PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID
D E W ,
SEVERAL APPEARANCES CONNECTED
BY WILLIAM CHARLES WELLS, M.D.,
HASWELL, BARRINGTON, AND HASWELL,
293 MARKET STREET.
OF THE PHENOMENA OP DEW.
SECT. I. Of circumstances which influence the production of Dew, . 6
SECT. II. Of the old connected with the formation of Dew, . . 16
OF THE THEORY OF DEW.
Former Theories, 27
A NEW THEORY PROPOSED. Dew is the Production of a preceding Cold
in the Substances upon which it Appears, . . 28
That Cold precedes the formation of Dew, ascertained by experiment, . , 29
This fact applied to explain several Natural Appearances:
1. The variety in the quantities of Dew on different Bodies exposed to
the Air during the same time of the night, but ia different situations, 31
2. The Cold connected with Dew not being always proportional to the
quantity of that fluid, ib.
3. The production of Heat by the formation of Dew, . . . ib.
4. The fact of more Dew being acquired in very calm nights, by sub-
stances placed upon a raised board, than by others of the same kind
on the grass; and that of a slight agitation of the atmosphere, when
very pregnant with moisture, increasing the quantity of Dew, . 33
5. The fact of Dew never being formed in temperate climates upon
the naked parts of a living and healthy human body, . . . ib.
6. The fact of Hygrometers formed of Animal and Vegetable sub-
stances, when exposed to a clear sky at night, marking a degree of
moisture beyond what is actually resident in the Atmosphere, . ib.
The Cold which produces Dew is itself produced by the Radiation of Heat
from those Bodies upon which Dew is deposited, . . ib.
The Cold produced by the radiation of Heat from substances upon the sur-
face of the earth, is compensated or over-balanced in the day-time by the
Heat from the Sun, and lessened at night by various causes, . . .35
The Cold originating in the nightly radiation of Heat from Bodies upon the
surface of the Earth, though lessened by various causes, is often very con-
siderable, .... 36
Some of the useful effects of the radiation of Heat from the Earth at night, 38
4 CONTENTS TO ESSAY ON DEW.
Observations upon, or explanations of, the under-mentioned circumstances:
1. The prevention, wholly or in part, of Cold from radiation, in sub-
stances on the ground, by the interposition of any solid body between
them and the sky, 39
2. The prevention, wholly or in part, of Cold from radiation, in sub-
stances on the ground, by the interposition of clouds, . . . ib.
3. The prevention, wholly or in part, of cold from radiation, by fogs, 40
4. The prevention, wholly or in part, of cold from radiation, by conduc-
tion from warmer substances in contact with the radiating sub-
5. The effect of wind in compensating the Cold from radiation, and
sometimes in lessening, and sometimes in increasing, the production
of Dew, 43
6. The Cold from radiation, of a thermometer placed on a board, being
less diminished than that of one suspended in the air, . . . ib.
7. The hurtful effects of Cold occurring chiefly in hollow places, ac-
cording to a remark of Theophrastus, . . . . . ib.
8. Frost being less severe upon hills than in neighbouring plains, in
calm and serene nights, . . . . , . . .44
Reasons assigned for believing that Air is actually heated by the sunbeams
which enter it, and that it not only absorbs, but radiates Heat, . . . ib.
9. The leaves of trees often remaining dry throughout the night, while
those of grass are covered with Dew, ...... 49
10. Bright metals exposed to a clear sky in a calm night being less
dewed on their upper surface than other solid bodies; and those
metals which radiate Heat most, being most attractive of Dew, . ib.
11. The difference between black and white bodies with respect to
radiation, when exposed to the sky at night, ..... 52
Whether Dew is the product of vapour emitted during the night by the
earth and plants upon it, i;^*r'-. : . 53
OF SEVERAL APPEARANCES CONNECTED WITH DEW.
