(3.) While the mercury in the barometer stands above 30 vhe air must
oe very dry or very cold, or perhaps both, and no rain may be ixpected.
(4.) When the mercury stands very low indeed, there will never be much
rain, although a fine day will seldom occur at such times.
(5.) In summer, after a long continuance of fair weather, the barometer
#ill fall gradually for two or three days before rain falls ; but, if the fall
of the mercury be very sudden, a thunder-storm may be expected.
(G.) When the sky is cloudless and seems to promise fair weather, if the
barometer is low, the face of the sky will soon be suddenly overcast.
(7.) Dark, dense clouds will pass over without rain when the barometer
is high ; but if the barometer be low it will often rain without any appeal
ance of clouds.
(8.) The higher the mercury, the greater probability of fair weather.
(9.) When the mercury is in a rising state, fine weather is at hand ; but
wrhen the mercury is in a falling state, foul weather is near.
(10.) In frosty weather, if snow falls, the mercury generally rises to
30 J , where it remains so long as the snow continues to fall; if after this the
weather clears up, very severe cold weather may be expected.
It will be observed that the barometer varies more in winter than in
Bummer. It is at the highest in May and August; then in June, March,
September and April. It is the lowest in November and February; then in
October, July, December and January.
[These rules are from Dr. Brewer's work called " The Science of Familiar
543. OP THE DIFFERENT STATES OF THE BAROMETER. Of the Fall of tk
Barometer, In very hot weather the fall of the Barometer indicates thun-
ier. Otherwise, the sudden fall of the barometer leads to the expectation
)f high wind.
In frosty weather the fall of the barometer denotes a thaw.
If wet weather follow soon after the fall of the barometer, but little oi
uch weather may be expected.
In wet weather, if the barometer falls, expect much wet.
In fair weather, if the barometer falls and remains low, expect much wet
In a few days, and probably wind.
The barometer sinks lowest of all for wind and rain together; next to
that for wind, except it be an east or north-east wind.
54i. Of the Rise of the Barometer. In winter the rise of the barometer
In frosty weather, the rise of the barometer presages snow.
If fair weather happens soon after the rise of the barometer, expect but
little of it.
In wet weather, if the mercury rises high and remains so, expect continued
fine weather in a day or two.
In wet weather, if the mercury rises suddenly very high, fine weather
will not last long.
The barometer rises highest of all for north and west winds; for all other
winds, it sinks.
645. The Barometer in an Unsettled State. If the motion of the mercury
b* unsettled, expect unsettled weather.
If it stand at "much rain" and rise to "changeable," expect fair weather
of short continuance.
If it stand at "fair" and fall to "changeable," expect foul weather.
Its motion upwards indicates the approach of fine weather j its motion
downward indicates the approach of foul weather.
Wlt.at is ike ^46. THE THERMOMETER. The Ther-
1 Thermometer, mometer * is an instrument to indicate the tem-
^rindple is it perature of the atmosphere. It is constructed
Constructed? O n the principle that heat expands and cold
contracts most substances.
547. The thermometer consists of a capillary tube, closed at
the top and terminating downwards in a bulb. It is filled with
mercury, which expands and fills the whole length of the tube or
contracts altogether into the bulb, according to the degree of
heat or cold to which it is exposed. Any other fluid may be
used which is expanded by heat and contracted by cold, instead
of mercury. Fig . 81
b48. On the side of the thermometer is a scale to ^\
indicate the rise and fall of the mercury, and conse-
quently the temperature of the weather.
WJiat scale is ^49. There are several different scales
adopted for the applied to the thermometer, of which those
^ihis^oun of ^ahrenbeit, Reaumur, Delisle and Gel-
fry * sius, are the principal. The thermometer
in common use in this country is graduated by Fahren-
neit's scale, which, commencing with 0, or zero, extends
upwards to 212 degrees, the boiling point of water, and
downwards to 20 or 30 degrees. The scales of Reau-
mur and Celsius fix zero at the freezing point of water ;
and that of Delisle at the boiling point.
What is the 550. THE HYGROMETER. The Hygrom-
Hygromete*- ? e t e r is an instrument for showing the degree
of moisture in the atmosphere.
* The word "Thermometer" is from the Greek, and means "a meaaurt
of heat." " Hygrometer " means "a measure of moisture."
