Simple Variation Compass. 81 



of surveying, it is necessary to direct the theodolite telescope 

 to a meridian mark, or other proper object, and to read off its 

 horizontal limb ; and it is desirable that this should be done 

 both before and after observing the magnet. The collar A 

 should be adjusted to the telescope before taking the observa- 

 tions of the meridian mark ; and the magnet and its cover 

 should be put in their places, and removed again, with as de- 

 licate manipulation as possible, in order to avoid disturbing 

 the theodolite, — the cover for that purpose being made to slide 

 off and on with very little friction. Practically, I have found 

 no sensible discrepancy in the readings for the meridian mark 

 arising from disturbances caused by handling the magnet and 

 its cover ; but if it be deemed desirable to avoid altogether 

 the chance of such errors, it may be done by furnishing the 

 aperture in the cover, which illuminates the collimator cross, 

 with a piece of parallel plate-glass. The meridian mark may 

 then be seen through this glass, and observed without remov- 

 ing the cover, immediately after observing the magnet. Any 

 error due to refraction will be eliminated by reversing the 

 cover, when it is replaced after reversing the magnet, and 

 again observing the meridian mark ; but a good piece of glass, 

 such as that which is used for making the mirrors of sextants, 

 will cause no error from refraction appreciable with the mag- 

 nifying power of an ordinary theodolite telescope. 



It is always proper, however, to reverse the cover, in order to 

 eliminate the effects of any attraction it may exert on the mag- 

 net ; and for the same purpose, I have always observed with the 

 vertical limb of my Kater's circle facing alternately east and 

 west. I may add, with reference to the observations of mag- 

 netic declination given in the sequel, that I have since ascer- 

 tained that when that instrument was brought as near as was 

 possible to a collimating magnet, suspended by a very delicate 

 silk thread, and observed through a telescope, it caused no per- 

 ceptible deflection. 



The practical limit to the accuracy of observations made by 

 such an instrument as that which I have described, is the fric- 

 tion of the point of suspension. When the needle shows any 

 symptom of not swinging freely, the point should be carefully 

 sharpened on a hone, — a process which any one may learn to 

 perform for himself. 



VOL. I. to. i. — JAN. 1855. f 



