138 



Dr. T. E. Thorpe. 



These results are perfectly satisfactory, and show that the Dover 

 circle is calculated to afford the true dip at the various stations along 

 the line of survey. The results of the American observations are 

 contained in Table II. 



Table II. 



Station. 



Date. 

 1878. 



Local time. 



No. 1. 



CCU1C 



No. 2. 



Mean. 







h. m. 



h. m. 











New Haven . . 



July 17 



11 to 



1 3 



73 05-8 



73 05-0 



73 05 



•4 



La Junta .... 



„ 25 



9 55 „ 



1 25 



66 01-3 



66 03 -2 



66 02 



3 



Manitou .... 



Aug. 4 



11 8 „ 



4 39 



66 58-5 



66 59 9 



66 59 



•2 





8 



10 27 „ 



12 6 



67 31 -2 



67 34-3 



67 32 



•s 



Cheyenne .... 



„ 9 



3 19 „ 



4 23 



69 01 -5 



69 02-5 



69 02 



•o 



Salt Lake City. 



„ 14 



10 13 „ 



12 55 



67 01 -9 



67 02 -6 



67 02 



•3 





„ 24 



12 31 „ 



2 31 



67 23-0 



67 26-1 



67 24 



5 



Green River . . 



„ 26 



2 59 „ 



3 57 



68 20 -5 





68 20 



•5 



Grand Island . 



„ 28 



3 52 „ 



5 24 



70 19-1 



70 16-2 



70 17 



•7 



Council Bluffs. 



„ 30 



9 26 „ 



10 51 



71 05 -2 



71 06-2 



71 05 



•7 





Sept. 2 



12 33 „ 



2 21 



72 40-1 



72 38-7 



72 39 



•4 



Allegheny .... 



5 



12 15 „ 



2 10 



72 07 -0 



72 08-0 



72 07 



•5 



Washington . . 



„ 8 



11 22 „ 



1 11 



70 47 -8 



70 45 -9 



70 46 



9 



Only a single complete set of observations with needle 1 could be 

 taken at Green River. This was just concluded when a sudden and 

 violent wind-storm, which filled the air with dust and nearly upset the 

 tripods, put an end to the work. In the absence of the requisite data 

 it is scarcely possible to attempt to correct the foregoing numbers for 

 secular variation, even if that should be thought necessary ; the time 

 occupied by the survey was hardly long enough to affect the results to 

 any appreciable extent. We may, therefore, safely take 1878*7 as the 

 epoch of all the observations. 



Of the thirteen stations named in the foregoing table, New Haven, 

 Washington, Allegheny, and Chicago alone would' seem to afford data 

 for a discussion of the yearly rate of change in the inclination ; these 

 four towns (if we may consider Pittsburg and Allegheny, which are 

 merely separated by the Monongahela River, as practically identical) 

 are the only ones included in the long list given by Sir Edward 

 Sabine for the epoch 1842*5. (Contributions to Terrestrial Mag- 

 netism, No. XIII, "Philosophical Transactions," 1872.) The dif- 

 ference in dip at the four stations at the two epochs may be seen from 

 the following table : — 



Dip 1842-5. Dip 1878*7. 



Station. 



New Haven. . 73 30 



Washington 71 18 



Pittsburg .. 72 40*5 



Chicago .... 72 44*3 



73 05*4 

 70 46*9 

 72 07*5 

 72 39*4 



Diff. of Epoch. 

 36*2 years 



Diff. of Dip. 



24-6 

 31*1 

 33-0 

 4-9 



