INSTRUMENTS FOR ASCERTAINING THE HUMIDITY OF THE AIR.
=97. Hygrometric Substances.=--The instruments devised for the purpose of
ascertaining the humidity of the atmosphere are termed _hygrometers_. The
earliest invented hygrometers were constructed of substances readily acted
upon by the vapour in the air, such as hair, grass, seaweed, catgut, &c.,
which all absorb moisture, and thereby increase in length, and when
deprived of it by drying they contract. Toy-like hygrometers, upon the
principle of absorption, are still common as ornaments for mantel-pieces.
A useful little instrument of this class, formed from the beard of the
wild oat, is made to resemble a watch in external appearance, and is
designed to prove the dampness or dryness of beds: a moveable hand points
out on the dial the hygrometric condition of the clothes upon which the
instrument is laid.
=98. Saussure's Hygrometer=, formerly used as a meteorologic instrument,
but now regarded as an ornamental curiosity, is represented in fig. 75.
Its action depends upon a prepared hair, fixed at one end to the frame of
the instrument, and wound round a pulley at the other. The pulley carries
a pointer which has a counterpoise sufficient to keep the hair stretched.
By this means the shrinking and lengthening of the hair cause the pointer
to traverse a graduated arc indicating the relative humidity.
[Illustration: Fig. 75.]
Such instruments, however ingenious, are not of scientific value; because
they do not admit of rigid comparison, are liable to alter in their
contractile and expansive properties, and cannot be made to indicate
precisely alike.
=99. Dew-Point.=--The amount of water which the air can sustain in an
invisible form increases with the temperature; but for every definite
temperature there is a limit to the amount of vapour which can be thus
diffused. When the air is cooled, the vapour present may be more than it
can sustain; part will then be condensed as dew, rain, hail or snow,
according to the meteorologic circumstances. The temperature which the air
has when it is so fully saturated with vapour that any excess will be
deposited as dew, is called the _dew-point_.
=100. Drosometer.=--"To measure the quantity of dew deposited each night,
an instrument is used called a _Drosometer_. The most simple process
consists in exposing to the open air bodies whose exact weight is known,
and then weighing them afresh after they are covered with dew. According
to Wells, locks of wool, weighing about eight grains, are to be preferred,
which are to be divided [formed] into spherical masses of the diameter of
about two inches."--_Koemtz._
=101. Humidity.=--The proportion existing between the amount of vapour
actually present in the air at any time, and the quantity necessary to
completely saturate it, is called _the degree of humidity_. It is usually
expressed in a centesimal scale, 0 being perfect dryness, and 100 complete
saturation.
The pressure, or tension, of vapour at the dew-point temperature, divided
by the tension of vapour at the air temperature and the quotient
multiplied by 100, gives the degree of humidity. (Regnault's Tables should
be used.)
Hence the utility of instruments for determining the dew-point.
=102. Leslie's Hygrometer.=--This instrument consists of a glass syphon
tube, terminated with a bulb or ball at each end, turned outwards from
each other, as in fig. 76. The tube is partly filled with concentrated
sulphuric acid, tinged by carmine. One of the balls is covered smoothly
with fine muslin, and is kept continually moistened with pure water, drawn
from a vase placed near it by the capillary attraction of a few strands of
clean cotton-wick. The descent of the coloured liquid in the other stem
will mark the diminution of temperature caused by the evaporation of the
water from the humid surface. The drier the ambient air is, the more
rapidly will the evaporation go on; and the cold produced will be greater.
When the air is nearly saturated with moisture, the evaporation goes on
slowly; the cold produced is moderate, because the ball regains a large
portion of its lost heat from surrounding bodies; and the degree of
refrigeration of the ball is an index of the dryness of the air.
[Illustration: Fig. 76.]
