1891.] President's Address. 227 



Geodetic Union, at its last meeting in the autumn of 1890, on the 

 motion of Professor Foerster, of Berlin, resolved to send an astro- 

 nomical expedition to Honolulu, which is within 9 of the opposite 

 meridian to Berlin (171 west from Berlin), for the purpose of making 

 a twelve months' series of observations on latitude corresponding to 

 twelve months' analogous observations to be made in the Royal 

 Observatory, Berlin. Accordingly Dr. Marcuse went from Berlin, 

 and, along with Mr. Preston sent by the Coast and Geodetic Survey 

 Department of the United States, began making latitude observations 

 in Honolulu about the beginning of June. In a letter from Professor 

 Foerster, received a few weeks ago, he tells me that he has already 

 received from Honolulu a first instalment of several hundred deter- 

 minations of latitude, made during a first three months of the pro- 

 posed year of observations ; and that, in comparing these results 

 with the corresponding results of the Berlin Observatory, he finds 

 beyond doubt that in these three months the latitude increased in 

 Berlin by one-third of a second and decreased in Honolulu by almost 

 exactly the same amount. Thus, we have decisive demonstration 

 that motion, relatively to the Earth, of the Earth's instantaneous axis 

 of rotation, is the cause of variations of latitude which had been 

 observed in Berlin, Greenwich, and other great observatories, and 

 which could not be wholly attributed to errors of observation. This, 

 Professor Foerster remarks, gives observational proof of a dynamical 

 conclusion contained in my Presidential Address to Section A of the 

 British Association, at Glasgow, in 1876, to the effect that irregular 

 movements of the Earth's axis to the extent of half a second may be 

 produced by the temporary changes of sea-level due to meteorological 

 causes. 



It is proposed that four permanent stations for regular and con- 

 tinued observation of latitude, at places of approximately equal lati- 

 tude and on meridians approximately 90 apart, should be established 

 under the auspices of the International Geodetic Union. The reason 

 for this is that a change in the instantaneous axis of rotation in the 

 direction perpendicular to the meridian of any one place would not 

 alter its latitude, but would alter the latitude of a place 90 from it in 

 longitude by an amount equal to the angular change of the position 

 of the axis. Thus two stations in meridians differing by 90 would 

 theoretically suffice, by observations of latitude, to determine the 

 changes in the position of the instantaneous axis ; but differential 

 results, such as those already obtained between Berlin and Honolulu, 

 differing by approximately 180 in longitude, are necessary for elimin- 

 ating errors of observation sufficiently to give satisfactory and useful 

 results. It is to be hoped that England, and all other great nations 

 in which science is cultivated, will co-operate with the International 

 Geodetic Union in this important work. 



