66"^ GAUSS AND WEBER OX TERRESTRIAL MAGNETISM. 



here, because these pages are intended not merely for the limited 

 number of those who participate in the simultaneous observa- 

 tions, but for all who are engaged in investigating the laws of 

 magnetic phenomena. 



Those less dehcate instruments which were employed for 

 magnetic measurements before the invention of the magneto- 

 metei', may not only be used for the same purposes as formerly, 

 but may also be applied to those absolute measurements of in- 

 tensity which owe their origin to the invention of the magneto- 

 meter. It is true that these instruments ai'e far from affording 

 such accurate residts as the magnetometer; but the results 

 they give are more easily obtained. On this account they have 

 not lost all their value by the later invention ; they may still be 

 usefully employed, though in a more limited sphere. Wherever, 

 from want of means or time, or from any other circumstances, 

 magnetometers cannot be employed, these instruments may still 

 be used with advantage. This will be the case most frequently 

 in voyages and journeys to remote parts of the world. It is true 

 that magnetometers may be carried on journeys, as was done by 

 M. von Waltershausen and Dr. Listing in their Italian tour; 

 but this is only possible for those who are highly favoured 

 by external cu'cumstances ; and it is therefore not to be ex- 

 pected that many will follow this praiseworthy example. If, 

 therefore, we wish to collect observations from the whole sur- 

 face of the earth, we must be content with such as are not 

 made with magnetometers ; and it is important to extend the 

 application of portable instruments to the absolute measure- 

 ment of the intensity, which has been hitherto performed Avith 

 the magnetometer onlj'. The difference in respect to accuracy 

 between the absolute measures with such instruments and those 

 made with the magnetometer, is nearly the same as between 

 measurements of decHnation with the two kinds of instruments. 

 A skilful hand will be able to obtain useful results even with ^he 

 smaller apparatus; and it appeal's desirable, therefore, that it 

 should be extensively employed. 

 We shall consider successively, 



1. The parts of the small apparatus. 



2. The observations to be made with it. 



3. The application of the observations. 



4. The calculations required. 



5. The result of the calculation. 