"Should the water become frozen on the ball, this hygrometer will still
act; for evaporation goes on from the surface of ice in proportion to the
dryness of the air. Leslie estimates, that when the ball is moist, air, at
the temperature of the ball, will take up moisture equal to the
sixteen-thousandth part of its weight, for each degree of his hygrometer;
and as ice in melting requires one-seventh of the caloric consumed in
converting water into vapour, when the ball is frozen, the hygrometer will
sink more than when wet by 1 deg. in 7 deg.; and hence, in the frozen state, we
must increase the value of the degrees one-seventh: so that each of them
will correspond to an absorption of moisture equal to
one-fourteen-thousandth part of the weight of the air.
"When this hygrometer stands at 15 deg., the air feels damp; from 30 deg. to 40 deg.,
we reckon it dry; from 50 deg. to 60 deg., very dry; and from 70 deg. upwards, we
should call it intensely dry. A room would feel uncomfortable, and would
probably be unwholesome, if the instrument in it did not reach 30 deg.[8] In
thick fogs it keeps almost at the beginning of the scale. In winter, in
our climate, it ranges from 5 deg. to 15 deg.; in summer often from 15 deg. to 55 deg.;
and sometimes attains 80 deg. or 90 deg. The greatest degree of dryness ever
noticed by Leslie was at Paris, in the month of September, when the
hygrometer indicated 120 deg."--_Professor Trail, in "Library of Useful
Knowledge."_
In estimating the value of the indications of this hygrometer, it should
be borne in mind that the scale adopted by Leslie was _millesimal_, that
is to say, from the freezing to the boiling-point of water was divided
into a thousand parts; ten millesimal degrees are therefore equal to one
of the scale of Celsius.
103. DANIEL'S HYGROMETER.
This instrument was invented about the year 1820, by Professor Daniel, the
distinguished author of _Meteorological Essays_; and it entirely
superseded all hygrometers depending upon the absorption of moisture. The
form of the instrument is shown in fig. 77.
[Illustration: Fig. 77.]
It consists of a glass tube, about one-eighth of an inch in diameter of
bore, bent twice at right angles, and terminated, at each end, in a bulb
about one inch and a quarter in diameter. In one limb of the tube is
enclosed a delicate thermometer, which descends to the centre of the
adjoining bulb, which is about three-parts filled with sulphuric ether.
All the other parts of the tube are carefully freed from air, so that they
are occupied by the vapour of the ether. This bulb is generally made of
black glass; the other is transparent, but covered with a piece of fine
muslin. The support for the tube has a thermometer attached, which shows
the temperature of the external air. The tube can be removed from the
stand, and the parts are made to pack, with a necessary phial of ether, in
a small box, which can easily be got into the pocket.
_How to use the Hygrometer._--This instrument gives the dew-point by
direct observation, which must be made in the following manner:--Having
fixed the tube upon the stand, with the bulbs vertically downward, the
ether is all caused to flow into the lower ball by inclining the tube. The
temperature of the air is noted by the exposed thermometer. Then some
ether is poured, from a dropping tube fitting into the neck of the phial,
upon the muslin-covered bulb. The rapid evaporation of this ether cools
the bulb and causes condensation of the ethereal vapour in its interior.
This gives rise to rapid evaporation of the ether in the lower bulb,
whereby its temperature is greatly reduced. The air in the vicinity is
deprived of its warmth by the cold bulb, and is soon cooled to the
temperature at which it is perfectly saturated with the vapour which it
contains. Cooled ever so little below this temperature, some aqueous
vapour will be condensed, and will form a dew upon the black-glass bulb.
At the first indication of the deposit of dew the reading of the internal
thermometer is taken: which is the dew-point.
This hygrometer has undeniable disadvantages. The surface upon which the
dew condenses is small, and requires a peculiar direction of light in
which to see it well. The observer, having his attention on the bulb and
the thermometer, cannot always fix with precision the dew-point; and hence
he is recommended to note the temperature at the appearance and at the
disappearance of the dew, in order that the chance of error may be
diminished. Without doubt, the necessarily long continuance of the
observer near the instrument influences, to some extent, the observed
temperatures; and the difficulty of not being always able to procure pure
ether for the experiments is not the least of the drawbacks to the use of
the instrument. Some of these disadvantages are obviated in Regnault's
hygrometer.
