December i, i8go.] 
THE TROPICAL AGRICULTURIST. 
453 
57 deg. and 58 deg. mean shade temperature. But 
on exceptional nights the temperature goes down to 
freezing point. For instance, in January and 
Deoember 1859, the minimum nocturnal temperature 
went so low as 32 deg, ; in February 33 deg.; in 
November 33 deg. 3 ; and in March 34 deg. UoHee 
trees growing in swamps, at elevations far below 
that of Nuwara Bliya, were sometimes blighted 
(“killed by frost’' was the popular idea) in very 
clear cold weather. Of course the injury 
arose from air near the ground chilled by the 
combined effects of evaporation of moisture and 
radiation of heat into space. There can bo cold 
at night and in the early mornings, intense enough 
to injure vegetation, without the presence of actual 
frost. But to return to Mr. Aitken’s conclusions 
regarding dew. It will be obvious that dew, owing 
its origin always to the earth, as he has proved, 
is not always condensed by plants or other objects 
near the surface. Much of it occasionally ascends 
into the atmosphere, until it meets with strata 
cold enough to condense its minute particles. It then 
falls back to the earth whence it arose, if there is a 
calm; but if wind is blowing it may be carried 
and added so as to fall on surfaces so hard and 
dry, that obviously from them no moisture could 
be derived. This is the explanation of dew on 
arid ground and on rooks which possess no moisture 
or none near their surface. It follows from all 
that has been stated, that, unless we carefully 
observe and distinguish, we may be deceived by 
leaf-excreted moisture into assuming the presence 
of dew when it is absent. But I submit that 
when we see such subjects as spiders’ webs of 
the moat beautiful and elaborate geometrical forms 
made conspicuous in every detail by means of 
diamond-like reflections of sunlight from copious 
moisture on every filmy thread, we cannot be 
mistaken as to the existence of genuine dew which 
has been condensed near the surface of the earth 
or in one of “the fields of upper air.” 1 believe 
I have correctly represented the main results of 
Mr. Aitken’s interesting experiments as embodied 
in Dr. Maopherson’s paper ; but the paper is so 
intereiting and in some aspects so important, that 
I trust you may be able to reproduce it in its 
entirety. In any case I cannot deny myself the 
pleasure of quoting the concluding paragraph, in 
which the scientific facts are clothed with the 
language of poetry, thus ; — 
These two facts, then, have now been established: 
that what was long considered to be dew is merely 
the exudation of vigorous plants, and that true dew 
rises from the ground. Brilliant globules are produced 
by the vital action of the plant — the liquid being the 
elixir vita of vegetation — showing life in one of the 
most charming forms in the phenomena of nature, 
especially when the deep-red setting sun makes them 
glisten all a-tremble wiih gold light ; while an infinite 
number of minute but glistening particles of moisture 
bedeck the blade-surfaces in the form of gentle dew, 
which has risen in water-vapOur from the warm bosom 
of Mother Earth, to rorfosh the thirsty plants and 
diffuse fragrance all around. 
I wish I knew as much about one of the earliest 
meteorological phenomena mentioned in the sacred 
record, — the ‘‘mist” which “arises out of the 
ground ” in a moist state of the earth, as, 
by means of the paper referred to, I now 
do about the dew, which is peculiar to weather 
in which the atmosphere contains the minimum 
of moisture. In Nuwara Eliya, for instance, rain 
and mist prevail iu June, when the average 
mean relative humidity is represented by so high 
a figure as ‘Jo deg. ; dew, in its frozen state, giving 
hoar-frost in Feb. ; the average humidity of 
which month is so low as 73 deg. Mr. Blanford 
in his valuable work on the weather and climates 
of India and Ceylon is less satisfactory in deal' 
ing with haze and fog, than in any other de- 
partment of the work. He does not seem to have 
studied the phenomenon of mist in mountain 
regions, such as that whence I write, and where 
for days together sometimes the sky is darkened 
by dense vapour at and above 5, COO feet altitude, 
while brilJiant sunlight prevails below the limit 
mentioned. While dew is condensed by cold, it 
would seem as if heat were the agent required 
to dissipate fog or mist. And yet there is a 
haze which owes its origin to heat. I always 
feel that above 5,000 feet hero the prevalence 
of mists should count a good deal in qualifica- 
tion of our comparatively limited annual rainfall 
of 90 inches. That quantity must be equivalent 
to over 100 inches where mist dees not prevail ? 
Why does it prevail, and why is it ocoasionally 
so unpleasantly persistent, and why is mist- 
moisture not rapidly condensed and precipitated 
by the cold of the atmosphere, as dew vapour 
is ? For above the mist line the temperature is 
appreciably colder than at the lower altitude, where, 
if mist does form, it soon “ lifts” or is dissipated. 
But for dust in the atmosphere, wo are told, rain 
would not resolve itself into drops, but would 
saturate every material thing including the interiors 
of dwellings as much as their exteriors. In that 
case does the absence of dust at high altitudes 
account for the frequent prevalence of the perva- 
sive form of moisture known as fog or mist ? 
Blanford does not help me with answers to such 
questions, except as answers may be found in terms 
so brief as the following, under the heading “ Fog”:— 
“ It is a well known physical fact that two masses 
of air at different temperatures, and both completely 
charged with invisible vapour, when intermingled, 
can no longer hold the whole quantity in suspension ; 
the excess must therefore be deposited as fog.” But 
the diffused particles of moisture which constitute 
fog are surely not deposited ? They sometimes 
remain suspended in the atmosphere for days. The 
Rev. Joseph Burnet, a very careful observer, men- 
tioned to me the curious fact that the line of pre- 
valent mist and of connected success in coffee culture 
in the Matale districts of our hill region was 
found to be 4,500 feet. Here the line is about 5,000 
feet, and on the eastern side of the mountain 
system it is higher still, coffee having flourished at 
5,500 in Haputale. Except in oases of bron- 
chial affections, misty weather does not seem 
to be insalubrious, but on the contrary 
healthier than hot, bright, dry weather. It is a 
popular belief that, as mountains are denuded of 
forest, mist will disappear, but my experience does 
not confirm this belief. It seems mainly a question 
of altitude and, no doubt, of temperature of the 
air as the result of altitude. In cold, north-east 
monsoon weather, however, we often look down 
from our clear heights on lower ranges and valleys 
enveloped in a sea of white mist, I have no 
meteorological work at hand to refer to, except 
Blanford’s, and the Penny Oyelopedia is now some- 
what ancient. Still the following seems worthy of 
quotation ; — “ Mist. The vapour of water, when 
mixed with air of the same or a higher temperature, 
is invisible ; but when the temperature of the air 
is reduced below that of the vapour, the vapour 
becomes visible and forms a mist. * ’* It has 
been found that the quantity of vapour in the air 
diminishes nearly uniformly with the temperature, 
from the equator to the poles. But as the quantity 
of vapour which the air will hold at any given 
temperature is limited, whenever that quantity is 
at or near the point of saturation, a very small 
reduction of temperature renders the air misty, and 
a further reduction converts the vapour into rain.’' 
