December i, 1890 .] 
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 
December J.889, the minimum nocturnal temperature 
went so low as ‘62 deg, ; in February 33 deg.; in 
November 33 deg. 3 ; and in March 34 deg. Coffee 
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 
■ to the earth whence it arose, if there is a 
’ hinwing it may be carried 
-- naiv) and 
of sublre?ding^“ ‘‘ line 
page 453. ist column, line 2W 
“ eddied,” and in line 34 <<’ u- read 
" objects. ” s«i>Jects » should he 
has been oonueu.^,.- 
or in one of ‘ ‘ the fields ot uppo. _ 
1 have correctly represented the main resuiis 
Mr. Aitken’s interesting experiments as embodied 
in Dr. Maopherson’s paper ; but the paper is so 
intereibing 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 with 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-valour from the warm bosom 
of Mother Earth, to rerfesh 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 clew, which is peculiar to weather 
in which the atmosphere contains the minimum 
of moisture. In Nuwara Eliya, for instance, rain 
and mist prevail in June, when the average 
mean relative humidity is represented by so high 
a figure as 95 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 olimates 
of India and Ceylon is less satisfactory in deal’ 
lag 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 5,000 feet altitude, 
while brilliant sunlight prevails below the limit 
mentioned. While dew is condensed by cold, it 
would seem as if beat 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 here 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 does not prevail ? 
Why does it prevail, and why is it occasionally 
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 dees form, it soon “ lifts” or is dissipated. 
But for dust in the atmosphere, we are told, rain 
’ ’ not resolve itself into drops, but would 
,e every material thing including the interiors 
llings as much as their exteriors. In that 
does the absence of dust at high altitudes 
Qt for the frequent prevalence of the perva- 
form ot moisture known as fog or mist ? 
prd does not help me with answers to such 
ions, except as answers may be found in terms 
ief as the following, under the heading “ Fog": — 
is a well known physical fact that two masses 
it at different temperatures, and both completely 
fged with invisible vapour, when intermingled, 
no longer hold the whole quantity in suspension ; 
excess must therefore be deposited as fog.” But 
■ diffused particles of moisture which constitute 
; are surely not deposited ? They sometimes 
. .naiii suspended in the atmosphere for days. The 
Rev. Joseph Burnet, a very careful observer, men- 
tioned to me the curious fact that the line ot pre- 
valent mist and of connected success iu 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, exoept 
Blanford’s, and the Penny Cyclopedia 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 email 
reduction of temperature renders the air misty, and 
a further reduction converts the vapour into rain.” 
