A Magnetic Survey of the Fortieth Parallel. 



145 



For the present epoch, the calculated value is 4'4292. My deter- 

 minations gave 4'3608, in close accordance with Mr. Schott's observa- 

 vation made on June 15, 1878, of 4'366. The difference between the 

 observed and computed values affords further evidence of the fact 

 pointed out by Mr. Schott, that the secular changes in the neighbour- 

 hood of Washington are of a mixed progressive, and periodic 

 character. The divergences exhibited both in this case and in that of 

 the dip, further show how very unsafe it is to employ numerical 

 formulae of this kind beyond the limits of the time within which the 

 observations upon which they are based are comprised. 



Declination. 



The unifilar magnetometer was provided with a mirror and hori- 

 zontal telescope in accordance with the plan first proposed by Dr. 

 Lloyd in the "Proceedings of the Royal Irish Academy " for 1858; 

 hence the observations could be carried out in the manner described 

 by Mr. Welsh in the " Admiralty Manual of Scientific Inquiry." 



As a rule, six readings of the position of the magnet were taken ; 

 three of these were with the scale erect and three with the scale 

 inverted : at Cheyenne, however, owing to lack of time, only four 

 observations were possible. 



Care was always taken that the transit-mirror was in adjustment 

 before commencing the observations. The operation of adjustment 

 for collimation-error was greatly facilitated by a neat contrivance 

 which Dr. Balfour Stewart had caused to be introduced in the origi- 

 nal construction of the instrument. In order to eliminate any error 

 which might arise from the plane of the mirror not being parallel to 

 its axis, the observations were always made first with the mirror in its 

 normal position and then after reversal on its bearings. Care, too, 

 was always taken to remove, as far as practicable, all torsion from the 

 suspending thread. When time permitted, the magnet was replaced 

 by a torsion-plummet of equal weight, the position of which was 

 noted after an interval of an hour or so, a suitable correction being- 

 applied to the original observation if it were found that any torsion 

 still remained in the thread. 



As the correct determination of the sun's azimuth was mainly 

 dependent upon an accurate knowledge of local time, every oppor- 

 tunity was taken to ascertain the errors and rates of the chronometers 

 employed. In order to guard against the possible accidents of travel 

 over a country so rough and difficult as that of many parts of Western 

 America, I provided myself with three of these instruments. That 

 lent to me by Dr. Stewart was a large-sized marine chronometer, by 

 Frodsham, No. 4,066 ; in addition, I hired a pocket instrument from 

 the same makers, No. 5,674, which had behaved well as a "hack" 

 chronometer for some months previously: this was always carried 



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