MAGNETIC 0BSE11VA T 1 N S . 



199 



r<g and r^ are neither equal to each other, nor yet to r x and r e , but are, the one 



nearly three, and the other four, times as great as 



1 L6 



Assuming the theory 



employed in this discussion to be correct, we should have expected to find r s , r e , 



T 



r T , r e sensibly equal to each other, and all sensibly equal to -7--. Such, however, 



V 16 



is not the case; and, as the results for each compass all tend in precisely the same 



direction as the mean result, a doubt naturally arises whether or not the theory 



really represents the semi-circular deviation as accurately as it docs the quadrantal. 



As this doubt is founded upon observations which may possibly have been affected 



by some unknown cause of constant error — as they were all made on a single vessel 



during a single cruise — perhaps it would not be well to insist upon it too strongly; 



but at all events, it shows the necessity for further investigation of the subject, and 



especially the great want of more observations. 



The probable errors of the coefficients 5?, Q, £), (£, for each compass, when com- 



c P AP f AO 



puted from the values of A u , — , , J , — , — — , S\ and (£, given in the table 



A A A* A, A* At 



on page 193, are as follows: 



Compass. 



B 



C 



D 



e 



Admiralty Standard . 

 After Binnacle . 

 After Ritchie 

 After Azimuth . 

 Forward Alidade 

 Forward Binnacle 

 Forward Ritchie. 





± O.OOIO 

 ± O.OOI2 

 ± O.OO3O 

 ± O.OO35 



± o'ooi6 

 ± 0.0014 

 rfc 0.0022 



=1= 0.0017 

 ± 0.0023 

 ± 0.0051 

 ± 0.0035 

 db 0.0019 

 ± 0.0026 

 ± 0.0040 



± O.OOIO 



± 0.0009 

 ± 0024 

 ± 0.0033 



± O.OOI I 



± O.OOI2 

 ± 0.0018 



± O.OOIO 



± 0.0009 



± 0.0035 



± 0.0026 



± O-OOIO 



± 0.0014 

 ± 0-0015 



The following table shows, for each compass, the place at which the maximum 

 value of its deviation, b, was the greatest, together with the point on which that 

 maximum value occurred, and its amount. Also, the place at which the maximum 

 value of its deviation was the least, together with the point on which that maximum 

 occurred, and its amount. These deviations are given on the compass points, and 

 in computing them the true A was used. 



Compass and Station. 



Admiralty Standard. 



Hampton Roads 



Sandy Point 

 After Binnacle. 



Hampton Roads 



Acapulco . 

 After Ritchie. 



Hampton Roads 



Panama . 

 After Azimuth. 



Hampton Roads 



St. Thomas 



E. by N. 

 N. E. by E. 



N. W. by W. 

 N. W. by W. 



W. N. W. 



N. W. by W. 



S. E. by E. 

 S. E. 



4. 9 29' 



4- 2 3 



— 9 15 



— 5 2I 



— 12 45 



— 5 4i 



— 10 5 



— 8 45 