104. REGNAULT'S CONDENSER HYGROMETER
(Fig. 78) consists of a tube, _C_, made of silver, very thin, and
perfectly polished; the tube is larger at one end than the other, the
large part being 1.8 inches in depth, by 0.8 in diameter; this is fitted
tightly to a brass stand, _B_, with a telescopic arrangement for adjusting
when making an observation.
[Illustration: Fig. 78.]
The tube, _C_, has a small lateral tubulure, to which is attached an
India-rubber tube, with ivory mouth-piece; this tubulure enters _C_ at
right angles near the top, and traverses it to the bottom of the largest
part.
A delicate thermometer, _D_, is inserted through a cork, or India-rubber
washer, at the open end of the tube, _C_, the bulb of which descends to
the centre of its largest part.
_G_ is an attached thermometer for taking the temperature of the air, and
_F_ is a bottle containing ether.
_To use the Condenser Hygrometer_, a sufficient quantity of ether is
poured into the silver tube to cover the thermometer bulb: on allowing air
to pass bubble by bubble through the ether, by breathing in the tube, _E_,
an uniform temperature will be obtained; if the ether continues to be
agitated, by breathing briskly through the tube a rapid reduction of
temperature will be the result; at the moment the ether is cooled down to
the dew-point temperature, the external surface of that portion of the
silver tube containing ether will become covered with a coating of
moisture, and the degree shown by the thermometer at that instant will be
the temperature of the dew-point.
This form of hygrometer, for ascertaining by direct observation the
dew-point, is so superior to Daniell's, both from its being more certain
in its indications and economical in use, that Messrs. Negretti and
Zambra have been induced to modify it, and reduce its price to little more
than that of a good Daniell's Hygrometer.
=105. Temperature of Evaporation.=--When the air is not saturated with
vapour, evaporation is going on with more or less activity, according as
the temperature is high or low, rising or falling. Now vapour cannot be
formed without an expenditure of heat; as we invariably find that the
process of evaporation lowers the temperature of the liquid from which the
vapour is produced, and, by communication, that of contiguous substances
also. Thus the emigrant, crossing the line under the scorching influence
of the vertical sun, wraps a wet towel round his can of water, swings it
in the breeze, to evaporate the moisture of the towel, and obtains a glass
of cool water. So also, European residents in India, during the hot
season, spread out mats in their apartments, and keep them wet, in order
that the evaporation may cool the air. This principle has been applied,
for the purpose of ascertaining the hygrometric condition of the air, in
the instrument known as Mason's hygrometer, or psychrometer, which is now
in general use, from its simplicity, accuracy, and ease of observing.
106. MASON'S HYGROMETER.
=The Dry and Wet Bulb Hygrometer, or Psychrometer=, known also as Mason's
hygrometer (fig. 79), consists of two parallel thermometers, as nearly
identical as possible, mounted on a wooden bracket, one marked _dry_, the
other _wet_. The bulb of the wet thermometer is covered with thin muslin,
and round the neck is twisted a conducting thread of lamp-wick, which
passes into a vessel of water, placed at such a distance as to allow a
length of conducting thread, of about three inches; the cup or glass is
placed on one side, and a little beneath, so that the water within may not
affect the reading of the _dry bulb thermometer_. In observing, the eye
should be placed on a level with the top of the mercury in the tube, and
the observer should refrain from breathing whilst taking an observation.
[Illustration: Fig. 79.]
The _dry_ bulb thermometer indicates the temperature of the air itself;
while the wet bulb, cooled by evaporation, shows a lower temperature
according to the rapidity of evaporation.
