DIURNAL VARIATIONS OF TERRESTRIAL MAGNETISM. 
45 
The numerical value of N ir/Z-n-p is 1'99 . 10 1 " (N = 86400, v* = 1 l'O . 10 8 ) or 31 3R, 
where R is the radius of the earth. 
The calculated values of the semi-diurnal components of velocity to east and south, 
at the latitude of St. Helena (16° S.), are approximately given by 
(34) 
f (East) —21 sin (2nt + 154°) cm./sec., 
] (South) 9 sin (2n£ + 244°) cm./sec. 
J. S. Dines has determined the actual values at St. Helena to be 
(35) 
f(East) — 22 sin (2nt+ 158°), 
\(South) 35 sin ('2nt + 237 ). 
The agreement in phase is therefore very good, and also in amplitude for the 
easterly component; the southerly component variation is, on the contrary, much 
larger than the simple theory would predict. 
It has already been remarked that Sp/p seems to diminish upwards, so that in 
accordance with (31), the value of $ in (33) should diminish in amplitude with 
increasing height.* The phase should also vary with height in the same way as for 
the pressure variation, t 
§ 20. The Lunar Diurnal Barometric Variation. 
Laplace, in the ‘ Mecanique Celeste,’ xiii., ch. 1, seems to have been the first 
to mention that tidal motions should be present in the atmosphere as well as in the 
oceans. He also discussed a series of barometric observations made in France and 
found definite evidence of a very small lunar semi-diurnal variation, which he inclined 
to attribute to the indirect (rather than direct) tidal action of the moon working 
through the lunar tidal motion of the sea. Sabine (‘Phil. Trans., 1847) proved 
from the discussion of two years’ barometric observations at St. Helena that the 
magnitude of the lunar semi-diurnal barometric variation was of the order of 0T mm. 
of mercury. The most complete determination of the effect, however, lias been made 
at Batavia (‘Observations,’ 1905); as in the case of the solar diurnal variations, the 
* As regards tlie magnitude of the diminution, some European mountain observations discussed by IIaxn, 
‘Wien. Denkschriften,’ 59, 1892, maybe quoted, although mountain observations may not be altogether 
representative o£ the conditions in the free atmosphere at the same height. Considering the whole 
amplitude of the barometric variation, reduced to seadevel according to the formula Sy/p, the results from 
heights of R to 2-|- km. were found to lie O'28 mm., or even less, whereas the normal seadevel value 
in the same latitude is O'.32 mm. 
t Hann (ibid.) shows that, at the mountain stations referred to, the phase angles range from 
110 or 120:degrees to 140 degrees, in place of 154 degrees. 
