THE SMITHSONIAN INSTITUTION. 



387 



scale-readings are obtained on fixed scales attached to the reading 

 telescopes t, t, t, the permanence of the direction of which is checked 

 by reference marks on the opposite wall of the room. 



The three instruments are supported on brick piers 16 X 16 inches, 

 having no connexion with the floor, while the reading telescopes and 

 the record cylinders for D and H rest on wooden brackets bolted to 

 the wall; the record cylinder V is supported on a wooden stand which 

 rests on the floor, as do also the tripods which support the gas- 

 burners. The centres of the mirrors and of the cylinders are all in 

 one horizontal plane three feet above the floor. The distance from 

 each mirror to its record cylinder is 9 feet 6 inches, affording a scale 

 value of one-fifteenth of an inch to one minute of angular motion of 

 the magnet. The distance from burner to mirror is about 28 inches. 



DECLINATION INSTRUMENT. 



Fig. 2. 



Pig. 2 is a side view, on a scale of 

 one-tenth the actual size, of the in- 

 strument for recording the changes 

 in the declination. A marble slab 

 sustains a frame made of brass tubes, 

 supporting a top piece, from which 

 the magnet is suspended by a bundle 

 or skein of parallel silk fibres. The 

 suspension skein is enclosed in a glass 

 tube, and the magnet, with its attach- 

 ments, is also enclosed in a case with 

 sides of plane plate glass. The top 

 pane is likewise covered over by a 

 round glass cap. The magnet is 9 

 inches long, 1£ inch wide, £ inch 

 thick, and is held by a clamping 

 frame, the upper part of which 

 carries the hook by which it is sus- 

 pended, w r hile to the lower part is 

 attached the frame which holds the 

 speculum and the plane mirror for 

 scale readings. In Fig. 2 the specu- 

 lum is seen edgewise; compare, also, 

 Fig. 4 below, which gives a front 

 view. The mirror frame is pivoted 

 to the carrier, and is held in position 

 by a tangent screw, by means of 

 which its direction can be nicely 

 adjusted to any part of the record 

 cylinder. 



The magnet is closely surrounded 

 by a copper "damper," which, acting 

 by induction when the magnet is in 

 motion, checks the vibrations occa- 

 sioned by the changes of direction, 

 and keeps the oscillations within nar- 



J2ia:n ofjjamoei-. 



Declination instrument. 



