of a perturbation of this nature, is not distin- 

 guishable from the direction assumed under the ordinary magnetic 

 influence, by any other criterion yet known than by the magnitude 

 of its deflection from the mean or normal position in the same 

 month and at the same hour ; the magnitude of the abnormal de- 

 flection may be thus taken, to a certain extent, as a means of recog- 

 nizing the existence of a perturbing force, and it is the only one we 

 possess. If we employ this criterion of magnitude as manifesting a 

 disturbed observation, and separate the observations so disturbed 

 from the others, we must still be aware that there may exist, and 

 that probably there do exist, amongst the body of observations from 

 which the large disturbances have been separated, some which may 

 be affected by the same disturbing cause or causes operating in a 

 minor degree ; and assuming the disturbances to have different 

 laws from the general body, the unseparated minor disturbances 

 may still impede the perfect deduction of the laws of the other class 

 with which they are so intermixed. 



But though the criterion of magnitude may not enable us to effect 

 a complete separation of the two classes, it will suffice to accomplish 

 an approximation to that end ; it will separate a sufficient body of 

 disturbed observations, disturbed beyond the limit of any other 



