Manipulations of the Dipping Compass. 237 



slight pressure with the pencil against the side of the needle ) 

 say, when the face of the needle is westward the pivot is shoved 

 over till it is checked on the west side, and when the face of the 

 needle is eastward it is drawn over until checked on the eastern 

 side. 



4. When the needle has been placed in the compass and its 

 reading noted, it should not be immediately taken out and revers- 

 ed, but be suffered to remain exactly on its bearings, and the 

 compass be carefully reversed in azimuth, and with it, the needle, 

 when the second reading should be made, after which the needle 

 itself may be taken out and reversed. The reversals and read- 

 ings will then have the following order : 



1. Face of the compass east, and face of the needle east. 



2. Face of the compass west, and face of the needle west. 



3. Face of the compass west, and face of the needle east. 



4. Face of the compass east, and face of the needle west. 



In the above, it will be seen that although the needle in rela- 

 tion to the face of the instrument and to the " points of compass" 

 assumes four positions, yet to attain them it is removed from its 

 place in the Vs but once, and that is between the second and 

 third reading. 



I am aware that it may be objected to all of this, that although 

 I may obtain consistent results, still they may not indicate the 

 true dip, for if the pivots of each needle be alike, although not 

 perfectly cylindrical, and alike placed with reference to the axis 

 of the needle, the mean results would agree to the same error. 

 To this objection we observe first, that it is by no means proba- 

 ble that two pivots intended to be cylindrical should both assume 

 another form, and both be exactly alike, and that these forms, 

 even supposing them to be alike, should be alike placed in refer- 

 ence to the needle's axis ; and secondly, we would remark that 

 although we may involve an error by our consistent mode of ob- 

 servation, it would be a constant one, both in quantity and kind, 

 might be ascertained by a great number of comparisons with the 

 results of other instruments, or by other obvious means, and be 

 corrected by a constant equation. 



As an example of the order of reversals observed by me, I 

 give below the result of my observations at Baltimore, April 28, 

 1841. 



