various Properties of Terrestrial Magnetism. 83 



self-correction, which may be relied upon for the certainty of its 

 results, gives this mode of taking the magnetic variation a de- 

 cided advantage over the common method with the azimuth 

 compass. 



The true meridian may, of course, be found in the same 

 manner as in the azimuth compass; but as the vertical box 

 has a graduated circle at its back agreeing with those within 

 it, and a fixed pivot, the axis of which is in the common 

 centre, (for reasons immediately to be stated,) I intend to 

 adapt to it a tube on the principle of those used in theodolites, 

 having a broad flanch at each end, the solar rays being trans- 

 mitted through slits at right angles to each other in the flanch 

 at the object end, and received by the other at the opposite 

 end, which is faced with ivory to render the lines of light 

 more distinct ; and I may fix to one or more of these slits the 

 segment of a glass cylinder, as adopted by Capt. Kater in his 

 azimuth compass, for the purpose of increasing this effect. This 

 tube, being fitted to a bar which turns on the central pivot, 

 and furnished with verniers at each end, suited to the gradu- 

 ated circle, will enable the observer to find the sun's meridian 

 and altitude at the same time. The true adjustment of the 

 tube may be ensured, or any needful corrections ascertained, 

 by taking the observations with the face of the box turned in 

 opposite directions. The inside of the tube may be provided 

 with cross wires and glasses, so as to render the instrument a 

 good theodolite, if desired. 



But the most valuable property of the instrument, because 

 the most wanted, is the facility with which it will indicate the 

 intensity of the earth's magnetism in every latitude. To 

 accomplish this object, steel magnets are employed to de- 

 flect the needle from its natural dip, the greater or less in- 

 tensity existing at the place of observation being determined 

 by the extent of the deflection. I have employed two mag- 

 nets, each three inches in length (or about one half that of 

 the needle), and after having been exposed to the heat of 150° 

 at least, I have inclosed them in two brass tubes. These are 

 made to slide in a larger tube, six inches in length, on the 

 principle of the spy-glass, one being fixed at each end, so 

 that the magnetic poles alternate with each other. The large 

 tube is perforated through the middle, and exactly fits the 

 central pivot at the back of the box, where it may be con- 

 fined in its place by means of a nut and screw at the end of 

 the pivot, or taken off' at pleasure. This arrangement admits 

 of the tube or deflector being turned round in a vertical plane 

 parallel to that in which the dipping-needle moves, and of its 

 being firmly fixed in any required position within that plane. 



M 2 



