Horizontal Component of the Eartlis Magnetic Field, 495 



excited by a dynamo having a low-resistance armature, and 

 giving a potential of 100 volts. The relative strengths of the 

 different magnets remained the same, and the strength only 

 very slightly increased, thus showing that they were nearly 

 magnetized to saturation. 



The greatest trouble in the determination of the intensity 

 of the earth's magnetic field arises out of the variations of the 

 field itself — variations in declination causing changes of zero, 

 and variations of intensify between different determinations 

 causing apparent inaccuracy in the results. 



This trouble was in the later measurements got over by 

 means of a permanent magnetic vibrator, the period of vibration 

 of which, at different times, gives a comparison of the intensity 

 of the magnetic field at these times. In order to render the 

 result of any one determination free from error due to diurnal 

 variation, the period of the vibrator is taken at the beginning 

 of the experiment, between the deflection and the oscillation 

 experiment, and again at the end of the experiment. If these 

 agree well and the results also agree well among themselves, the 

 value found is considered reliable ; if not, the whole experi- 

 ment is discarded. This vibrator has only lately been adopted, 

 and hence the results up to those taken on June 11th are not 

 corrected for diurnal variation. The results are tabulated in 

 the order in which they were taken on each day ; and it will 

 be observed that the earlier results are generally the smaller, 

 this being due to diurnal variation. The last three results 

 given in the Table are corrected to noon for diurnal variation. 



It has been found impossible to get consistent results at all 

 on several occasions, owing to extraordinary variations, and 

 this was notably the case on the 1st of September. It is 

 interesting to note how readily the vibrator shows extra- 

 ordinary variations which take place suddenly. The observer 

 has only to watch for sudden changes of amplitude. These 

 changes of amplitude are sometimes extremely marked, the 

 vibrator being almost stopped on some occasions, and after- 

 wards gradually worked up to a large swing. 



Table I. gives the results of the various determinations. 

 The meanings of the numbers in the different columns are 

 clearly indicated in the headings. 



