Luke Hebert.

The engineer's and mechanic's encyclopædia, comprehending practical illustrations of the machinery and processes employed in every description of manufacuture of the British Empire .. (Volume 1) online

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but Mr. Waring makes the effect to be equal only to that of a good under-shot
wheel, when driven by an equal quantity of wafer falling fi-om the same height.

BARK STOVE is a kind of hot-house, containing a bed of tanners' spent bark,
mixed, according to circumstances, with a proportion of earth and other matters ;
in which are placed the plants, in pots or otherwise. The bed is usually gently
heated from a flue underneath, which together with its disposition to ferment of
itself (when kept moist), forms a powerful stimulant to accelerate and perfect
the growth of exotic plants.

BARLEY. A well known kind of grain, chiefly used in this country in the



BAROMETERS. 145

preparation of malt, for which see MALTING. The meal is also used in bread
and soups. Whiskey, gin, and other spirits, are distilled from it. The barley-
sugar of the shops is merely sugar boiled in barley-water to a consistency that
will cause it to solidify in the cold state ; it is poured upon a stone slab, anointed
with oil of sweet almonds, rolled out into little cylinders, and twisted.

BARM, or YEAST, is a substance which separates under the form of a froth,
more or less viscid, from all the juices and infusions which experience the
vinous fermentation. It is commonly procured from the beer manufactories,
and is hence called the barm of beer. If left to itself for some days, in a close
vessel, at a temperature of from 55 to 70, it is decomposed, and undergoes
the putrid fermentation. To prevent this decomposition, it is the custom, in
Paris, to evaporate it to a solid form, in which state it is sold under the name
of levure, which article, however, comprises not only yeast, but the bottoms
of the beer in the working-tun and store-casks. This is purchased by the yeast
merchants, the beer drained from it through sacks, and the remainder of the
beer washed out by putting the sacks in a stream of water ; the solid matter
left in the sacks is then dried in the open air. The true yeast, or froth of the
beer, is also dried for use in the same manner, as the bakers in Paris prefer it in
a solid state. Dry levure ought to be yellow, brownish, or greyish white, by no
means black or bitter. It should not yield to pressure by the fingers, and be
equally dry throughout, so as to break with a smooth surface. When dissolved
in hot water, and a few drops of the solution are poured into boiling water,
they should immediately rise to the surface. See BEER, and BREAD.

BAROMETER. An instrument by which the pressure or elasticity of the
air is ascertained. It consists essentially of a glass tube, not less than 34 inches
in length, closed at one end. The tube is completely filled with mercury, and
then inverted in a cup or box of the same metal. By inverting the tube, a
portion of the mercury will fall out, but a column varying in height from 28 to
31 inches above the surface of the metal will still remain, and which, being
supported by the pressure of the air, will be an indication of its amount at the
time. The barometer has assumed a variety of forms, the construction of the
most important of which we shall endeavour to explain ; but we deem it neces-
sary, previously, to enter a little more minutely into the nature and operation of
that form of the instrument to which we have just referred. The common baro-
meter was the discovery (for it can scarcely be called an invention) of Torricelli,
a disciple of the celebrated Galileo. It is, from this circumstance, frequently
called the Torricellian tube ; and the vacuous space above the surface of the
mercury in the tube, is called the Torricellian vacuum. In the construction of
the barometer, the principal object to be attained is a perfect vacuum in the
upper part of the tube. To effect this, we must first make the tube perfectly
free from moisture by exposing it gradually to the heat of a charcoal fire. The
tube must have a bore sufficiently large to render the effect of capillary attraction
insensible. The mercury employed to fill the tube must be rendered as pure
as possible, by pressing it through the pores of chamois leather, and afterwards
distilling it. A small portion of the purified mercury may now be put into the
tube, and heat gradually applied to it, till the mercury boils. Another portion
may then be added, and boiled in a similar manner ; and this process continued
till the tube is completely filled. By these means, the mercury itself will be
purged of any air it may have acquired, and the air which adheres to the sides
of the tube will also be effectually expelled. If the tube be now inverted into
a cup or other vessel of mercury, the barometer, as far as its construction is
concerned, will be complete. In the annexed cut, a c represents the barometric
tube inverted in its cup. The mercury has subsided to the point /; and the
next object is, to ascertain the precise length of the column from c to /. For
this purpose, a scale a 6 is attached to the barometer near the top. In the
ordinary use of this instrument as a weather-glass, the rise and fall of the mercury
seldom exceeds a range of 3 inches, the height of the column varying with
the atmospheric pressure from 28 to 31 inches. If, therefore, a scale of about
4 inches long, carefully graduated to tenths of an inch, be attached at the height
that accords with the number of inches we have mentioned, it will be sufficient to