1. Of the greater moisture sometimes observed in winter mornings upon
the insides of the panes of glass in windows covered with inside shut-
ters, than upon those not covered by them, 57
2. Of the greater sensation of Cold, which is sometimes experienced upon
exposure to the sky in a clear night, than is to be explained by the tem-
perature of the Atmosphere, . . 58
3._Of the effect of those means employed by gardeners to protect tender
plants from Cold during the night, which screen them from the sky, . 59
4. Of the effect of a covering of snow, or of other matters, during still
and serene nights, in protecting vegetables from Cold, . . . .61
5. Of the putrefaction which has been supposed to take place in animal
substances exposed to Moonshine, .62
6.- Of the formation of Ice during the night, in Bengal, when the tempera-
ture of the air is above 32, 63
ESSAY ON DEW
I was led, in the autumn of 1784, by the event of a rude experi-
ment, to think it probable, that the formation of dew is attended with
the production of cold. In 1 788, a paper on hoarfrost, by Mr. Patrick
Wilson of Glasgow, was published in the first volume of the Transac-
tions of the Royal Society of Edinburgh, by which it appeared, that
this opinion had been entertained by that gentleman before it had oc-
curred to myself. In the course of the same year, Mr. Six of Canter-
bury mentioned, in a paper communicated to the Royal Society, that,
on clear and dewy nights, he always found the mercury lower in a ther-
mometer laid upon the ground, in a meadow in his neighbourhood,
than it was in a similar thermometer suspended in the air, six feet
above the former ; and that, upon one night, the difference amounted
to 5 of Fahrenheit's scale. Mr. Six,however,did not suppose, agree-
ably to the opinion of Mr. Wilson and myself, that the cold was
occasioned by the formation of dew ; but imagined, that it proceeded
partly from the low temperature of the air, through which the dew,
already formed in the atmosphere, had descended, and partly from
the evaporation of moisture from the ground, on which his ther-
mometer had been placed. The conjecture of Mr Wilson, and the
observations of Mr. Six, together with many facts, which I after-
wards learned in the course of reading, strengthened my opinion ;
but I made no attempt, before the autumn of 1811, to ascertain by
experiment if it were just, though it had, in the mean time, almost
daily occurred to my thoughts. Happening, in that season, to be
in the country on a clear and calm night, I laid a thermometer on
grass wet with dew, and suspended a second, in the air, two feet
above the other. An hour afterwards, the thermometer on the
grass was found to be 8 lower, by Fahrenheit's division, than the
one in the air. Similar results having been obtained from several
similar experiments, made during the same autumn, I determined
in the next spring to prosecute the subject with some degree of
steadiness, and with this view went frequently to the house of one
of my friends, who lives in Surrey. At the end of two months,
I fancied that I had collected information worthy of being published
but fortunately, while preparing an account of it, I met by accident
with a small posthumous work of Mr. Six, printed at Canterbury
in 1794, in which are related differences observed on dewy nights,
OCT. 1838. Q 2
6 ESSAY ON DEW.
between thermometers placed on grass and others in the air, that
are much greater than those mentioned in the paper presented by
him to the Royal Society in 1788. In this work, too, the cold of
the grass is attributed, in agreement with the opinion of Mr. Wilson
altogether to the dew deposited upon it. The value of my own
observations appearing to me now much diminished, though they
embraced many points left untouched by Mr. Six, 1 gave up my
intention of making them known. Shortly after, however, upon
considering the subject more closely, I began to suspect, that Mr.
Wilson, Mr. Six, and myself, had all committed an error, in regard-
ing the cold, which accompanies dew, as an effect of the formation
of that fluid. I, therefore, resumed my experiments, and having,
by means of them, I think, not only established the justness of my
suspicion, but ascertained the real cause both of dew, and of several
other natural appearances, which have hitherto received no sufficient
explanation, I venture now to submit, to the consideration of the
learned, an account of some of my labours, without regard to the
order of time in which they were performed, and of various con-
clusions which may be drawn from them, mixed with facts and
opinions already published by others.
OP THE PHENOMENA OF DEW.
OP CIRCUMSTANCES WHICH INFLUENCE THE PRODUCTION Oi 1 DEW.
