DEPARTMENT OF TERRESTRIAL MAGNETISM. 307 



are so numerous as to warrant drawing the definite conclusion that an appre- 

 ciable variation in the Earth's magnetic field occurs during a solar echpse. 

 This particular variation is termed here the " solar-echpse magnetic varia- 

 tion." 



(6) The range of the solar-eclipse magnetic variation, according to the par- 

 ticular magnetic element, is about 0.1 to 0.2 that caused by the solar-diurnal 

 variation on undisturbed days. The effects are of a more or less compUcated 

 character, according to location of observation-station in the zone of visibility. 

 The effects caused during the local eclipse-interval are superposed upon those 

 caused by the continued disturbance of the Earth's magnetic field in the 

 region over which the shadow-cone has already passed. It is thus possible 

 to discern effects having a period approaching that of the local ecHpse-interval 

 and others having a period approximately that of the entire or terrestrial 

 echpse-interval. 



(c) The general character of the system causing the solar-eclipse magnetic 

 variation is the reverse of that causing the day-light portion of the solar- 

 diurnal magnetic variation. The range of the ecHpse variation is comparable 

 with that of the lunar-diurnal variation, and, like the latter, the variation 

 usually consists of a double oscillation during its period of development. 



(d) The range of the apparent effect on the intensity of magnetization of 

 the Earth during the solar-eclipse magnetic variation, is about equal to that 

 found associated with a 10 per cent change in the solar radiation, as shown by 

 changes in the solar-constant values. 



(e) The results at the high mountain-station. Corona, Colorado, indicate 

 that the magnetic effects during a solar eclipse may be modified and even 

 intensified by altitude of station, topography, and meteorological conditions. 

 In view of the bearing of these results upon the theoiy of the solar-eclipse 

 magnetic variation and possibly upon the theory" of other variations of the 

 Earth's magnetic field as well, it will be highlj^ desirable in the planning of 

 future ecUpse work to include as many mountain-summit stations as con- 

 veniently possible. 



Part 2 gives a synopsis of the meteorological and miscellaneous observations 

 obtained at the various stations. 



Part 3 deals mainly with the atmospheric-electric observations made at 

 Lakin, Kansas, the observing-station of the Department of Terrestrial Mag- 

 netism. The apparatus and methods employed are described and various 

 curves and tables are given to set forth the results obtained. The chief con- 

 clusions derived from the atm.ospheric-electric observations during the total 

 solar ecHpse of June 8, 1918, at Lakin, located in the belt of totahty, at an 

 elevation of 900 meters, on an irrigated, grassy plain, far from either sea or 

 mountains, are summarized as follows : 



(a) A decrease of about 20 per cent in the value of the potential-gradient 

 at the time of totality and continuing for a period of about 20 minutes there- 

 after. 



(6) The short-period fluctuations which usually characterize the potential- 

 gradient and which were very marked both before and after the eclipse were 

 almost totally absent during the period of minimum potential-gradient, 

 namely, during totality and the 20 minutes immediately following. 



(c) The unipolar conductivities, X+ and X_, each showed an increase, of 

 the order of 20 per cent, during a period beginning several minutes before 

 totahty and continuing until about 30 minutes after totahty. Inasmuch as 

 7^ + and X _ were similarly affected, the remark concerning them apphes also to 

 the total conductivity. 



(d) The air-earth current-density, as computed from total-conductivity 

 and potential-gradient data, showed a rapid increase for about 10 minutes 



