1869.] Mr. C. Chambers on Magnetic Dip- Observations. 427 



week. Up to August 17, 1867, the observations commenced with either 

 end (A or B) of the needle dipping, and without remagnetizing the needle ; 

 i. e. the magnetization for the latter half of one observation was made to 

 serve for the first half of the next observation with the same needle, the 

 two needles having been kept during the interval with contrary poles ad- 

 jacent in a zinc box ; but after August 17, 1867, the needle was always re- 

 magnetized, so as to make the end A dip during the first half of the obser- 

 vation. The effect of this change of practice was to produce a marked 

 increase in the accordance of successive observations. Tables are given 

 containing every complete observation made up to the end of 1868, and 

 showing, as well as the mean dip, the partial results in each position of the 

 circle, and with each end of the needle dipping, and also the mean weekly 

 and mean monthly values. The mean dip obtained for the months April 

 to December 1867 was 19° 2'-00, and for the year 1868 was 19° 3' 87. 

 The period embraced by the observations is too limited to allow of an exact 

 determination of the rate of secular change ; nevertheless the observations 

 show distinctly that the dip is increasing. The author takes + l'*3 as the 

 rate of annual change. 



For the probable error of a single weekly determination, including the 

 effect of actual magnetic disturbance of an irregular character, the author 

 obtains for the period from April 29 to August 16, 1867, 0''67; from 

 August 23 to December 31, 1867, 0'*26 ; from January 1 to December 31, 

 1868, 0'*24. Notwithstanding the extreme smallness of these probable 

 errors, the indications of needle No. 2 exceeded those of needle No. 1 by 

 quantities ranging, in the means of periods of a few months, from about 

 to -\-5''0. An endeavour is made in another communication to explain a 

 possible cause of these differences. 



III. " On the Uneliminated Instrumental Error in the Observations 

 of Magnetic Dip." By Charles Chambers, Esq., Superinten- 

 dent of the Government Observatory, Bombay. Communicated 

 by Lieut.-General Sabine, U.A., President. Received April 15, 

 1869. 



(Abstract.) 



A single reading of one end of a dipping-needle placed in a dip- circle pro- 

 vided with microscopes for observing is liable to a variety of instrumental 

 errors, which are eliminated by taking the mean of the sixteen readings of the 

 two ends in the eight different positions included in a complete observation. 

 Nevertheless it is found that with the best modern instruments a mean 

 value results from these sixteen observations different for each different 

 needle, and that the difference between the results obtained with two 

 different needles is not the same at all times. 



The irregularities in the values of the dip observed at Bombay with two 

 needles of excellent character made by Barrow of London, led the author 



