178 TERRESTRIAL MAGNETISM. 



witli instruments wliiclihave been compared witli Kew, but these meas- 

 urements, tliongb amply sufficient for the purposes of our research, were 

 not numerous enough to serve as a firm basis for determining the dis- 

 crepancies between the various standards, so tliat the exact rehitions 

 between these important sets of apparatus are still unknown. 



The first point, therefore, to which I wish to draw the attention of 

 the section is the necessity for a full primary comparison between the 

 standard magnetic instruments in use at our different observatories. 



But if this were satisfiictorily accomplished the question would arise 

 as to whether it should be repeated at regular intervals. We have at 

 jireseut only a presumption in favor of the view that the standards 

 which W'e know are discordant are nevertheless constant. A single 

 instance may suffice to show how necessary it may be — at all events in 

 the case of outlying and isolated observatories — to put this belief to the 

 test. 



In the most recent account of the work of the observatory of the 

 Bombay government at Colaba, the dips are discussed for the period 

 of twenty years between 1872 and 1892. During this interval the 

 adjustment of the agate i^lates upon which the dip needle rolls has 

 thrice been modified. In 1877 the plates were renewed. In 1881 and 

 1887 the dip circle was taken to pieces and rebuilt. In the intervals 

 the dip as determined by several needles, but always with this circle, 

 remained approximately constant, but after each overhauling it sud- 

 denly altered, increasing by 12 minutes on the first occasion, by 23 min- 

 utes on the second, and by 20 minutes on the third. Mr. Chambers 

 states that he "can give no satisfactory account of this behavior of 

 the instrument," but suggests that "the needle gradually hollows out 

 a depression in the agate i^lates on which it rolls, and that this charac- 

 teristic of the dip circle" has not before been discovered owing to the 

 reluctance of magnetic observers to interfere with the. adjustment of 

 instruments which are apparently working well. 



I do not think that this explanation will suffice. Dr. Thorpe and I 

 employed a new dip circle in the earliest part of our survey work, which 

 has remained in accord with Kew for ten years. During that time the 

 dip has been measured some seven hundred times with it. This cor- 

 responds, I believe, to more than the amount of work done with the 

 circle at Colaba in six years, which in turn is longer than some of the 

 intervals in which the Colaba instruments gave results erroneous to the 

 extent of 20 minutes. I feel, therefore, quite sure that the difficulties 

 which have been experienced at Bombay are not due to any "charac- 

 teristic defect of the dip circle." But whatever the cause may have 

 been, surely the lesson is that, if such things can happen in so well- 

 known an institution, it is desirabh^ that we should take the moderate 

 pains required to assure ourselves whether smaller — but, possibly, not 

 unimportant — errors are gradually aflecting the results at any of our 

 observatories. 