ARISTOTLE* and many other writers have remarked, that dew
appears only on calm and serene nights. The justness of this
observation, however, has not been universally admitted. For
Musschenbroekt says, that dew forms in Holland, while the surface
of the country is covered with a low mist ; but, as he mentions at
the same time that it is deposited upon all bodies indiscriminately,
the moisture, of which he speaks, connot properly be called dew, as
will be more distinctly seen hereafter. Other writers of considerable
reputation have also regarded clearness of the atmosphere, as not
being requisite for the production of dew, misled, I believe, partly
by theory, and partly by observing on misty mornings copious dews,
which had been produced during preceding clear nights. Respect-
ing this point I can aver, after much experience, that I never knew
dew to be abundant, except in serene weather.' In regard to the
* Meteor. Jjib. I, c. x. et De Mundo. c. iii.
f Nat. Phil. T. ii. De Rore.
THE PHENOMENA OF DEW. 7
necessity of the air being still, I know of no person who rejects it,
except M. Prieur,* a late French author of little consideration, and
he affirms, in opposition to the most common observation, that a
fresh wind is requisite for the production of dew.
The remark of Aristotle, however, is not to be received in its
strictest sense, as I have frequently found a small quantity of dew
on grass, both on windy nights, if the sky was clear, or nearly so,
and on cloudy nights, if there was no wind. If, indeed, the clouds
were high, and the weather calm, I have sometimes seen on grass,
though the sky was entirely hidden, no very inconsiderable quantity
of dew. Again ; according to my observation, entire stillness of
the atmosphere is so far from being necessary for the formation of
this fluid, that its quantity has seemed to me to be increased, by a
very gentle motion in the air. Dew, however, has never been seen
by me, on nights both cloudy and windy.
If, in the course of the night, the weather, from being calm and
serene, should become windy and cloudy, not only will dew cease
to form, but that which has formed will either disappear or di-
In calm weather, if the sky be partially covered with clouds, more
dew will appear than if it were entirely covered, but less than if it
were entirely clear.
Dew probably begins, in this country, to appear upon grass, in
places shaded from the sun, during clear and calm weather, soon
after the heat of the atmosphere has declined. My opportunities,
however, for making such observations have not been numerous,
since, while pursuing this subject, I seldom went into the country
till late in the afternoon ; but I have frequently felt grass moist, in
dry weather, several hours before sunset. On the other hand, I have
scarcely ever known dew to be present in such quantity upon grass,
as to exhibit visible drops before the sun was very near the horizon,
or to be very copious till some time after sunset. It also continues
to form, in shaded places, after sunrise ; but the interval between
sunrise, and its ceasing to form, is, according to my observation,
which, upon this point, has not been extensive, considerably shorter
than that between its first appearance in the afternoon and sunset.
Contrary, however, to what happens at sunset, if the weather be
favourable, more dew forms a little before, and, in shaded places, a
little after sunrise, than at any other time. Musschenbroek, there-
fore, errs greatly when he says, that dew does not form after the sun
has risen. The preceding observations, on the early appearance of
dew in the afternoon, are to be restricted to what happens to grass,
or other substances highly attractive of dew placed on the ground ;
for it occurs much later on similar substances which are elevated a
few feet above the ground, though upon these it continues to form
as long after the rising of the sun as upon the others, if they be
equally sheltered from the rays of that body.
* Journal de PEcole Polytechnique, Tom. ii. 409.
8 ESSAY ON DEW.
The formation of dew, after it has once commenced, continues
during the whole night, if the weather remain still and serene. M.
Prieur, indeed, of whom I have already spoken, asserts, that dew
forms only in the evening and morning, and that any which occurs
in the former season always disappears in the course of the night.
I can affirm, however, from long experience, that grass, after having
been dewed in the evening, is never found dry until after sunrise,
unless the weather has, in the mean time, changed. Upon one
serene and still night, I placed fresh parcels of wool upon grass every
hour, and by weighing each of them, after exposure for an hour,
found that they had all attracted dew.
When dew forms upon a smooth dense body as glass, and it is
only by means of such a body that the process can be accurately
observed, the appearances are altogether similar to those which oc-
cur on a like body when exposed to the steam of water a little
warmer than itself. The exposed surface has first its lustre dimi-
nished by a slight damp uniformly spread over it. As the moisture
increases, it gathers into irregularly shaped flat drops, which are, at
first, very small, but afterwards enlarge and run into one another,
forming streamlets, by means of which a great part escapes from the
body which had received it.
