Inclinometer. liii 



each extremity of the needle is observed, there are thus eight readings obtained. 

 The poles being changed, and the other end dipping, other eight readings are similarly 

 obtained. The means of the two readings for each position are given in this volume. 

 In changing the poles, the needle was placed on a small wooden block having 

 a hole to receive the axle ; it then received eight strokes on each face (as in the 

 method of double touch) from two magnets, each 9 inches long, | inch broad. 



83. Observations were made on April 18 and May 2, 1843, in different azi- 

 muths, in order to determine the correction due to the irregularity of the needle's 

 axle, or perhaps to the presence of iron in the vertical circle ; these observations 

 have been already given (Table 21 and Table 22, Introduction, 1841-2.) The cor- 

 rection deduced was about — 11' for needle No. 1. No correction has been applied 

 to the results in this volume. In 1846, the vertical circle was removed from the 

 instrument and placed horizontally, the dip needle was suspended by a silk fibre 

 within the circle, the needle and circle being in the same plane, the needle was then 

 vibrated horizontally, and the zero of the graduations was placed in different azi- 

 muths ; the time of vibration was found very little affected by the varying positions 

 of the circle ; it seems probable, therefore, that the correction above mentioned is 

 due solely to the imperfections of the axle. 



The inclinometer occupied a strong wooden pillar in the intensity house uncon- 

 nected with the floor. 



84. From various instrumental causes, the observations of magnetic dip in 1844 

 appear to be of little value, the difficulties in connection with the lifter already 

 noticed in the Introduction for 1843 were frequently experienced, much care was 

 bestowed upon the observations, but, without some alteration in the instrument, no 

 care seemed capable of giving consistent results. Upwards of 60 hours were ex- 

 pended in observing alone in 1844, and a half may be added for the necessary pre- 

 parations, &c. Yet, it is conceived, that a single good observation would be as 

 valuable as the mean of the whole.* The observations are given, pages 162-164. 



The mean of all the observations of magnetic dip in 1844 = 71 0, 28"7. 



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

 in February 1 846. 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. 



MAG. AND MET. OBS., 1844. 



