Magnetic Declination and Horizontal Force and Solar Spots. 65 



diurnal range of declination in each individual month is taken to 

 represent (relatively to other months) the magnetic energy of the 

 month, and similarly for horizontal force. By the mean diurnal 

 range is to be understood a number formed as follows. Means of 

 the indications at each separate hour of the day being taken through 

 a month (omitting days of extreme disturbance) the difference 

 between the least and the greatest of the twenty-four mean values is 

 the monthly mean diurnal range. The numbers are obtained from 

 the Greenwich annual volumes, but those for the years 1895 and 

 1896 not having been yet published, the Astronomer Royal has 

 kindly allowed me to use them as necessary for the purposes of this 

 paper. The very small correction for temperature required by the 

 horizontal force results from 1878 to 1882 as printed, has been duly 

 applied : beginning with 1883, the values are printed corrected for 

 temperature. 



Thus is obtained, both for declination and horizontal force, 

 results strictly comparable with those of the previous paper, giving 

 in all a series of results for fifty-six years. In any graphical repre- 

 sentation of unexplained phenomena, it is most important that 

 there should be ready reference to the numerical data on which it is 

 founded, to enable those who might wish to test the work the means 

 of so doing without great inconvenience, otherwise the graphical 

 representation alone can carry no proper conviction. The numbers 

 for the years 1841 to 1877 are to be found in the previous paper; it 

 is therefore necessary to give here only the corresponding numbers 

 from 1878 to 1896. These are contained in Table I. The numbers 

 for horizontal force are given, as in the previous paper, in parts of 

 the whole horizontal force taken as unity. The relation in magni- 

 tude of the westerly force (declination) to the northerly force (hori- 

 zontal force) will be understood by considering that one minute of 

 arc of declination corresponds to 0*00029, that is 29 of the hori- 

 zontal force unit of Table I. Examining now this table, it will 

 be seen that there is an annual inequality in the magnitude of the 

 diurnal range, the summer numbers being much greater than the 

 winter numbers. In order, therefore, to estimate progressive 

 change, it is convenient to form a number for each month that shall 

 be free of annual inequality, to allow the progressive change better 

 to appear. Assuming the different months to be of the same length 

 this is done, as before, by taking the mean of each twelve consecu- 

 tive monthly numbers, beginning successively with each individual 

 month, first, say, with January, next with February, and so on, 

 taking afterwards the mean of each two consecutive numbers so 

 found, thus producing annual values free of annual inequality, 

 which may be presumed to apply to the middle of each successive 

 month. The process is equivalent, suppose for the number for 



G 2 