146



BAROMETERS.



determine the ordinary changes of atmospheric pressure. There is, Fig. 1.
however, one circumstance to which it is necessary to attend, in
measuring the height of the mercurial column. It will be seen,
that as the column fc falls, it will necessarily raise the level of the
fluid in the cistern ; and contrariwise, when the column rises, it will
depi'ess the level of the cistern. On this account, the initial point of
the scale is perpetually changing its position, and, consequently, the
numbers on the scale cannot indicate the true distance between the
surface of the mercury in the cistern and that of the supported column.
To obviate this inconvenience, the horizontal sectional area of the
cup is made considerably greater than a like section of the tube,
by means of which a considerable fall in the tube produces but an
inconsiderable alteration in the level of the fluid in the cup. For
example, if the diameter of the tube be of an inch, and that of the
cup be 2 inches, the respective areas will be as to 4, or as 1 to
64 ; hence a fall of I inch in the tube will cause a rise of ^ of an
inch in the cup. In ordinary observations, this degree of accu-
racy would be sufficient ; but in the nicer operations of science, a
greater degree of precision is essential ; hence different contri-
vances have been employed for the purpose of ascertaining more
exactly the difference of levels. The most general mode of
effecting this object is to make the bottom of the cistern movable,
so as to be raised or depressed by means of a screw e. An ivory
index d may be attached to the top of the cup, and which being
finely pointed at the lower end, will serve to indicate a fixed level.
Before an observation is made with the barometer, the screw e is
turned until the surface is brought exactly to coincide with the point of the index, by
raising or depressing the bottom according as the surface was below or above that
point. By this means, the surface of the mercury always stands at the same level,
and hence the divisions on the scale a b will represent the actual changes in the
height of the barometric column. Having attained a method of fixing the initial
point of the scale, the next object to be attained is a means of reading offits indica-
tions with the utmost precision. In our description, we have supposed the scale
divided into tenths of an inch, but this is not sufficiently accurate for experimental
research. It would therefore be necessary to divide each of these tenths into hun-
dredths of an inch, and adapt a small microscope to the instrument, in order to
obtain the precise height of the column. The usual mode, however, of obtaining
these smaller subdivisions of the scale, is by means of a vernier, or small gra-
duated plate, which is movable by a screw, or otherwise, on p- ff
the divided scale of the barometer. The principle of the ver- to 30

nier will be seen by reference to the accompanying engraving.
Let a b represent a portion of the scale divided into tenths of
an inch ; let c d be the sliding scale, or vernier, equal in
length to 11 divisions on the principal scale, but divided
into only 10 equal parts. From this arrangement, it will be
seen that every division on the vernier will be the tenth part
of 11 tenths of an inch, or every division is equal to 11
hundredths of an inch, and, consequently, every division on
the vernier exceeds, by one hundredth of an inch, every
division on the principal scale. Suppose the vernier placed,
as in the diagram, so that its upper edge d may be exactly
even with the surface e of the mercury in the tube. If we
examine the principal scale, we shall perceive that the mer-
cury stands somewhat higher than 29 inches and 4 tenths. |g
If we now look at the vernier, we shall find that the divi-
sion 4, on its surface, coincides with a particular division on
the scale. Now, as we have seen that each division on the
vernier is one hundredth of an inch greater than one division
on the scale, it follows that the space from 29 up to the level
of the mercury, is 4 tenths and 4 hundredths of an inch,



BAROMETERS.



