Febeuaey 7, 1902.] 



SCIENCE. 



223 



international polar year 1882-3 made at 

 KingTia-Fjord, South Georgia and Fort 

 Rae. 



The discussion of the earth-currents is based 

 upon the tabulated hourly ordinates from the 

 curves. Instead of measuring a single ordi- 

 nate for each hour, a planimeter was em- 

 ployed, covering a region on each side of the 

 ordinate sought. A further reduction, by 

 means of trigonometric series, was carried out, 

 in order to get a still closer approximation 

 to the true hourly values. 



The diurnal variation of the earth-currents 

 was well marked, showing two principal 

 maxima, and two secondary. An examination 

 of the equations for the mean diurnal varia- 

 tion for the diiierent years shows a slight sys- 

 tematic change from year to year. The mean 

 variation for each year is prettily shown in 

 the excellent vector diagrams, which are a 

 feature of the work. All of the curves show 

 a motion in the direction of the hands of a 

 watch, and in the details of configuration the 

 agreement is also good. A number of inter- 

 esting deductions are drawn, indicating the 

 dependence of the phenomena upon the Sun's 

 position. 



This dependence is no less clearly shown by 

 the annual change in the diurnal variation. 

 A principal maximum of current intensity 

 occurs at the time of the vernal equinox, a 

 secondary one at the summer solstice. The 

 principal minimum is at the winter solstice. 

 The east-west and south-north components for 

 the diurnal variation are very similar through- 

 out the year. As the Sun moves north, the 

 principal waves in the diurnal variation be- 

 come more pronounced, the secondary waves 

 less so. In winter the reverse is the case, 

 making the winter curves the more compli- 

 cated. Similar fluctuations are shown in the 

 coefficients of the trigonometrical representa- 

 tion, as well as by a series of vector diagrams- 

 for the months and the seasons. The latter 

 are particularly interesting, showing that the 

 mean current in winter is only about half as 

 strong as in summer. Changes of a few days' 

 duration in the character of the curves also 

 occur frequently, which the author attributes 

 to the varying relative position of nonhomo- 



geneous portions of the Svm, with reference 

 to the earth. 



A patient study was made of the diurnal 

 variation, bringing to light the existence of 

 36 secondary waves in the course of a day. 

 These occurred about 11 minutes later in the 

 north and south than in the east and west 

 line. The exact number of wavelets may be 

 open to doubt, for the personal equation car- 

 ries great weight in such investigations; but 

 at least the existence of a system of regularly 

 occurring secondary waves seems established. 



The second part of the work is devoted to 

 a discussion of the magnetic records from 

 the stations already mentioned, and the con- 

 nection between them and the earth-currents. 

 The method of treatment is essentially the 

 same as with the earth-currents, the three 

 rectilinear components of the total intensity 

 being considered. A study of the diurnal 

 variation by means of vector diagrams reveals 

 a more or less definite connection with the 

 Sun's motion. In discussing the direction of 

 the variation, two systems of coordinates are 

 used: First, the 'geopolar,' given by the hour- 

 angle and latitude of the point where the 

 direction at any hour cuts the Earth's surface ; 

 and second, the ' heliopolar,' in terms of the 

 angle with the Sun's direction (heliopolar dis- 

 tance), and the angle which the plane through 

 the direction at any hour and the Sun makes 

 with the equator. The track of the diurnal 

 variation upon the Earth's surface is de- 

 scribed in detail, and shows interesting sim- 

 ilarities between the different stations. The 

 vector diagram of the total variation is also 

 resolved into components in the directions of 

 the planes of the equator, the meridian, and 

 a plane perpendicular to both ; in each case the 

 dependence upon the Sun's position is well 

 marked. The vector diagram in heliopolar 

 coordinates takes the form of a conical sur- 

 face around the Sun. The variation vector 

 sometimes makes an angle as great as 90° with 

 the direction of the Sun, but never points 

 directly toward it, from which the conclusion 

 is drawn, that if the Sun is the cause of the 

 variation, the influence can not be exerted 

 along a straight line from the Sun to the 

 Earth. We must pass over the many interest- 