During nights that are equally clear and calm, dew often appears
in very unequal quantities, even after allowance has been made for
any difference in their lengths. One great source of these differ-
ences is very obvious. For, it being manifest, whatever theory be
adopted concerning the immediate cause of dew, that the more
replete the atmosphere is with moisture previously to the operation
of that cause, the more copious will the precipitation of water be
after this operation has commenced, all the circumstances which
tend to increase the quantity of moisture in the atmosphere, must
likewise tend to increase the production of dew. Thus dew, in
equally calm and clear nights, is more abundant shortly after rain,
than during a long tract of dry weather. It is more abundant, also,
throughout Europe, with perhaps a few exceptions, and in some
parts of Asia and Africa, during southerly and westerly winds, than
during those which blow from the north and the east. Aristotle*
says, that Pontus is the only country in which dew is more copious
during a northerly than during a southerly wind. But a similar
fact occurs in Egypt ; for dew is scarcely ever observed there, ex-
cept while the Etesian winds prevail. Both cases, however, though
contrary to the letter, are consonant with the spirit of the rule ;
since the north wind in one country proceeds from the Euxine sea,
and in the other from the Mediterranean. Another circumstance,
of the same kind with the blowing of wind from the south and the
west, as showing that the air contains much moisture, is the lessen-
ing of the weight of the atmosphere. My experience on this point
has not, indeed, been great, as the falling of the mercury in the
* Meteor. Lib. i. c. x.
THE PHENOMENA OF DEW. 9
barometer is very commonly attended with wind or clouds, both
unfavourable to the production of dew ; but still the greatest dew, I
have ever witnessed, occurred while the barometer was sinking. A
corresponding observation is made by M. de Luc, who says, that
rain may be foretold, when dew is uncommonly abundant in relation
to the climate and season.*
To the greater or less quantity of moisture in the atmosphere, at
the time of the action of the immediate cause of dew, are likewise
to be referred several other facts respecting its copiousness, the
explanation of which is, perhaps, not so apparent as in the preced-
In the first place ; dew is commonly more plentiful in spring and
autumn than in summer ; the reason is, that a greater difference is
generally found between the temperatures of the day and the night,
in the former seasons of the year than in the latter. In spring this
circumstance is prevented often from having a considerable effect,
by the opposite influence of northerly and easterly winds ; but, during
still and serene nights in autumn, dew is almost always highly
In the second place ; dew is always very copious on those clear
and calm nights which are followed by misty or foggy mornings ;
the turbidness ot the air in the morning showing, that it must have
contained, during the preceding night, a considerable quantity of
Thirdly ; I have observed dew to be unusually plentiful on a
clear morning which had succeeded a cloudy night. For the air,
having in the course of the night lost little or no moisture, was in
in the morning more charged with watery vapour, than it would
have been if the night had also been clear.
Fourthly ; heat of the atmosphere, if other circumstances are
favourable, which according to my experience they seldom are in
this country, occasions a great formation of dew. For, as the power
of the air to retain watery vapour in a pellucid state increases con-
siderably faster while its temperature is rising, than in proportion
to the heat acquired, a decrease of its heat, in any small given
quantity during the night, must bring it, if the temperature be
high, much nearer to the point of repletion, before it be acted upon
by the immediate cause of dew, than if the temperature were low,
We read, accordingly, in the writings of those who have travelled
into hot climates, of a copiousness of dew frequently observed by
them there, which very much exceeds what occurs any time in this
country. But even here, dew, though for the most part scanty in
our hottest season, is sometimes very abundant during it, an example
of which occurred to me on the night common to the 29th and 30th
of July 1813 ; for on that night, notwithstanding its shortness, more
dew appeared than has ever been observed by me on any other.
In the last place ; I always found, when the clearness and
* Rech. sur les Mod. de PAtm. 725,
10 ESSAY ON DEW.
ness of the atmosphere were the same, that more dew was formed
between midnight and sunrise, than between sunset and midnight,
though the positive quantity of moisture in the air must have been
less in the former than in the latter time, in consequence of a pre-
vious precipitation of part of it. The reason, no doubt, is the cold
of the atmosphere being greater in the latter, than in the prior part
of the night.
