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Article XVIII. 



On a Transportable Magnetometer. By Wiliielm Weber. 



[This article is translated partly from the Resulfafe aus den Beohachtiingen des 

 magmtischen Vereins im Jahre 1838, and partly from manuscript commu- 

 nications from M. Weber to Major Sabine.] 



A SMALL travelling apparatus for the absolute measurement 

 of the force of the earth's magnetism has been described in the 

 Resultate for 1836*. That apparatus was not a magnetometer, 

 but rather served as an illustration of the mode in which this 

 measurement, which had previously been executed only with 

 a magnetometer, might be made with an ordinary compass 

 needle. 



The degree of accuracy attainable with such a small appa- 

 ratus, and the occasions on which it ought to be employed, were 

 examined in the memoir x'eferred to. But for the limitation 

 imposed by the want of time, or by other external circum- 

 stances, it would of course be always preferable to use the 

 magnetometer; the small apparatus being only intended to 

 serve as a substitute, on occasions when the use of the more per- 

 fect instrument is impracticable. It is very desirable to reduce 

 the number of such occasions as much as possible, by devising 

 means of removing the difficulties which often oppose them- 

 selves to the use of the magnetometer ; and this will appear the 

 more desirable, the more we consider the great difference in 

 the degree of precision attainable by the two instruments ; and 

 the more we reflect on the importance that would be given to 

 a class of observations in which magnetometers have not hitherto 

 been used, (namely, those made during distant and extensive 

 journeys and voyages,) if they could be rendered susceptible of 

 a higher degree of accuracy, certainty, and completeness. 



If the final aim of such obsei*vations were simjjly that of 

 constructing magnetic maps on which no ulterior investigation 

 was to be based, the degree of exactness to which such maps 

 should be carried might be arbitrarily determined; and pos- 

 sibly such an amount of accuracy as can be obtained without 



* Translated in the Scientific Memoirs, Part V. 

 VOL. ir. PAIlT VIII. 2 P 



