DISCUSSION OF THE VERTICAL FORCE. 5 



Before putting the reflecting vertical force magnetometer in its place in January, 

 1841, observations were made for its correction for temperature by means of deflec- 

 tions ; the result, however, was not satisfactory, owing to the small difference in 

 the deflections at high and low temperatures, and the necessity of keeping the bar 

 at a proper distance from the declinometer to prevent the possibility of a permanent 

 change of magnetism. The weight of the mirror and other fixtures of the bar ren- 

 dered the method of horizontal oscillations impracticable without their removal, and 

 it was finally decided to determine the value of q by means of a subsidiary instru- 

 ment kept at a imiform temperature in a separate building, while the vertical force 

 instrument at the observatory was subject to considerable fluctuations of tempera- 

 ture. The subsidiary instrument consisted of a small dipping needle mounted on a 

 knife edge, and rendered horizontal by weighting it. The indications, however, 

 did not prove very satisfactory; 14 scale divisions were indicated as the correction 

 for 1 change in temperature. Subsequently an inclinometer, according to Prof. 

 Lloyd's plan, was mounted as a subsidiary instrument, and observed twice a day 

 with the vertical force instrument at the observatory. The mean values, expressed 

 in scale divisions, thus found between February, 1843, and January, 1844, are as 

 follows: 



13.3 14.3 14.4 12.3 12.2 13.1 and 15.4. 



Average value 13.56 + 0.25. In the first reduction the value 13.5 was used. 



(B.) Investigation of the temperature coefficient from the regular series of 

 observations. We will first examine the principal series observed between 1841 

 (February), and 1845 (June), with the reflecting magnetometer. In February, 

 March, April, and May, 1841, the readings gradually increased and approached the 

 end of the scale, requiring a readjustment of the instrument after May 22. It was 

 supposed that 529 scale divisions would be an approximate correction for refer- 

 ring the observations to the indications of the scale subsequent to May 22, the 

 uninterrupted series of observations commencing with June 1, 1841. The following 

 table contains the uncorrected monthly means of the vertical force magnetometer 

 together with the observed mean monthly temperature taken directly from the 

 record. The tabular means for January, February, and March, 1843, when the 

 instrument was read only once a day (at 2 h 17 m P. M.), were obtained as follows: 

 The difference between the daily mean and the mean at 2 h 17 m P. M. was ascer- 

 tained for each month, from the records of the preceding year (1842) and the fol- 

 lowing year (1844). The mean correction to the average reading at 2 h 17 ra P.M. 

 to refer the same to the mean of the day and month is +18.6, +14.4, and +11.2 

 scale divisions for the months of January, February, and March, respectively. 

 These corrections have been applied. 