But there are many circumstances influencing the quantity of
dew which, though much more open to accurate observation than
those hitherto mentioned, are yet much less easy to be understood.
In my first attempts to compare the quantities of dew formed
during different times, or in different situations, I attended only to
the appearance which it made on bodies having smooth surfaces.
But quickly seeing this method to be very imperfect, I next employed
wool to collect dew from the atmosphere, and found it well adapted
for my purpose, as it readily admits amongst its fibres the moisture
which forms on its outer parts, and retains what it receives so firmly,
that I never but once had occasion to suspect that it suffered any
portion of what it had thus acquired to pass entirely through it.
The wool which I used was white, moderately fine, and already
imbued with a little moisture, from having been long exposed to
the air of a room in which no fire was kept. I divided it into
parcels of 10 grains each, and, immediately before exposure, pulled
the fibres of every parcel somewhat asunder, so as to give it the
form of a flattened sphere, the greatest diameter of which was about
2 inches. As in doing this I went by the judgment of my sight
alone, some inequality, in point of size, must have existed among
different parcels, but none, I think, sufficient to affect the accuracy
of my conclusions from the experiments in which they were em-
ployed, more especially as my conclusions scarcely ever rested upon
Previously to mentioning the results of any of my experiments
with these parcels of wool, I think it right to describe the place
where by far the greater part of my observations on dew were
made. This was a garden in Surrey, distant, by the public road,
about three miles from the bridge over the Thames at Blackfriars, but
not more than a mile and a quarter from a densely built part of the
suburbs on the south side of that river. The form of the garden
was oblong, its extent nearly half an acre, and its surface level.
At one end was a dwelling-house of moderate size, at the other a
range of low buildings ; on one side a row of high trees, on the
other a low fence, dividing it from another garden. If this
fence had been absent, the garden would have been on the latter
side entirely open. Within it were some fruit trees, but, as it had
not been long made, their size was small. Towards one end there
was a grassplat, in length 62 feet, and nearly 16 broad, the herbage
of which was kept short by frequent mowing. The rest of the
garden was employed for the production of culinary vegetables.
All of these circumstances, however trifling they may appear, had
influence on my experiments, and most of them, as will hereafter be
THE PHENOMENA OF DEW. 11
seen, must have rendered the results less remarkable than they
would have been if they had occurred on a wide open plain, con-
siderably distant from a large city.
I now proceed to relate the influence which several differences
in the situation, mechanical state, and real nature of bodies, have
upon the production of dew.
I. One general fact relative to situation is, that whatever dimin-
ishes the view of the sky, as seen from the exposed body, occasions
the quantity of dew, which is formed upon it, to be less than would
have occurred if the exposure to the sky had been complete.
I placed on several clear and still nights, 10 grains of wool upon
the middle of a painted board, 4i feet long, 2 feet wide, and 1 inch
thick, elevated 4 feet above the grassplat, by means of 4 slender
wooden props of equal height ; and, at the same time, attached,
loosely, 10 grains of wool to the middle of its underside. The
two parcels were consequently only an inch asunder, and were
equally exposed to the action of the air. Upon one night, however,
I found that the upper parcel had gained 14 grains in weight, but
the lower only 4. On a second night, the quantities of moisture,
acquired by like parcels of wool, in the same situations as in the
first experiment, were 19 and 6 grains ; on a third, 11 and 2 ; on
a fourth, 20 and 4 ; the smaller quantity being always that which
was gained by the wool attached to the lower side of the board.
I bent a sheet of pasteboard into the shape of a house-roof, mak-
ing the angle of flexure 90 degrees, and leaving both ends open.
This was placed one evening, with its ridge uppermost, upon the
same grassplat, in the direction of the wind, as well as this could
be ascertained. I then laid 10 grains of wool on the middle of that
part of the grass which was sheltered by the roof, and the same
quantity on another part of the grassplat fully exposed to the sky.
In the morning, the sheltered wool was found to have increased in
weight only 2 grains, but that which had been exposed to the sky
.In these experiments, the view of the sky was almost entirely
cut off from the situations in which little dew was formed. In
others, where it was less so, the quantity gained was greater. Thus,