150 NATURAL PUTT/)SOPUY.
How is it con- 551. The hygrometer may be constructed of
tt7-u$ted ? any material which dryness or moisture expand*
or contracts ; such as most kinds of wood, catgut, twisted cord,
the beard of wild oats, &c. It is sometimes also composed of a
scale balanced by weights on one sMe, and a sponge, or ot 1 er
substance which readily imbibes moisture, on the other.
552. By the action of the sun's heat upon the surface of the
earth, whether land or water, immense quantities of vapor are raised
into the atmosphere, supplying materials for all the water which is
deposited again in the various forms of dew, fog, rain, snow, and
bail. Experiments have been made to show the quantity of moist-
ure thus raised from the ground by the heat of the sun. Dr. Wat-
son found that an acre of ground, apparently dry and burnt up by
the sun, dispersed into the air sixteen hundred gallons of water in
the space of twelve hours. His experiment was thus made : He put
a glass, mouth downwards, on a grass-plot, on which it had net
rained for above a month. In less than two minutes the inside was
covered with vapor ; and in half an hour drops began to trickle down
its inside. The mouth of the glass was 20 square inches. There
are 12% square inches in a square yard, and 4840 square yards in
,m acre. When the glass had stood a quarter of an hour, he wiped
it with a piece of muslin, the weight of which had been previously
ascertained. When the glass had been wiped dry, he again weighed
the muslin, and found that its weight had increased six grains by
the water collected from 20 square inches of earth ; a quantity equa'
to 1600 gallons, -from an acre, in 12 Lours. Another experiment,
after rain had fallen, gave a much Larger quantity.
553. When the atmosphere is colder than the earth, the vapor
which arises from the ground, or a body of water, is condensed and
becomes visible. This is the way that fog is produced. When the
darth is colder than the atmosphere, the moisture in the atmosphere
condenses in the form of dew, on the ground, or other surfaces.
Clouds are nothing more than vapor condensed by the cold of the
upper regions of the atmosphere. Rain is produced by the sudden
cooling of large quantities of watery vaj>or. Snow and hail are
produced in a similar manner, and differ from rain only in the de-
gree of cold which produces them.
What is the 554. THE DlVER ; S BELL OR DlVING-BELL.
Diving-bell, The Diving-bell is a large vessel shaped like
and on what . , b , . f. .
principle is it an inverted goblet, in which a person may
constructed? safely descend to great depths in the water.
It is constructed on th'e principle of the impenetrability of
555. It has already been stated that air, being a material sub
stance, possesses all the given essential properties of -wcer, and
among them the property of impenetrability. The weight of the
air giving it a pressure in every direction, or the property of fluidity,
it penetrates and fills all things around us, unless by mechanical
means it be carefully excluded. An open vessel, of whatever kind,
is always full either of air or of some other substance, and unless
the air is first permitted to escape no other substance can take the
place of the air.
556. If a tumbler be inverted and immersed in water, the water
will not rise in the tumbler, because the air in the tumbler fills it.
[f the tumbler be inclined so as to let the air ascend in obedience to
the laws of the equilibrium of fluids, the water will rush in and dis-
place the air, while the lighter air. ascending, rises to the surface of
the water. If this experiment be made with a bottle, the air will
rise in bubbles with a gurgling sound. The same experiment may be
made with a tube closed at one end by the finger ; the water will not
enter the tube until bv the removal of the finger the air be permitted
to escape. It is on this principle that the diving-bell is constructed
557. Fig. 82 represents a Fig> 82 '
Explain the con- ... ... . _
struction of the diving-bell. It consists of a
diving-bell by large heavy vessel, formed
of various shapes), with the mouth open. It
descends into the water with its mouth down-
wards. The air within it having no outlet,
it is compelleu by the order of specific grav-
ities to ascend in the bell, and thus (as water
and air cannot occupy the same space at the
same time) prevents the water from rising
in the bell. A person, therefore, may de-
scend with safety in the bell to a great depth
in the sea, and thus recover valuable articles
that have been lost. A constant supply of
fresh air is sent down, either by means of
barrels, or by a forcing-pump. In the Fig.
P represents the bell with the diver in it.
rif tube attached to one side and reaching the air within ; and
P is the forcing-pump through which air is forced into the bell.
