OF THE MAGNETIC NEEDLE. 



5 



necessarily be a great circle of the sphere, or the magnetic pole a mathe- 

 matical point. 



The nex.t Table contains the details of some magnetical observations 

 made at the observatory of the Surveyor General's Office, under the direc- 

 tion of Lieutenant-Colonel Hodgson. There being no inclination instru- 

 ment, or dipping needle, in the depot, the investigation was necessarily 

 confined to the determination of the declination. The paper gives all the 

 particulars, and details the precautions taken to' insure a correct result. 

 One verification which is not touched on in either of the papers I may 

 notice, as it is an important one, and is seldom adverted to. 



In making observations with a declination circle — the following is 

 the proceeding. The true azimuth of an object, or its angular distance 

 from the meridian, being determined by other methods, we observe its 

 magnetic bearing by the declination instrument, that is, the angle which 

 the object forms with the direction of the needle. Now this supposes 

 that we can determine the precise point on the limb of the instrument 

 situated in the vertical plane, passing through the line of collimation of the 

 telescope, and also in that passing through the axis of the needle. The 

 first can be done by reversing the telescope, and repeating the intersec- 

 tion of the object, taking the mean of the two readings as the true place 

 of the telescope on the limb. But the second has this difficulty attend- 

 ing it as these instruments are ordinarily made, that the needle being 

 referred to a different set of divisions, unless we are sure that the line 

 marked zero on each accurately correspond, there will be error. Tiiis is a 

 point the verification of which is not provided for by any instrument 

 I have seen, although the remedy is obvious enough — that of making the 

 same set of divisions answer for both needle and telescope. This 

 I have understood was the case in M. De Blossville's instrument. To 

 determine the amount of the error, if any, in the instrument used at the 

 Surveyor General's Office, I proceeded as follows : — 



c 



