5G8 WEBEU ox A TRANSPORTABLE MAGNETOMETER. 



to the great magnetometers, one of about three or four inches 

 may be used without disadvantage, being at once more conve- 

 nient and more CEconomical, and still allowing the measurements 

 to be depended upon to within from 10 to 20 seconds of arc. 

 In considering the subject further, it will be seen, that admitting 

 the necessity in the travelling apparatus of diminishing the ob- 

 servation distance, a diminution in the size of the magnetometer 

 (which would not be admissible under other circumstances), does 

 in no degree detract from the accuracy of the observations. For 

 with a distance four times less, the degree to w'hich the reading 

 can be depended on (and which it is desired to preserve), is not 

 affected, though the proportion of the magnetic force of the 

 magnetometer to external disturbing influences be lessened in 

 the same proportion. It may be assumed, that the magnetic force 

 decreases as the cube of the linear dimensions of the bar, and 

 external disturbing influences as the square, whence it follows 

 that the bar may be made four times less without diminishing 

 the dependence to be placed on the readings (which is to about 

 the one tenth part of one division of the scale). If, with this di- 

 minution, other arrangements are adopted for guarding against 

 external disturbing influences more carefully than has been 

 hitherto found necessary with the larger magnetometers, there 

 will be no material disadvantage in pushing the diminution in 

 size somewhat further, having in such case only to preserve the 

 degree of deijcndence w^hich may be placed on the readings. 

 In fact, the length of the bar of 600 millimeters has been 

 reduced to 100 millimeters; and observation has shown that 

 the readings may be equally depended upon ; with this differ- 

 ence only, that the divisions, as read off, have a four times 

 greater value of arc than in the case of the larger magnetome- 

 ters, so that one division of the scale is equivalent to 80 seconds 

 of arc instead of 20 seconds. Hence it appears, that by suit- 

 able arrangements, all the advantages of the magnetometer may 

 be secured to magnetic expeditions ; of course, without that 

 highest degree of precision attainable only in fixed observa- 

 tories, where nothing is wanting in construction and arrange- 

 ment. 



The instrument to be described affords these advantages in 

 respect to the absolute declination and its variations, and still 

 more in respect to the absolute measurement of the horizontal 

 intemifg ; for in the Resultate for 1836, p. 88, it has been 



