.4. D. Bache on the Horizontal Component of Magnetic Force. 377 



This table shows bj the change of sign and progress of values 

 that the epoch of change occurs some time after the equinoxes, 

 and that the nriaxiraum of variation is reached about the time of 

 the solstices, a result which corresponds closely with that de- 

 rived from the discussion of declination in Part II. Represent- 

 ing the average of the results for each half year by the usual 

 analytical formula, we find that the change takes place about 

 twenty-two days after the equinoxes which is about twelve days 

 later than we found for the declination. 



To obtain the best result from observations, the values of 

 table No. I were thrown into the usual analytical form, equa- 

 tions for each month, for the half year, and for the year's results 

 having been obtained. The analytical results for a determinate 

 hour of any month differ but slightly from the observed results. 

 For example the differences for August between the computed 

 and observed results, differ in no case more than three scale 

 divisions and generally range between and 2. Dingrams (C) 

 and (D) represent these results, the dots corresponding to the 

 observed quantities and the curves to the results of analysis. 

 The results for the summer months are given in diagram C, and 

 those for the winter in diagram (D). The comparison of the 

 two diagrams shows the much greater range of the variation of 

 horizontal force when the sun is north of the equator, as was 

 also found from the discussion of the magnetic declination. 



Diagram (E) gives the curves of regular solar diurnal variation 

 of the horizontal force for the summer, the winter, and for the 



Table VIII, contains the computed values of the time and 

 amount of the morning maximum and minimum and of the after- 

 noon maximum. The values for the secondary afternoon mini- 

 mum are taken from the diagrams. The time of the a. m. maxi- 

 i^um and minimum is within the nearest eighth minute, that of 

 the p. M. maximum within the nearest tenth minute, that for the 

 P- ii. secondary minimum within the nearest hour. The change 

 of force is expressed in scale divisions. 



The extreme variation in the epoch of the A. M. maximum is 

 2^ loni. The variations for the A. M. minimum is l^ bo^, for the 

 ^■}^. maximum it is 2^30™ and for the secondary afternoon 

 rninimum between three and four hours. In all cases the earlier 

 hours occur in the summer season. Table IX shows the diurnal 

 ^nge, expressed in scale divisions, parts of the horizontal force 

 and in absolute measure. In the second column the range be- 

 tween the A. M. maximum and minimum is given ; in the third 

 column that between the a. m. minimum and the P. M. maximum. 

 These two amplitudes for A. M. and for A. M. and P. M., are further 

 illustrated in diagram (F), which shows the curve to be double 

 crested with maxima near the time of the equinoxes and the 

 greater of these near the autumnal equinox. 



