566 WEBER ON A TRANSPORTABLE MAGNETOMETER. 



the use of magnetometers might be deemed sufficient. But if 

 these maps are not themselves the final object sought, — if they 

 are to form the basis of a new investigation, — if determinate 

 rules and laws ai-e to be recognised, — if the maps are to serve 

 as the means of comparing experiment with the general theoiy 

 of the earth's magnetism, — and if the elements of the theory 

 are to be deduced from them, — then the degree of accuracy 

 to be demanded is no longer arbitrary, but is determined by the 

 nature of the subject. A minor degree of accuracy, such as these 

 maps now possess, has, it is true, served for a first attempt 

 at such a comparison ; but in order that they may afford an 

 adequate basis for an amended calculation, they must receive 

 a higher degree of exactness. Such is now the great purpose 

 of the magnetic observations to be made in distant expeditions, 

 and it is this which now gives to such expeditions pecuhar im- 

 portance and value. 



But the greater the importance which thus attaches to such 

 voyages and observations, in consequence of the demands of 

 theory, the more essential it becomes to examine what they are 

 capable of affording. 



Magnetic observations may be made at places widely remote 

 from each other, either at the same time or nearly so, or alter- 

 nately, so as to lessen the errors occasioned by regarding them 

 as simultaneous. At all the stations, or at the more important 

 at any rate, the observations may be continued with regularity 

 for at least one or more weeks, so as to afford mean values 

 freed in some measure from disturbing influences. But it is 

 still more desirable to give to such expeditions the advantage of 

 the recent improvements, by furnishing them with magnetome- 

 ters. This would probably be best accomplished, by the per- 

 sons who undertake magnetic expeditions making themselves 

 thoroughly acquainted, both theoretically and practically, m ith 

 the whole subject of magnetometric measurements, as they 

 would then be able to devise for themselves the best travelling 

 arrangements. But as there are not many opportunities of 

 acquiring this knowledge, the following memoir may be inter- 

 esting and useful to persons who cannot study the subject 

 more thoroughly in other ways. 



I proceed to describe a transportable magnetometer, which, as 

 it unites all the advantages proper to magnetometers, with faci- 

 lity of management and compendious construction, appears 



