276 



SCIENCE. 



[N. S. Vol. IV. No. 87. 



SCIENTIFIC JOURNALS. 

 TERRESTRIAL MAGNETISM, JULY. 



A Summary of the Results of the Recent Magnetic 

 Survey of Great Britain and Ireland Conducted 

 by Professors Riicker o,nd Thorpe : By A. W. 



RiJCKER. 



The writer divides his article into three parts. 



Part I : On the Accuracy of the Delineation of 

 the Terrestrial Isomagnetic Lines. Complete ob- 

 servations were made at 882 places in the British 

 Isles, thus averaging one station to every 139 

 square miles of land area. With the exception 

 of the recent magnetic survey of Holland by 

 Dr. Rijckevorsel, where the stations averaged 

 one to about every 39 square miles, this survey 

 of Riicker' s and Thorpe's is the most detailed 

 one thus far made. A first survey embracing 

 205 stations was made for the mean epoch 1886, 

 and a second one covering 667 stations for the 

 epoch 1891. In no previous case have two such 

 detailed surveys for the same region been 

 made within so short an interval of time. They 

 they therefore present a good opportunity for 

 testing the accuracy with which the positions 

 of the terrestrial isomagnetics can be inferred 

 from the observations. The conclusion is 

 reached that the accuracy of the calculated 

 values is about equal to the probable error of 

 an observation (declination ±0.^6, dip ±0.^4, hor- 

 izontal intensity in C. G. S. units ±0.00006). 



Part II : On the Accuracy of the Determination 

 of the Local Disturbing Magnetic Forces. The 

 main object of so elaborate a survey was to 

 study local magnetic disturbances. For this 

 purpose the northerly, the westerly and the 

 vertical components of the earth's magnetic 

 force were determined from the observed ele- 

 ments, declination, dip and horizontal intensity. 

 The differences between these quantities and 

 those calculated from the formulae for the iso- 

 magnetics gave the corresponding components 

 of the disturbing force. When these were 

 plotted, and lines called respectively ridge and 

 valley lines were drawn through the loci of maxi- 

 mum and minimum vertical disturbing force it 

 was found that, with few exceptions, the hori- 

 zontal disturbing forces were directed to the 

 ridge lines. Eight such regions were detected 

 in the 1886 survey and again revealed in the 

 later survey. The range of the magnitude of 



the vertical disturbing force at places where the 

 surface is comprised of sedimentory rocks is 

 about 0.00600 C. G. S. units. On granite and 

 gneiss the range is doubled, and in the neighbor- 

 hood of basalt it may be enormously increased. 

 One of the ridge lines could be traced with- 

 out difficulty for 170 miles and more. Distur- 

 bances of a secondary order were also revealed. 

 Part III : On the Relation between the Magnetic 

 and the Geological Constitution of Great Britain 

 and Ireland. The most probable causes of the 

 disturbing forces are electrical earth currents 

 and magnetic rocks, or both of these combined. 

 Riicker is led to the belief that the presence of 

 magnetic rocks is the more potent cause. The 

 article is illustrated by maps and concludes 

 with a summary of the results with regard to 

 the relationship between geological and mag- 

 netic features. 



Die magnetischen St'orungen der Yahre 1890-5, 



nach den Aufzeichnungen des Magneto graphen in 



Potsdam: By G. Ludeling. 



This is an investigation of the magnetic pertur- 

 bations for the interval 1890-95, as revealed by 

 the self- registering instrviments at the Royal 

 Magnetic Observatory, Potsdam. According 

 to the tabular and cartographic presentation, 

 there is a characteristic increase in the number 

 of disturbances towards evening and decided 

 decrease at noon. The equinoctial months are 

 the most disturbed, and in these the diurnal 

 distribution of disturbances is most pronounced, 

 a double maximum showing itself most decid- 

 edly. The annua;l distribution as determined 

 from this six-year series shows as yet no pro- 

 nounced parallelism with the distribution of 

 sun spots over the same period. There is, how- 

 ever, a nearly perfect concordance in both the 

 diurnal and in the annual period of magnetic 

 disturbance and of polar lights. 



A letter to the editor by Prof. Hellmann of 

 Berlin, with regard to an old work containing 

 magnetic declinations, notes by the editor and 

 reviews of Paulsen's papers 'On the Nature 

 and the Origin of the Avtrora Borealis, ' of Carl- 

 heim-Gyllenskold's ' Determination of the Mag- 

 netic Elements in Sweden and of Weyer's Re- 

 searches,' 'On the Magnetic Declination and 

 its Secular Variation,' conclude the number. 