The forcing-pump is attached to the tube by a joint at D. When
die bell descends to a great depth, the pressure of the water
C is a bent metal-
condenses the air within the bell, and causes the water to asoen;!
in the bell. This is forced out bv constant accessions of fresh
air, supplied as above mentioned. Great care mast be taken
that a constant supply of fresh air is sent down, otherwise the
lives of those within the bell will be endangered. The heated
and impure air is allowed to escape through a stop-cock in the
upper part of the bell. (See par. 1462.)
558. THE COMMON WATER PUMP.
Hew is water , Tr ^ . . , . ,, ,
raised in a com- Water is raised in" the common pump by
How high may
water be raised
means of the pressure of tho atmosphere
on the surface of the water. A vacuum
common being produced by raising the piston or
pump-box,* the water below is Fig. 83.
forced up by the .atmospheric pressure, on the
principle of the equilibrium of fluids. On this
principle the water can be raised only to the
height of about thirty-three feet, because the
pressure of the atmosphere will sustain a column
of water of that height only.
559. Fig. 63 represents the common
pump, generally called the suction-
pump. The body consists of a large tube,
or pipe, the lower end of which is immersed in the
water which it is designed to raise. P is the piston,
V a valve t in the piston, which, opening upwards,
admits the water to rise through it, but prevents its
return. Y is a similar valve in the bodj. of the
* In order to produce such a vacuum, it is necessary that the piston 01
box should be accurately fitted to the bore of the pump ; for, if the air
above the piston has any means of rushing in to fill the vacuum, as it i*
produced by the raising of the piston, the water will not ascend The pis
ton is general'j worked by a lever, which is the handle of the pump, not
represented in the figure.
f A valve is a lid, or cover, so contrived as to open a communication ic
one way and close it in the other. Valves are made in different ways^
according to the use for which they are intended. In the common pumj
they are generally made of thick leather partly covered with wood I.i
the air-pump they are made of oiled silk, or thin leather softened wit*
oil. The clapper of a pair of bellows is a familiar specimen "i v<V
The valves of a pump are commonly called b-.txta
pump, below the piston. When the pump is not in action, the
Calves are closed by their own weight ; but when the piston is
raised it draws up the column of water which rested upon it
producing a vacuum between the piston and the lower valve Y
The water below immediately rushes through the lower valve
and fills the vacuum. When the piston descends a second time,
the water in the body of the pump passes through the valve
V, and on the ascent of the piston is lifted up by the piston,
and a vacuum is again formed below, which is immediately
filled by the water rushing through the lower valve Y. In
this manner the body of the pump is filled with water, until it
reaches the spout S, where it runs out in an uninterrupted stream.
560. In the description here given of the common pmnp, as
well as in the figure, it will be observed that the common form
of the handle of the pump is not noticed. The handle of the pump
is merely a lever of the first kind ; the fulcrum is the pin which
attaches it to the pump, and the iron rod connected with the
upper valve of the pump is raised or depressed by means of the
561. Although water can be raised by the atmospheric pressure
only to the height of thirty-three feet above the surface, the com-
mon pump is so constructed that after the pressure of the atmos-
phere has forced the water through the valve in the body of the
pump, and the descent of the piston has forced it through the valve
in the piston, it is lifted up, when the piston is raised. For this
reason, this pump is sometimes called the lifting pump. The dis-
tance of the upper valve from the surface of the water must never
exceed thirty-two feet ; and in practice it must be much less.
562. THE FORCING-PUMP. The Forcing-
How does the T/V r ,1
Forcing-pump pump differs from the common pump in
differ from the having a forcing power added, to raise the
common pump? . ,'..
water to any desired height.
563. Fig. 84 represents the forcing - pump. The
body and lower valve V are similar to those in the
common pump. The piston P has no valve, but is
solid; when, therefore, the vacuum is produced above the
pl - 84 lower valve, the water, on the dcationt
of the piston, is forced through the tube
into the reservoir or air-vessel B, where
it compresses the air above it. The air,
by its elasticity, forces the water out
through the jet J in a continued stream,
and with great force. It is on this prin-
"TT" }} ciple that fire-engines are constructed.
TL**^' Sometimes a pipe with a valve in it is
1 1 S substituted for the air-vessel ; the water
k-^ is then thrown out in a continued stream,
but not with so much force.
How & the
564. THE FIRE-ENGINE consists of two forcing-
pumps, worked successively by the elevation and
depression of two long levers of the second kina,
called " Brakes."
