398 
Proceedings of the Royal Society 
4 o’clock lunar time were the average time of high water for the 
whole earth. It is obvious that the resultant force of the moon, 
on the whole mass of the solid and liquid constituting the earth, is 
not a single force, exerted in the line MC, but that, after the 
manner of Poinsot, it may be represented by a single force in this 
line, and a couple in a direction opposite to that of the arrows, 
indicating in the diagram the direction of the earth’s rotation. 
Thus the lunar attraction produces, as it were, the action of a 
friction brake resisting the earth’s rotation. The same is no 
doubt also the case in respect to the sun and the water of the 
ocean. 
If HIT were inclined to the line of the attracting body, on the 
other side from that shown in the first diagram, the effect of the 
attraction would be to accelerate the earth’s rotation. Now this, 
which is represented in the second diagram (fig. 2), is found by 
observation to be actually the case in respect to the sun and (not 
the waters of the ocean, but) the earth’s atmosphere. The accom- 
panying table and formula show the result of the Pourier 
Harmonic Analysis applied for the diurnal period by Mr. G. H. 
Simmonds to barometric observations collected from all parts of the 
world. In the formula, E denotes the excess of the barometric 
pressure above its mean value for the day, at the time 6 reckoned 
in degrees from midnight, at the rate of 15° per mean solar hour : 
R-pq, R 2 c 2 , -^ 3 c 3 denote the ranges and angles, corresponding to 
the times of maximum height, for the first three terms of the 
Fourier expression which the formula exhibits. The table shows 
the values of Rpq, R 2 c 2 , R 3 c 3 , calculated for the different places, 
from observations at the times stated in column 5. 
It is a very remarkable result of this analysis that the amplitude 
R 2 of the semidiurnal term is for most places, especially those 
within 40° of the equator, considerably greater than the R 2 of the 
diurnal term. The cause of the semidiurnal variation of barometric 
pressure cannot be the gravitational tide-generating influence of the 
sun, because, if it were, there would be a much larger lunar influ- 
ence of the same kind, while in reality the lunar barometric tide is 
insensible or nearly so. It seems therefore certain that the solar 
diurnal variation of the barometer is due to temperature. Now the 
diurnal term, in the Harmonic Analysis of the variation of 
