Inclinometee. li 



Cot 2^ = cot ^71 + cot 2^' 



Azimuth, 0° n = 71° 26'-75 Azimuth, 90° n' = 89° 57-25 6 = 71° 26'-75 



30° ^ = 73° 25'-50 120° ;j/ = 80° 27'- 56 ^ = 71° 7'-73 



60° 7, = 80° 22'-25 150° yf = 73° 30'-62 6 = 71° 9'-65 



The dip from the observations in the magnetic meridian differs considerably 

 from the vakies of obtained from the observations in other planes ; the latter agree 

 pretty well with those deduced from needle No. 1, April 18, 1843 (Introduction, 

 1841-42, Table 22, first series.) This coincidence of the results from two needles 

 seems to place the source of error in the metal of the instrument, rather than in the 

 axles of the needles. 



March 31, 1846. The vertical circle of the inclinometer was removed from 

 the instrument and placed horizontally, the dip needle. No. 1, was suspended by a 

 silk fibre within the circle, the needle and circle being in the same plane, the needle 

 was then vibrated horizontally, the zero of the graduations being placed in different 

 azimuths ; taking the zero on the right of the horizontal diameter (the circle being 

 in its usual vertical position) as the commencement of the graduations, and counting 

 downwards and onwards to 360, the following are the means for different positions 

 of about 80 vibrations (commencing with a semiarc of 18°, and ending with a semiarc 

 of 6°) :— 



s. 



N. end of needle at graduation, 50° Mean time of one vibration, corrected for arc, 6'257 



20° 6282 



350° 6-275 



320° 6-278 



290° 6-278 



260° 6-278 



The time of vibration varies little with the exception of that for the first posi- 

 tion ; the last two means are for positions occupied by the needle in the observations 

 of inclination, given above. 



84. The true dip, and the cause of the differences in different azimuths, is still 

 a question. No correction has been applied to the observations in this volume. The 

 difficulties noticed in connection with the lifter in the Introduction for 1843 were 

 experienced more or less in 1845 and 1846, but considerably less than in 1844.* 



* Observations were made in the following manner for the determination of the magnetic dip 

 in February 1846. The dipping needle having been placed on its supports in the inclinometer, it was 

 deflected by a magnet placed at known distances, in order to determine the ratio of the magnetic 

 moment of the deflecting bar to the vertical component of the earth's magnetism ; the moment of the 

 bar was obtained from observations of deflection and vibration for the absolute horizontal intensity, 

 whence the vertical component could be determined, and the dip from the ratio of the two com- 

 ponents. The advantage of this method over others, consists in the capability of using a powerful 

 deflecting bar whose moment can be determined with the accuracy of the observations for the hori- 

 zontal intensity. 