565. THE AIR-PUMP. The Air-pump 13
a machine constructed on the principle of the
and'on what ^ elasticity of the air, for the purpose cf ex
constructed f nausting the air from a vessel prepared for
the purpose. This vessel is called a receiver.
and is made of glass, in order that the effects of the removal
of the air may be seen.
566. Air-pumps are made in a great variety of forms ; but all
are constructed on the principle that, w,r en any portion of confined
air is removed, the risudue, immediately expanding, by its elasticity
fills the space occupied by the portion that has been withdrawn.
Explain the con-
567. Fig. 86 represents a single-barrel air
Ttruciion of the pump, used both for condensing and exhausting.
air-pump by A D is the stand or platform of the instru-
ment, which is screwed down to the table by
means of a clamp, underneath, HCQ>
which is not represented in the
figure. R is the glass vessel,
or bulbed receiver, from which
the air is to be exhausted. P
is a sol*d piston, accurately fit-
ted to the bore of the cylinder,
and H the handle by which it
is moved. The dotted line T
represents the communication
between the receiver H and the
barrel B ; it is a tub hrough
which the air, entering at the opening I, on the plate of the
pump, passes into the barrel through the exhausting valve E v.
c v is the condensing valve, communicating with the barrel B
by means of an aperture near E, and opening outwards through
the condensing pipe p.
Explain the op- 568 ' The operation of the pump is as follows
eration of the The piston P being drawn upwards by the han-
air-pump by ,jle H, the air in the receiver R, expanding bv
its elasticity, passes by the aperture I througt
the tube T, and through the exhausting valve E v, into the bar-
rel. On the descent of the piston, the air cannot return through
that valve, because the valve opens upwards only : it must,
therefore, pass through the aperture by the side of the valve,
and through the condensing valve c v, into the pipe j9, where it
passes out into the open air.* It cannot return through the con-
densing valve c v, because that valve opens outwards only. By
continuing this operation, every ascent and descent of the piston
P must render the air within the receiver B. more and wore
156 NATURAL PHILOSOPHY.
rare, until its elastic power is exhausted. Tne receiver L< tbeo
said to be exhausted; and, although it stiii contains a smaiJ
quantity of air, yet it is in so rare a state that the space within
the receiver is considered a vacuum.
569. Prom this statement it will appear that a perfect vacuum
can never be obtained by the air-pump as at present constructed.
But so much of the air within a receiver may be exhausted that the
residue will be reduced to such a. degree of rarity as to subserve
most of the practical purposes of a vacuum. The nearest approach
made to a perfect vacuum is the famous experiment of Torricelli,
which has been explained in No. 530. That would be a perfect
vacuum, were there not vapor rising from the mercury.
570. Prom the -explanation which has been
&be cwLfed S iven of the operation of this air-pump, it will
by means tf the readily be seen that, by removing the receiver
pump which has ft an( j screw i n g any vessel to the pipe p, the
l >een described? . J m,
air may be condensed in the vessel. Thus the
pump is made to exhaust or to condense, without alteration.
(Vtifii is a con- *^1. Air-pumps in general are not adapted
densing syr- for 3ondensation ; that office being performed by
tn S e an instrument called " a condensing syringe,"
which is an air-pump reversed^ its valves being so arranged as
to force air into a chamber, instead of drawing it out. For
this purpose, the valves open inwards in respect to the chamber,
while in air-pumps they open outwards.
572. A guage, constructed on the principle of the barometer, ia
sometimes adjusted to the air-pump, for the purpose of exhibiting
ihe degree of exhaustion.
How does the ^^' ^ ne Double air-pump differs from tho
double air-pump single air-pump, in having two barrels and two
differ from the pjgtons ; which, instead of being moved by the
hand, are worked by means oi a toothed wheel,
pla-ying in notches of the piston-rods.
Fig. 87 represents an air-pump of a different construction. In
this pump the piston is stationary, while motion is given to the
barrel by means of the lever H. The barrel is kept in a proper
position by means of polished steel guides.
574. By means of the air-pump many interesting experiments
may be performed, illustrating the gravity, elasticity, fluidity, *and
inertia of air.
