THK 



LONDON and EDINBURGH 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



FEBRUARY 1884. 



XVII. Notice of an Instrument for ascertaining various Pro~ 

 perties of Terrestrial Magnetism, and affording a permanent 

 Standard Measure of its Intensity in every Latitude. By 

 Robert Were Fox*. 

 TNTERESTING as is the subject of terrestrial magnetism, 

 *- our acquaintance with many of its phenomena is very li- 

 mited, owing principally to the imperfection of our instru- 

 ments, and the time and minute attention which the best of 

 them require, to enable us to obtain results which can be con- 

 sidered even as approximations to accuracy. Impressed with 

 these defects, I have directed some attention to the subject, 

 and have succeeded in constructing an instrument, which I 

 think will be found highly useful in the prosecution of mag- 

 netical researches in any part of the world. 



It consists of a dipping-needle with a transverse axis, and 

 pivots working in jewel holes, like those used for the ba- 

 lance of a chronometer: the needle is contained in a vertical 

 box of the usual form, having a concentric brass plate inserted 

 in the back, which admits of its being turned round on its 

 axis independently of the box. The jewels are set precisely 

 in this axis, one in the concentric plate itself, and the other 

 in an arm connecting the axis with the plate. This contri- 

 vance admits of the needle's taking any direction without in- 

 terruption by the arm, which is moveable by pins affixed to 

 the back of the plate. In the construction of this instrument, 

 every precaution usually taken for the true adjustment of a 

 dipping-needle should be observed, the friction of the pivots 



* Communicated l>v the Author. 

 Third Scries. Vol. 4. No. 20. Feb. 1834. M 