147



and, consequently, the true altitude of the column
is 29 inches, 44 hundredths. In using the vernier,
considerable precision must be employed in making
its upper edge a tangent to the curved surface of the
mercury in the tube, and, consequently, its accuracy
will still depend upon the skilful manipulation of
the observer. To obviate this, and render the baro-
meter self-regulating, Mr. Christie, Secretary to
the London Mechanics' Institution, has invented
and constructed a barometer, in which, by means
of a float, the vernier is set at its proper height
by the rising or falling of the mercury itself.
The annexed sketch will show in what the
improvement consists. Here a b c represents a
glass barometer tube about 35 inches long, exclu-
sive of the part b c. It is not less than a quarter
of an inch diameter inside, and enlarged at the
top a to about two inches. On the surface of the
mercury, shown by the shading, rests a small glass
ball, or float d, supporting a slender steel wire, d e
with its attached vernier /. This wire passes loosely
through a guide hole in the projection g to keep it
close to the scale of inches h i. In other respects,
the scale and vernier are similar to those of the
common barometer. The upper part of the en-
larged tube being vacuous, the mercury is pre-
vented from descending by the pressure of the
atmosphere on the surface at d; and as the pres-
sure increases, it will force down the mercury in
the part c b of the tube, and, consequently, cause
it to rise in the leg a b. As the surface at d falls,
the float carrying the vernier falls with it, and thus
the edge of the vernier being always kept at a
given distance from the surface of the mercury,
will indicate the precise amount of any changes
that may occur in atmospheric pressure.

In order to increase the extent of the baro-
metric changes, a contrivance is sometimes adopted,
called the diagonal barometer, which is represented
in Fig. 4. b c d is the glass tube bent at c, the
altitude of which is less than 28 inches ; hence c b
includes the whole barometric range in the present
form, while a c is the range it would have were
the whole of the tube vertical. Now it is mani-
fest, that by decreasing the angle at c, so as to
bring b c nearer the horizontal position, we can
make its proportionate length to a c as great as we
please. Suppose b c so inclined that its length
shall be three times greater than a c, then every
rise or fall of 1 inch in a c would be equivalent
to a rise or fall of 3 inches in b c. The difficulty,
however, of observing the precise height of the
mercury in this arrangement, more than counter-
balances the advantage resulting from the extended
range, and this form is, therefore, seldom adopted.

The wheel barometer is another contrivance for
enlarging the scale, and rendering minute changes
more easily observed. This, which is the common
domestic barometer, is represented in Fig. 5, in which
d b is the longer leg, a b the shorter, in which the



Fig- 3.




Fig.




148



BAROMETERS.



Fig. 5.



changes of altitude occur : for as the diameter of the
bulb d at the top is very large, compared with the
diameter of the tube at c, a small fall in d will be
equivalent to a considerable rise in the tube a b. Thus
the surface at c may rise 3 inches, and thereby
shorten to that amount the distance between the two
surfaces of the column, which is the true height sus-
tained by atmospheric pressure, without sensibly af-
fecting the level at d. At c there is an iron ball,
floating on the surface of the mercury, and partly
supported by means of a thread passing over the
pulley p, and carrying a small counterpoise at w. On
the axis of the pulley, an index z is fastened, which
moving with it, branches the circumference of the
circular plate, on which a scale is drawn, to repre-
sent the rise or fall in inches ; and also the terms
fair, change, rain, fyc. t which certain altitudes have
been improperly considered to indicate. In this
arrangement, it is manifest that as the column is
supported by the air's pressure on the surface c,
any diminution of that pressure will cause the mer-
cury in the longer leg to fall, and that in the shorter
to rise. On the other hand, if the atmospheric

Eressure increases, the mercury will rise in the
mger, and fall in the shorter, leg ; but, as before
observed, the change of level will be scarcely per-
ceptible in the former, on account of the enlarged diameter of the upper part.
The changes, then, that occur in the shorter leg, may, without material error,
be considered the representatives of the changes that are continually occurring
in the atmosphere. Now it will be evident, on inspection, that as the ball c is
partly supported by the mercury, it will partake of its motion. If the pressure
of the air increase, the surface at c will be depressed, and as the iron ball must
sink with it, the thread to which it is attached will, at the same time, communicate
its motion to the pulley^?, and through it, to the index, which will, consequently,
move from the right towards the left of the graduated circle. On the other
hand, if the atmospheric pressure decrease, the surface at c will rise, and, with
the assistance of the counterpoise w force up the iron ball, and, by this means,
turn the pulley and index in an opposite direction. There are two sources of
error in this instrument, that render it inferior to the simple vertical barometer.
These are, the pressure of the iron ball on the surface of the mercury, which
necessarily increases the height of the shorter column, and the friction of the
pulley. It is impossible entirely to annihilate these causes of error, and hence,
for philosophical purposes, the straight barometer is preferred.