575. EXPERIMENTS ILLUSTRATING THE GRAVITY OP AIR.- -Having
adjusted the receiver to the plate of the air-pump, exhaust the air
end the receiver will be held firmly on the" plate. The forcn which
confines it is nothing more than the weight of the external air
which, having no internal pressure to contend with, presses with a
force of nearly fifteen pounds on every square inch of the external
surface of the receiver.
576 The exact amount of pressure depends on the degree or ex-
haustion, being at its maximum of fifteen pounds when there is a
perfect vacuum. On readmitting the air, the receiver may be readily
577. THE MAGDEBURGH CUPS, OR HEMI-
SPHERES. Fig. 88 represents the Magdeburgb
Cups, or Hemispheres. They consist of two hol-
low brass cups, the edges of which are accu-
rately fitted together. They each have a handle,
* The air is readmitted into the receiver by turning a screw which is in-
serted into the receiver, in which there is an aperture, through which the
external air rushes with considerable force.
What are the
Cups, and what
Jo they illus-
** 88 - to one of which a stop-cock is fitted. The stop-
cock, being attached to one of the cups, is to be
screwed to the plate of the air-pump, and left
open. Having joined the other cup to that on
the pump, exhaust the air from within them,
turn the stop-cock to prevent its readmission,
and screw the handle that had been removed to
the stop-cock. Two persons may then attempt
to draw the cups asunder. It will be found that
great power is required to separate them ; but,
on readmitting the air between them, by turning
the cock, they will fall asunder by their own
weight. When the air is exhausted from within them, the press-
ure of the surrounding air upon the outside keeps them united.
This pressure being equal to a pressure of fifteen pounds on every
square inch of the surface, it follows that the larger the cups ;
or hemispheres, the more difficult it will be to separate them.
578. The Magdeburg Cups derive their name from the city
where the experiment was first attempted. Otto Guericke con-
structed two hemispheres which, when the air was exhausted, were
helc together by a force of about three-fourths of a ton. Fig. 89
shows the manner in which such an experiment may be tried.
Fig ' 90 ' What principle 579 - THE HAND-GLASS. Fig.
does the Hand- 90 is nothing more than a tuui-
the top and bottom ground smooth, so as to fit
the brass plate of the air-pump. Placing it
upon the plate, cover it closely with the palm
of the hand, and work the pump. Tbo a ; j
within the glass being thus exhausted, the hand will be pressed
down by the weight of the air above it : on readmitting the air,
the hand may be easily removed.
What principle . 580 ' p BLADDER-GLASS.-
is illustrated by Fig. 91 is a bell-shaped glass,
the ^Bladder- COV ered with a piece of blad-.
der, which is tied tightly around
its neck. Thus prepared, it may be screwed
to the plate of the air-pump, or connected with
it by means of an elastic tube. On exhausting
the air from the glass, the weight of the external air on the
bladder will burst it inwards, with a loud explosion.
*** 92 ' What does the 581. THE INDJA-RUBBER GLASS.
India-rubber Fig. 92 is a glass similar to
the one represented in the last
figure, covered with india-rubber. The same
experiments may be made with this as were
mentioned in tke last article, but with different results. Instead
of bursting, the india-rubber will be pressed inwards the whole
depth of the glass.
What is illus- ^' ^ HE FOUNTAIN-GLASS AND JET. Fig.
trated by means 93 represents the jet, which is a small brass
of the Fountain- tllbc pi 94 ig the fountain-glass. The ex-
elass and Jet !
periment with these instruments is designed to
Kg. 93. ghow the pressure of the atmosphere on ri e- 94 -
the surface of liquids. Screw the straight
jet to the stop-cock, the stop-cock to the
fountain-glass, with the straight jet inside
of the fountain-glass, and the lower end of
the stop-cock to the plate of the air-pump,
and then open the stop-cock. Having ex-
nausted the air from the fountain-glass, close the stop-
sock, remove the glass from the pump, and, immersing
it in a vessel of water, open the stop-cock. The pressure
of the air on the surface of the water will cause it to rush up
into the glas? like a fountain.
160 NATURAL PHILOSOPHY.
How are the 583. PNEUMATIC SCALES FDR WEIGHING AIR.
Pneumatic Fig. 95 represents the flask, rig. 95.
Scales used? QJ . glagg yeggel and gcaleg for
vveighing air. Weigh the flask when full
of air ; then exhaust the air and weigh the
tiusk again. The difference between its