VARIATION OF TERRESTRIAL MAGNETISM. 
469 
The observations are publislied in very different form by the various observatories. 
Tlie units of force at Bombay and St. Petersburg are the Gaussian unit millimeter- 
milligram-second. At Lisbon it is the foot-grain-second. At Greenwich the variations 
are given in terms of the whole vertical and horizontal force. At Bombay, moreover, 
the observations are given not for a certain hour, but for a time which varies with 
different instruments between 12 and 19 minutes past each hour. For this there was 
some reason originally, but at present it would be far better if Bombay would adopt 
the practice of other observatories. The daily variations had, in the first place, to be all 
reduced to C.G.S. units, and, further, instead of variations in declination and horizontal 
force, we had to find the components of the periodic force towards the geographical 
North and West. I am much indebted to Mr. Wm. Ellis, of the Greenwich Observatory, 
for the help he has given me in the reduction of the observations to a form which I 
could use in my calculations. A good many of the computations were done under his 
direct superintendence, and much time and trouble was saved me in consequence. 
The daily variation of declination and horizontal force was expressed in the form 
cq cos t sin cq cos -Vh sin 2^ ff- cos 3^ ~|— sin 3^ —b rq cos 4i cos 4q 
where t represents astronomical time. The summer months, April to September, were 
treated separately from the winter months, October to March. The unit of force, for 
convenience’s sake, was taken as 10"*’ C.G.S. 
Tables I. and II. give the coefficients which were calculated according to a well- 
known method from the original observations. 
Tables III. and IV. give the same coefficients reduced to forces directed to the 
geographical North and West, instead of to the magnetic North and West. 
Before showing how, with the help of these coefficients, the surface potential can be 
calculated, I must deduce a few formulae which will be used hereafter. 
Table I.—Force to Magnetic West. 
Coefficients in the expansion 
cos t h-^ sin t cos '2t -f- sin 2t + ttg COS 3^ + &3 sin 3^ -f cos it sin it. 
The unit of force is 1 C.G.S. X 10"*’; t being astronomical tune. 
Bombay. 
Lisbon. 
Greenwich. 
St. Petersburg. 
Summer. 
Winter. 
Summer. 
W inter. 
Summer. 
Winter. 
Summer. 
Winter. 
ai 
+ 
64-3 
I- 6-3 
+ 125-5 
+ 121-5 
+ 168-0 
+ 154-5 
+ 109-0 
+ 95-7 
L 
I" 
116-5 
+ 33-2 
+ 213-7 
+ 132-6 
+ 192-3 
-t- 113-5 
4- 219-4 
+ 96-7 
dcf 
+ 
134-3 
+ 6-5 
+ 126-1 
+ 53-6 
+ 129-1 
+ 
32-7 
+ 104-2 
- 12-8 
h 
+ 
59-5 
+ 23-2 
+ 152-2 
+ 111-1 
+ 114-6 
+ 
84-0 
4- 113-2 
4- 69-6 
% 
+ 103-5 
+ 40-4 
+ 75-1 
+ 51-0 
+ 65-8 
+ 
38-4 
+ 37-9 
+ 21-8 
^3 
4- 
7-1 
+ 7-6 
+ 56-2 
+ 51-0 
+ 42-5 
+ 
30-6 
4- 59-2 
4- 28-9 
-t 
15-5 
+ 33-1 
+ 13-1 
+ 30-7 
+ 9-3 
+ 
21-3 
4- 2-8 
4" 6-2 
G 
+ 
18-7 
- 9-6 
- 5-9 
+ 20-3 
+ 2-6 
+ 
17-6 
+ 10-9 
+ 13-7 
. 
1 
