WEBER OX A TRAXSPORTABLE MAGXETOMETER. 567' 



well adapted for magnetic expeditions and journeys, and is not 

 more inferior to the magnetometers of fixed observatoi'ies, than 

 good portable astronomical instruments are to the larger ones 

 used in fixed astronomical observatories. I shall first give 

 some general remarks on this instrument ; then a description of 

 its several parts ; and lastly, observations of the Declination, 

 and its Variations, made simultaneously with the transportable 

 magnetometer and \A"ith that of the Gottingen Observatory, 

 and a measurement of the Intensity made for the purpose of 

 exhibiting its capability in that respect. 



§ I. General Remarks. 



The transportable magnetometer, figured in half size in 

 PL XXV,, fig. 1, requires in general but few explanations, as it 

 is onlj^ essentially distinguished fi'om other magnetometers by 

 its small size, and by its more compendious construction. All 

 the observations which are made with the larger magnetometers 

 may also be made with the one under consideration ; so that the 

 absolute declination, the variations of the declination, and the 

 absolute horizontal intensity, can all be measured by it; the 

 variations of the horizontal intensity can also be observed, by 

 suspending the bar employed in the experiments of deflection, 

 as a bifilar magnetometer. The exactness with which these 

 various measurements can be made is much greater than has 

 yet been attained in travelling observations ; it suffices for all 

 the purposes of magnetic travellers ; and it admits of as much 

 accuracy and certainty, in proportion to its size, as do the 

 largest magnetometers. 



The results obtained with the large instrument used in the 

 Gottingen Magnetic Observatory may be depended upon almost 

 to the immediate readings, which are to j^ of a division of the 

 scale, or to 2 seconds of arc. This supposes the scale to be at 

 least five meters from the mirror of the magnetometer, as other- 

 wise the arc value of the divisions of the scale (which are one 

 millimeter long), would be greater. Such a distance would not 

 answer in journeys, as much time would be lost in bringing all 

 the parts of the instrument into their proper positions. For 

 travelling purposes, the distances ought to be limited so as to 

 admit of the whole apparatus being placed on a table, and they 

 should therefore be about four times less. Consequently, in 

 lieu of the S-inch theodolite, which is required to do full justice 



