127 
mean monthly and the mean annual^ values of six years, 
and the same means of seven years, have similar if not 
identical ratios. When, however, the mean monthly ratios 
are examined it will be found that the greatest difference of 
the fall happens in May. The six years’ average places this 
difference in June. Before it can be seen to which of these 
months it really belongs, a larger area of averages is 
required. The fall of 1874 places it in May. Both the six 
and the seven years’ average place the least difference in 
January, and this in a most marked manner. On the theory 
then, as I have pointed out before, that the excess of rain- 
fall in the lower gauge is due to the particles of invisible 
vapour in the air between it and the higher gauge coalescing 
with, the falling raindrops, the result seems to show that 
in the spring and early summer months there is relatively 
less of this vapour in the air below a height of 34 feet, and 
that there is relatively more of it in the winter months, and 
particularly in January. Hence the maximum of dry air 
on the ground is in May or June, and the minimum in 
January. The time of this maximum is also the time when 
the maximum of ozone is found. 
Monthly and Annual Ratios of the Excess op Rainfall Measured 
AT 3 Feet from the Ground over the Amount Measured at 
34 Feet from the Ground. 
Ratios 
of such Rainfall 
for 187-1. 
Ratios 
of such Rainfall 
for an Average 
of 7 years from 
1868 to 1874. 
! January 
i 
•726 
•717 
February 
'812 
•755 
; March 
•764 
•782 
April 
•778 
•854 
May 
•966 
•908 
June 
•801 
•889 
J^hy 
•869 
•887 
August 
•796 
•836 
September 
•783 
•848 
October 
•778 
•823 
November 
•813 
•775 
December 
•621 
•804 
Annual ratios... 
1 
•792 
•823 
