A Lunar Period in the Bates of Evaporation and Bainfall? 105 
Eeport of the Cape Meteorological Commission for 1902. Hourly observa- 
tions of evaporation from the surface of this tank are available since 1899. 
The measures are taken to the third decimal place in inches, and averaged 
for each hour of each month and year. 
The great labour in the present case was the clerical one of rearranging 
the tabular measures of evaporation, so that they should conform to the 
lunar day instead of to the solar day. This was done by tabulating the 
hourly quantities of evaporation in twenty-five columns, the thirteenth, or 
central, column containing the amount of evaporation in the hour during 
which the moon crossed the meridian above the horizon (lunar noon) ; the 
mean of the first and twenty-fifth columns being regarded as the evapora- 
tion in the hour during which the moon crossed the meridian below the 
•horizon (lunar midnight). One hundred and twenty lunar months, from 
August, 1899, to April, 1909, have been dealt with in this way and 
averaged. 
Tables I and II will give an idea of the method of procedure. Table I 
is an extract from the record books, and shows the hourly evaporation for 
the five days, March 1st to 5th, 1908. On these days the moon made its 
upper meridian passage on — 
March 1. — Between 11 a.m. and noon. 
,, 2. — About noon. 
,, 3. — Between noon and 1 p.m. 
„ 4. — Between 1 and 2 p.m. 
„ 5. — Between 2 and 3 p.m. 
And in Table I the quantities of evaporation for these particular hours 
is put in italic type. The reariangement in lunar terms is given in 
Table II. 
The averages deduced from observations made on 3408 lunar " days," 
for each hour of the lunar " day," are given in Table III : 
Table III. 
Hourly Mean Evaporation arranged according to the Lunar Day. 
Hour. 
Inch. 
Hour. 
Inch. 
Hour. 
Inch. 
I 
•0070 
X 
. ^0069 
XIX 
. ^0070 
II 
69 
XI 
69 
XX 
71 
III 
69 
XII 
70 
XXI 
70 
IV 
70 
XIII 
69 
XXII 
70 
V 
70 
XIV 
69 
XXIII 
71 
VI 
69 
XV 
70 
XXIV 
70 
VII 
69 
XVI 
70 
XXV 
71 
VIII 
69 
XVII 
70 
IX 
69 
XVIII 
71 
Mean 
. •OOTO 
