84 
MR. C. CHAMBERS ON THE DIRECTION AND INTENSITY 
Table XV. 
Year or Half-year. 
As observed. 
Corrected to Mean Reading 
of large Horizontal-force 
Magnetometer. 
Horizontal 
Force. 
Annual 
Increment. 
Horizontal 
Force. 
Annual 
Increment. 
1867, July to December 
1868, „ „ 
1868, January to December 
1869, „ „ 
1870, „ „ 
1871, „ „ 
1872, „ 
1873, „ „ 
80550 
80595 
80601 
8 0650 
80723 
80777 
8 0802 
8-0841 
[+•0045] 
+•0049 
+ •0073 
+•0054 
+ 0025 
+ 0039 
80524 
8 0561 
8-0563 
80600 
80644 
80688 
8-0728 
80791 
[+■0037] 
+•0037 
+ 0044 
+ 0044 
+ 0040 
+ 0063 
Mean 
+ ■0048 
+ 0045 
The annual increments within brackets, being derived from half-yearly means, are 
allowed only half the weight of the others. The mean secular change of Horizontal 
Force is thus found to be a yearly increase of '0048 from the uncorrected observations, 
or ’0045 from the corrected observations. 
17. Diurnal Inequality at different heights above the ground . — In Table XVI. a 
comparison is drawn between the diurnal inequalities of Horizontal Force at corre- 
sponding hours, shown by the absolute observations and by the large Horizontal-force 
Magnetometer. 
The temperature- and scale-coefficients used in calculating the numbers in the last 
column from those in columns 6 to 9 have been already given. 
The general result (with which twenty-one half-yearly differences are in agreement, 
and four of contrary import) is that the diurnal variation of Horizontal Force is, between 
the hours in question, less in the top room of the Electrometer Tower, at a height of 
38 feet above the ground, where the absolute observations were taken, than it is at a 
height of 6 feet, where the large Horizontal-force Magnetometer is placed ; and on the 
average it is about one fifth less. The average time of the first observation is 12 h 22 m , 
and that of the second observation 14 h 39 m ; and during this interval, which is generally 
about the same in extent and has nearly the same limits, the mean for the whole year 
of the regular daily diminution of force is 0‘ 37 of the mean range for the year of the 
diurnal variation of force, so that we are dealing with a large fraction (more than a third) 
of the whole diurnal movement. The maximum Horizontal Force of the day occurs 
about half an hour before the middle of the first observation. 
The supposition that most readily suggests itself in explanation of a result of this 
nature is, that, being derived from observations with instruments of different construc- 
tion, it is due to error of observation in the one case, or to error in the scale-coefficient 
or temperature-coefficient employed in the other. 
Against the supposition that it is due to error of observation with the Unifilar Mag- 
netometer, it may be urged, first, that it recurs in twenty-one out of twenty-five half- 
