REPORT OF THE SECRETARY. 19 
deduce, if possible, the law by which they are governed. This is 
the most intricate part of the whole series, and is the final object to 
which the preceding investigations were preparatory. From a care- 
ful study of all the observations on the dip, it was found that 1,446 
might be considered as giving abnormal values, and of the total force 
of the earth’s magnetism 1,470 indicated abnormal changes, which 
amounted to about one-fifteenth part of the whole number of obser- 
‘vations. These abnormal disturbances were analyzed in relation to 
their frequency during the hours of the day, the month of the year, 
and successive years ; they were also studied as to their tendency to 
exhibit an increase or diminution in their variations and the times of 
the greatest and least action in relation to the periods above men- 
tioned. This part ends with a table of the relative magnitude of the 
disturbances and a comparison of those of Toronto and Philadelphia, 
from which it appears that in some cases there is an agreement in 
the character of the simultaneous changes in the two places, and at 
others not. 
The second part of the fourth section treats of the solar diurnal 
and annual inequality of the dip and total force, that is, of the changes 
due to the sun which take place in the dip and total force from 
hour to hour and from month to month. In this investigation all the 
greater perturbations are omitted and the laws of the simpler 
or more normal changes are sought. The diurnal changes in the dip 
are shown analytically and graphically for each month and for the 
whole year. The general character of the curve exhibits a maxi- 
mum at about 11 a. m. and a minimum atabout 5 a. m., with a range 
of one minute and two-tenths—a quantity too minute to be recognized 
by the ordinary dip circle, and which can only be observed by the 
differential reflecting instrument. In summer the epochs occur ear- 
lier, with a range of a minute and a half, while in winter they occur 
later, with a range of only one minute. There is also a secondary 
fluctuation of small magnitude. The diurnal deviation of the dip is 
greatest about the time of the equinoxes, and of these maxima that 
of the winter is the least. The diurnal changes in the total force as 
deduced from the average of the year are represented by a single 
crested curve, but from the average of the observations in winter 
alone this assumes the form of a double curve. The principal maxi- 
mum as deduced from the whole year coincides with the hour of 
_ 2p. m., and in winter occurs about an hour and ahalf earlier. The 
principal minimum coincides with 10 p. m., and occurs in winter about 
two hours earlier. This part ends with an attempt to deduce from 
the data the annual changes in dip and intensity. The result, how- 