_To find the Dew-point._--From the readings of the two thermometers, the
dew-point can be deduced by formulae (that known as Apjohn's is considered
the most theoretically true), or from the valuable Hygrometric Tables by
J. Glaisher, Esq., F.R.S.
For practical purposes in estimating the comparative humidity, the annexed
table, which is a reduction from Mr. Glaisher's elaborate work, will be
sufficient; it will at least serve to assist in familiarising the
inexperienced in the value of the psychrometer's indications:--
+------------------------------------------+
| | Difference between Dry-bulb |
| | and Wet-bulb Readings. |
|Temperature |-----------------------------|
| by the | 2 deg. | 4 deg. | 6 deg. | 8 deg. | 10 deg.| 12 deg.|
| Dry Bulb |-----------------------------|
|Thermometer.| Degree of Humidity. |
|------------------------------------------|
| 34 deg. | 79 | 63 | 50 | .. | .. | .. |
| 36 | 82 | 66 | 53 | .. | .. | .. |
| 38 | 83 | 68 | 56 | 45 | .. | .. |
| 40 | 84 | 70 | 58 | 47 | .. | .. |
| 42 | 84 | 71 | 59 | 49 | .. | .. |
| 44 | 85 | 72 | 60 | 50 | .. | .. |
| 46 | 86 | 73 | 61 | 51 | .. | .. |
| 48 | 86 | 73 | 62 | 52 | 44 | .. |
| 50 | 86 | 74 | 63 | 53 | 45 | .. |
| 52 | 86 | 74 | 64 | 54 | 46 | .. |
| 54 | 86 | 74 | 64 | 55 | 47 | .. |
| 56 | 87 | 75 | 65 | 56 | 48 | .. |
| 58 | 87 | 76 | 66 | 57 | 49 | .. |
| 60 | 88 | 76 | 66 | 58 | 50 | 43 |
| 62 | 88 | 77 | 67 | 58 | 50 | 44 |
| 64 | 88 | 77 | 67 | 59 | 51 | 45 |
| 66 | 88 | 78 | 68 | 60 | 52 | 45 |
| 68 | 88 | 78 | 68 | 60 | 52 | 46 |
| 70 | 88 | 78 | 69 | 61 | 53 | 47 |
| 72 | 89 | 79 | 69 | 61 | 54 | 48 |
| 74 | 89 | 79 | 70 | 62 | 55 | 48 |
| 76 | 89 | 79 | 71 | 63 | 55 | 49 |
| 78 | 89 | 79 | 71 | 63 | 56 | 50 |
| 80 | 90 | 80 | 71 | 63 | 56 | 50 |
| 82 | 90 | 80 | 72 | 64 | 57 | 51 |
| 84 | 90 | 80 | 72 | 64 | 57 | 51 |
| 86 | 90 | 80 | 72 | 64 | 58 | 52 |
+------------------------------------------+
The total quantity of aqueous vapour which at any temperature can be
diffused in the air being represented by 100, the per-centage of vapour
actually present will be found in the table opposite the temperature of
the dry thermometer, and under the difference between the dry-bulb and
wet-bulb temperatures. The degree of humidity for intermediate
temperatures and differences to those given in the table can be easily
estimated sufficiently accurately for most practical purposes.
The difference between the two thermometer readings taken from the reading
of the wet bulb, gives the dew-point very nearly, when the air is at any
temperature between freezing and 80 deg. This simple rule will be found
serviceable to horticulturists, since it will enable them to estimate the
chilling effect of dew or hoar-frost on tender plants.
_Use as an Indicator of Weather._--In our climate, the usual difference
between the thermometer readings,--in the open air, shaded from the sun,
reflected heat, and currents of air,--ranges from one to twelve degrees.
In hot and dry climates, as India and Australia, the range out of doors
has been found as much as 30 deg., occasionally.
When the moisture is frozen, the bulb should be wetted afresh, and the
reading taken just before it again freezes; but the observation then is of
little value, and for general purposes need not be taken, as the air is
known to be dry in frosty weather.
The muslin or cotton rag should be washed once or twice a week by pouring
water over the bulb; and it should be replaced by a fresh piece at least
once a month. Accuracy depends very much upon keeping the wet bulb clean,
and not _too_ wet.
