62 Proceedings of the Royal Society of Edinburgh. [Sess. 
R 
The very gradual reduction day by day in the value of ^ when the 
R 0 
surface mulch is over 1 inch deep shows how efficient even so slight a 
depth of dry soil is in reducing loss of water by evaporation, and it is 
doubtful if much advantage is obtained, as far as checking evaporation is 
concerned, by further depths of dry soil. Yet King has proved the value 
of a 4-inch mulch in conserving water in the soil,* and possibly it is the 
fact that such a depth of dry soil, acting as a poor conductor, prevents any 
great rise in temperature of the soil under the mulch, and so reduces the 
possibility of renewed percolation of water, due to the reduced viscosity 
and surface tension of the soil water consequent on high underground 
temperatures. 
Section III. — Rain. 
(a) Percolation of Water. 
R 
The normal value of ^ in wet soil a few hours after rain has ceased is 
Ro 
about '42, with a corresponding lag of about 3J hours ; but these values are 
subject to great fluctuations during the actual period of rainfall, as may be 
seen from Table IY. 
R 
Although such high values of ~ prevail during 
actual rainfall, the 
13 
Table IY. — Values of — 4 and Lag during Rain. 
R 0 
Date. 
Value 
of 
r 4 
R 0 
Value 
of Lag. 
Temperature Conditions of 
Surface and 4-inch Depth. 
Time of Day 
Rain fell. 
1919. 
Apr. 10 
•55 
24 hrs. 
Surface warmer than 4-inch depth 
11 a.m. 
May 4 
•50 
3 „ 
55 
„ 
55 
11 a.m. and 2-3 p.m. 
June 4 
•60 
2 
55 
55 
11 a.m.-3 p.m. 
„ 13 
•85 
50 mins. 
5 5 
55 
55 
8 a.m. -12 noon. 
Oct. 29 
•50 
2-| hrs. 
55 
55 
55 
8 a.m.-12 noon. 
Nov. 17 
50 mins. 
55 
55 
55 
8 a.m.-3 p.m., after 
snow lying. 
2 p.m. -8 p.m. 
„ 18 
*55 
2J hrs. 
55 
55 
55 
Apr. 13 
•70 
1 hr. 
Surface colder than 4- inch 
depth 
Noon. 
May 2 
•75 
14 hrs. 
.55 
55 
55 
Night. 
„ 17 
•50 
2-| >> 
?) 
55 
5 p.m. onwards. 
June 7 
•75 
1 hr. 
55 
55 
55 
5 p.m. -6.30 p.m. 
„ 12 
•70 
55 
55 
6 p.m.-7 p.m. 
„ 25 
•60 
1^ hrs. 
55 
55 
55 
3.30 p.m. 5.30 p.m. 
Oct. 23 
•71 
1 hr. 
>5 
55 
55 
2 p.m. -10 p.m. 
Dec. 21 
•68 
If hrs. 
55 
55 
55 
12 noon onwards 
„ 24 
•60 
If „ 
55 
55 
55 
2 p.m.-5 p.m. 
* F. H. King, Physics of Agriculture, p. 186. 
