NIPHER MAGNETIC OBSERVATIONS IN MISSOURI, 187S. S3 



a horizontal position, the parts ac and ab being bent in opposite 

 directions. With this damper vertical and horizontal vibrations 

 were very quickly checked. This damper was first used at the 

 Canton series, before which time no damper was used. 



At Mexico and Columbia the meridian was determined by alt- 

 azimuth observations, but, the axis of the vertical circle being 

 much worn, it was thought better to use the method of equal 

 altitudes. 



In the former case the meridian was determined by calculating 

 the solar azimuth from the formula : 



. ^ , cos s sin is -o) 



sin- \ a ^ ^ 



cos <p sni z 



where z = zenith distance, 



= declination of sun, 



(p =. latitude of the place, 



s = h (z-\-(p-{-d) . 



In the second case the reading of true south is given with suffi- 

 cient accuracy by the formula 



Jo 



h{A,-A)-h 



cos <f sni /. 



Where A and yl^ are the readings of the azimuth circle, 

 J is the change of solar declination during a time of 

 2 /, which is the elapsed time, 

 / being taken as the hour angle of the western observation, 

 (0 r= the latitude of the place. 



At Louisiana, the loss of the diagonal eye-piece prevented us 

 from taking the afternoon observations of a second determination. 

 At the next station, Hannibal, an eye-piece was extemporized, 

 and was used at all subsequent observations, although the lost 

 eye-piece was soon forwarded to us The solar image, as seen 

 through the prismatic eye-piece, was too brilliant. 



The extemporized eye-piece was made as follows : — A cylin- 

 drical pasteboard box had mounted in it a plate of very smooth 

 dark red glass, set at an angle of 45° with its axis. This was 

 done by cementing it to a cylinder very accurately cnt in a mitre- 