In connection with the barometer, this hygrometer is very useful, not only
on land, but especially at sea, where other kinds of hygrometers cannot be
practically used. A fall in the barometer is indicative of coming wind or
rain: if the hygrometer shows increasing dampness by the difference of the
readings becoming smaller,--rain may therefore be anticipated. On the
contrary, if the hygrometer shows continuing or increasing dryness, a
stronger wind is probable, without rain.
_Domestic Uses._--Mason's hygrometer is useful in regulating the moisture
of the air of apartments; a difference in the thermometer readings of from
5 deg. to 8 deg. being considered healthy. Many complaints require that the
temperature and humidity of the air which the invalid breathes should be
carefully regulated. Hence it is a valuable household instrument. In a
room, it should be placed away from the fire as much as possible, but not
exposed to draughts of air.
Figs. 80 and 81 show cheap arrangements of the instrument for domestic
purposes. Other arrangements are given to the instrument to make it
suitable for exhibiting the hygrometrical state of the air in hot-houses,
conservatories, malting-houses, warehouses, manufactories, &c.
[Illustration: Fig. 80.]
[Illustration: Fig. 81.]
[Illustration: Fig. 82.]
Fig. 82 shows the instrument arranged on brass tripod stand, with folding
legs and metal cover, to render it portable.
=107. Self-Registering Hygrometer.=--A maximum thermometer and a minimum
thermometer, each fitted up as a wet-bulb thermometer, record the highest
and lowest temperature of evaporation during the interval of observation.
Negretti's mercurial maximum, and an alcohol minimum, answer best.
=108. Causes of Dew.=--"The aqueous vapour of our atmosphere is a powerful
radiant; but it is diffused through air which usually exceeds its own mass
more than one hundred times. Not only, then, its own heat, but the heat of
the large quantity of air which surrounds it, must be discharged by the
vapour, before it can sink to its point of condensation. The retardation
of chilling due to this cause enables good solid radiators, at the earth's
surface, to outstrip the vapour in their speed of refrigeration; and
hence, upon these bodies, aqueous vapour may be condensed to liquid, or
even congealed to hoar-frost, while at a few feet above the surface it
still maintains its gaseous state."[9] The amount of moisture so deposited
will vary with different atmospheric conditions. If the sky be decidedly
cloudy or misty, the heat radiated from the earth will be partly restored
by counter-radiation from the visible vapour; the cooling of the earth's
surface will, therefore, take place slowly, and little dew will be
deposited. On the other hand, if the air contain transparent vapour, and
the sky appear clear, the counter-radiation will be less, the earth will
cool rapidly, and the deposit of dew will be copious; provided the night
be comparatively calm, for, when the wind blows, the circulating air
supplies heat to the radiating substances, and prevents any considerable
chilling.
The dew which falls in tropical countries greatly exceeds in abundance
what we experience in our climate; because the air is there, from the
great heat, capable of sustaining a large amount of vapour in the
transparent state, and the conditions most favourable for a maximum
reduction of temperature by radiation are present. At those places, or
upon those substances which cool the lowest and most readily, the dew
falls most copiously.
[Illustration: Fig. 83.]
=109. Plan of Exposing Thermometers=, &c.--Figure 83 is an illustration of
a convenient slab for supporting thermometers in an exposed position
attached to a stand (such as Glaisher's, described in Chapter XVI.) for
ordinary scientific observations. It has a projecting ledge, _B_, to carry
off rain from the instruments, the slab, _A_, being erected vertically.
The hygrometer is placed at _E_, with the vase of water at _F_. An alcohol
minimum thermometer is represented at _C_, in the position most favourable
to its certain action; and at _D_ is shown one of Negretti & Zambra's
maximum thermometers, the position of which may be more nearly horizontal
than there exhibited, although a slight depression of the bulb-end of the
frame is desirable, but not necessary, as this thermometer can be used in
any position.