TERRESTRIAL MAGNETISM. 177 



This point has been investigated by Dr. Van Eijckevorsel, who five 

 yeai\s ago visited Kew, Pare St. Maur, Wilhelmshaven, and Utrecht, 

 and, using his own instruments at each place, compared the values of 

 the magnetic elements determined by himself with those deduced from 

 the self registering apparatus of the observatory. 



The discrepancies between the so called standards which were thus 

 brought to light were quite startling and j^rove the necessity for an 

 investigation as to their causes. 



Magneticians had long been aware that the instruments used by 

 travelers should be compared at the beginning and end of a journey 

 with those at some fixed observatory to make sure that the compara- 

 tively rough usage to which they are subjected has not affected their 

 indications. But Dr. Van Eijckevorsel's expedition first drew general 

 attention to the fact that there are serious dififereuces between the 

 standard observatory instruments themselves. 



The importance of a careful comparison between them was at once 

 recognized. The magnetic subcommittee of the International Meteor- 

 ological Conferen(;e held at Munich in the autumn of 1891 resolved 

 that it is " necessary that the instruments employed for absolute meas- 

 urements at the difl'erent 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 sur- 

 veys. To cite instances from the northwest of Europe only, observa- 

 tions 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 can not 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 Avill take part in it. 



It may fairly be left to each nation to determine for itself the rela- 

 tions 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 jiresent moment in a position to do. As far 

 as I know nobody has ever carried a magnetometer backward and 

 forward 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 Stonyhurst, Falmouth, and Valentia, 

 SM 94 12 



