302 Yapp . — On Stratification in the Vegetation of a 
Dr. Mill 1 found at Camden Square that the curve of wind velocity has 
apparently little relation to that of evaporation. But air-currents have 
much freer play around the vertical evaporating surface of the instrument 
used in these experiments, than on the horizontal water surface of a sunken 
evaporating tank. It is therefore to be expected that the influence of wind 
would be considerable. This is also true of the plants themselves, especially 
of the more exposed ones. 
The effect of wind on the vegetation will be referred to again later. 
(4) Duration OF Sunshine. It is noteworthy that the total evapora- 
tion was high on all days when there was long continued sunshine (Tables 
I and II). This agrees with the results of Dr. Mill, 2 who states that on 
the whole, at Camden Square, the evaporation curve follows, in summer, 
the curves of duration of sunshine and black-bulb temperature ; and in 
winter the mean temperatures of water and soil (at one foot). 
(5) Rainfall. The effect of rain is well seen on July 10 and 
August 15, 1907, and August 5 and 6, 1908, &c. On the two latter 
occasions heavy rain after 10 a.m. on August 5 ? and slight showers after 
12 noon on August 6, may be seen to have immediately depressed the 
evaporation curves (Text-fig. 8). 
Of course the greatest effect is produced when several of the factors 
act in the same direction, as was the case on July 11 and August 15, 1907 ; 
and from July 31 to August 3, 1908. Thus, on August 15, 1907, a strong 
wind, reinforced by low relative humidity, 8-5 hours of sunshine, and so on, 
resulted in the highest total evaporation recorded for any single day. 
It must be noted that part of this total was due to high night evaporation ; 
the night also being unusually windy. On the other hand, on August 5, 
1908, in spite of a fairly strong wind, the total was one of the lowest of the 
whole series. But in this case the temperature was low, the relative humidity 
rather high, and it was rainy and dull. 
The above results show, I think, that the evaporimeters employed, like 
the similar ones of Livingston, are highly sensitive to even slight changes 
in the atmospheric conditions. 
With regard to the two particular years during which the experiments 
were carried on, 1907 had an unusually wet and cold summer. Dr. Mill 
recorded 3 less total evaporation for that year than for any year since 1902. 
The deficiency was chiefly from April to August. September, on the other 
hand, was exceptionally dry. 1908 was fairly normal. Dr. Mill has kindly 
furnished me with the following information (not yet published) regarding 
his records for the latter year. The total evaporation for the year almost 
exactly coincided with the average for the last twenty-three years. Further, 
the evaporation for August, during which most of my experiments were 
made, was slightly in excess of the average for that month. 
1 Mill (’ 08 ), p. 45. 2 Mill, 1 . c., p. 45. 3 Mill, 1 . c., p. 45. 
