August 9, 1894] 



NATURE 



345 



Magneticians had long been aware that the instruments used 

 by travellers should be compared at the beginning and end of a 

 journey wiih those at some fixed observatory, to make sure that 

 the comparatively rough usage to which they are subjected has 

 not aflfected their indications. But Dr. Van Rijckevorsel's ex- 

 pedition first drew general attention lo the fact that there are 

 serious differences between the standard observatory instru- 

 ments themselves. 



The importance of a careful comparison between them was 

 at once recognised. Ttie Magnetic Sub-Committee of the 

 International Meteorological Conference, held at Munich in the 

 autumn of 1S91, resolved that it is "necessary that the instru- 

 ments employed for absolute measurements at the different 

 observatories should be compared with each other and the 

 results published." As far as I am aware nothing has been 

 done to give effect to this resolution, but the necessity for such 

 an international comparison is urgent. The last few years have 

 been a period of unexampled activity in the conduct of local 

 magnetic surveys. To cite instances from the north-west of 

 Europe only, observations have recently been made on a more 

 or less extended scale in the United Kingdom, France, Holland, 

 North Germany, and Denmark. 



It will be absurd if these surveys cannot be collated and welded 

 into a homogeneous whole, because we are in doubt whether 

 the indications of our standard instruments for the measurement 

 of declination and dip differ by five or six minutes of arc. 



If, however, an official international comparison of the 

 magnetic standards in use in different countries is instituted it 

 is probable that only one observatory in each country will take 

 part in it. 



It may fairly be left to each nation to determine for itself 

 the relations between the results of measurements made in its 

 own institutions. Apart, therefore, from all other reasons, we 

 in England would only be able to make the best use of an 

 international comparison if we had beforehand set our own 

 house in order, and were able at once to extend the results of 

 experiments made at Kew or Greenwich to .Stonyhurst, Valentia, 

 and Falmouth. 



This we are not at the present moment in a position to do. 

 As far as I know nobody has ever carried a magnetometer back- 

 wards and forwards between Kew and Greenwich to test the 

 concordance of the published results. During the recent survey 

 single or double sets of observations have been made at Stony- 

 hurst, Falmouth, and Valentia, with instruments which have 

 been compared with Kew, but these measurements, though 

 amply sufficient for the purposes of our research, were not 

 numerous enough to serve as a firm basis for determining the 

 discrepancies between the various standards, so that the exact 

 relations 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 com- 

 parison between the standard magnetic instruments in use at 

 our different observatories. 



But, if this were satisfactorily accomplished, the question 

 would arise as to whether it should be repeated at regular inter- 

 vals. We have at present only a presumption in favour of the 

 view that the standards which we 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 Colaha, the dips are discussed for 

 the period of twenty years between 1S72 ami 1892. During 

 this interval the adjustment of the agate plates upon which the 

 dip needle rolls has thrice been modified. In 1877 the plates 

 were renewed. In 18S1 and 18S7 the dip circle was t.aken to 

 pieces and rebuilt. In the intervals the dip as determined by 

 several needles, but always with this circle, remained approxi- 

 mately constant, but after each overhauling it suddenly altered, 

 increasing by 12' on the first occasion, by 23' on the second, 

 and by 20' on the third. Mr. Chambers states that he " can 

 give no satisbctory account of this behaviour of the instrument," 

 but suggests that " the needle gradually hollows out a depression 

 in the agate plates on which it rolls, and that this char.acteristic 

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

 relnctance of magnetic observers to interfere with the adjust- 

 ments 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 



NO. 1293, VOL. 50] 



survey work, which has remained in accord with Kew for ten 

 years. During that time the dip has been measured some 700 

 times with it. This corresponds, I believe, to more than the 

 amount of work done with the circle at Colaha 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'. 

 I feel, therefore, quite sure that the difficulties which have been 

 experienced at Bombay are not due to any "characteristic 

 [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 desirable that we should take 

 the moderate pains required to assure ourselves whether smaller 

 — but, possibly, not unimportant — errors are gradually affecting 

 the results at any of our observatories. 



This brings me to my next point, namely, that if we are to 

 draw conclusions from the minor differences between measiire- 

 ments of secular or diurnal change made in the observatories, 

 it is not only necessary that we should know whether the 

 instruments are strictly comparable and constant, but the 

 observations must be reduced by precisely the same methods. 



In 1886 the late -Mr. Whipple drew the attention of the 

 British Association to the fact that there was a systematic dif- 

 ference between the diurnal ranges of declination at Greenwich 

 and Kew. His results were based on the three years 1870-72. 

 In 1890 two of my students, Messrs. Robson and S. W. J. 

 Smith, extended the comparison to three more recent years 

 (1S83-6-7), and obtained results in complete accord with those 

 of Mr. Whipple. 



It is well known that the average daily oscillation of the 

 magnet is affected by the magnetic weather. Sabine showed 

 that magnetic storms do not merely buffet the needle now in 

 this direction and now in that— they affect its average be- 

 haviour, so that the mean swing east and west is different 

 according as we deduce it only from days of magnetic calm or 

 include ttiose of storm. 



Mr. Whipple reduced the Kew observations by two methods,' 

 one of which depended on the calmest days only, while the 

 other included those which were moderately disturbed. Neither 

 agreed exactly with the method in u.e at Greenwich, but the 

 difference between the resultsdeJuced from them was so small when 

 compared with the difference between either and that obtained 

 at Greenwich, that it seemed possible that the diurnal variations, 

 even at these closely neighbouring places, might differ appreci- 

 ably. The question whether this is so has now been answered. 

 In 1890, at the resquest of the Kew Committee, the Astronomer 

 Royal undertook to select early in each year five quiet days in 

 each of the preceding twelve months. It was also agreed that, 

 whether they adopted other methods or not, the chief English 

 magnetic observatories should determine the diurnal variations 

 from these days alone. The Greenwich- and Kew observations 

 for 1890 have therefore been worked up in exactly the same 

 way, with the result that the discrepancy, which had persisted 

 for twenty year-, has entirely disappeared, and that the two 

 diurnal ranges at the two observatories are in as close accord as 

 could be expected. 



If, therefore, we may judge from a single year, the cause of 

 the difference lay in the choice of days. Greenwich will in 

 future give us two diurnal variations, one obtained from the 

 most quiet days only, the other from all days except those of 

 violent storm, and in these we shall have most valuable data for 

 stud)ing the mean effect of disturbances on the diurnal varia- 

 tion. 



To this satisfactory conclusion I have only one suggestion to 

 add. The Astronomer Royal and M. Mascart now publish for 

 the same stormy days the photographic traces by which the history 

 of a magnetic storm is mapped. Is it possible for Greenwich and 

 Paris also to agree in their choice of calm days for the calcula- 

 tion of the diurnal variation, so that a precise similarity of 

 method may obtain not only between the English observatories, 

 but between England and France? 



The importance of co-operation between institutions engaged 

 on the same tasks having been illustrated, I am glad 10 be able 

 to announce that another step is about to be taken in the same 

 direction. For some years, in spite, I believe, of great finan- 

 cial difficulties, the Cornwall Royal Polytechnic Society has 

 maintained a magnetic otiservatory at Falmouth. The results 

 of the observations have hitherto been printed in the Journal of 



1 S.ibine's and Wild's. 



- The Greenwich observations for subsequent years have not yet l>een 

 published. 



