THIRTY THOUSAND MILES IN CHINA 3 



of the Department of Terrestrial Magnetism. A third volume 

 is in course of preparation. 



At each station astronomical observations of the sun, 

 usually by means of a theodolite, are made so as to determine 

 the latitude and longitude of the station, and the azimuth 

 or true bearing of some distant fixed mark as seen from the 

 station. Elevation above sea level is determined by hypso- 

 meter or aneroid barometers. Standard mean time is given 

 by a battery of watches and chronometers very carefully 

 carried. With a magnetometer the magnetic declination or 

 deviation of the compass from the true or astronomic north 

 is determined and also the intensity of the earth's magnetic 

 force at that place, while with a dipcircle the dip or inclin- 

 ation of the magnetic needle in the vertical plane is observed. 

 These three elements — declination, dip, and intensity — give 

 a full knowledge of the earth's magnetic force at that place 

 both as to its magnitude and its direction for the time when 

 the observations were made. But the case is greatly com- 

 plicated by the fact that both the direction and the magnitude 

 of the earth's force at a given place undergo cyclical changes 

 throughout the day, with monthly and secular variations 

 superposed. It becomes then a difficult task to ascertain 

 the precise correction to apply to any given compass reading in 

 order to ascertain the true bearing at the time of observation. 

 Continuous records of these changes in the earth's field must 

 be secured at as many permanent stations as possible while 

 at a large number of well distributed points throughout the 

 intervening territory detailed observations must be made and 

 repeated from time to time, so that both the absolute values 

 and the rate of change may be determined. The only station 

 in China where continuous records are being secured is at 

 Lukiapang, under the Zikawei Observatory, itself an integral 

 part of the missionary establishment of the French Catholic 



Church. 



The extensive observations already made throughout the 

 world, both on land and sea, under the auspices of the 

 Carnegie Institution of Washington will, when combined with 

 the results of surveys maintained by the various govern- 

 ments, give the first comprehensive magnetic survey of the 

 alobe and thus afford the basis of a much more intelligent 

 solution of the problems of terrestrial magnetism than has 

 hitherto been possible. Any more detailed reference would 

 be out of place here; but it is a cause for congratulation that 

 China has been included in the areas studied even ahead oi 

 the time when her own government will be able to undertake 

 &uch work and thus contribute her just share as a full-nedged 

 modern nation to the solution of a world problem. 