Fig, 6.



The Syphon Barometer is an instrument used
to ascertain the pressure of air in the partially
exhausted receiver of an air-pump. It consists
of a tube bent as in the accompanying engrav-
ing, Fig. 6. Each leg may be about 4 inches in
length, and one a b completely filled with puri-
fied mercury. When the instrument is con-
nected with the air-pump by means of the screw
at d, and the air partly exhausted, the mercury
in the leg a b will begin to fall, and, conse-
quently, rise in the branch c b ; every inch,
therefore, that it falls in a b, must be reckoned
equal to two inches in the straight barometer.
This instrument does not begin to act till the
air is reduced to about i of its original density ;
but this is no inconvenience, as its indications are seldom required till the
exhaustion is nearly complete. If the leg a b be lengthened, and left open at the



BAROMETERS. 149

top, as in Fig. 7, this barometer becomes a useful appendage to the steam
engine, in ascertaining the pressure of steam within the boiler. When it is
attached to the boiler by the screw d, and the air or steam in the boiler has
the same elasticity as the external air, the mercury stands at the same height in
both legs ; but as soon as the steam increases in elasticity, the mercury will be
depressed in the leg c b, and rise in a b, and the difference between the two
levels will be proportional to the difference between the external and internal
pressure. If the common barometer stands at 30 inches, and the difference of
level between the two surfaces is 6 inches, the elasticity of the steam will be
^ or | greater than the pressure or elasticity of the atmosphere.

There are other forms of the barometer, but their comparative unimportance
renders it unnecessary to describe them here. We shall, therefore, proceed to
consider the most important purposes to which this instrument is applied.

The most immediate use of the barometer, for scientific purposes, is the ascer-
tainment of the amount and variation of atmospheric pressure. The fluctuations
in the pressure being observed in connexion with changes in the state of the
weather, a general correspondence is supposed to prevail between these effects.
The instrument has, from this circumstance, been called a weather glass. Rules
have been attempted to be established, by which the approaching state of the
weather may be predicted from the height of the mercury, and the words rain,
fair, changeable, &c. are engraved on the scales of common barometers. These
marks are, however, entitled to no attention, since it is the changes that occur
in the height, and not the absolute height, that indicates approaching changes
in the weather. The variation in the altitude of the barometer in a given place,
together with the corresponding changes of the weather, have been regularly
recorded for a considerable time ; and it is by an exact comparison of these
results that general rules are to be found. At present, the best rules are liable
to some uncertainty at times. Those which have been considered least liable to
error are the following: 1. Generally, the rising of the mercury indicates fair
weather ; its fall shows the approach of foul weather. 2. In sultry weather the
fall of the mercury indicates coining thunder. In winter, a rise indicates frost.
In frost, its fall indicates thaw, and its rise indicates snow. 3. Whatever change
of weather suddenly follows a change in the barometer, may be expected to last
but a short time. Thus, if fair weather follow immediately the rjse of the mer-
cury, there will be very little of it ; and in the same way, if foul weather follow
the fall of the mercury, it will last but a short time. 4. If fair weather con-
tinue for several days, during which the mercury continually falls, a long suc-
cession of foul weather will probably ensue ; and again, if foul weather continue
for several days, while the mercury continually rises, a long succession of fair
weather will probably succeed. 5. A fluctuating and unsettled state in the mer-
curial column indicates changeable weather. The other important purpose to
which the barometer is applied, is the measurement of altitudes. If the atmo-
sphere were a liquid of nearly equal density, like water, the measurement of
heights by the barometer would be the simplest process imaginable : for we
should have then only to make one experiment to ascertain how much the mer-
cury would fall, in rising to the height of 100 feet for example, and then the fall
for 200 or 300 feet would, of course, be double or triple the former one. But
the density of air is well known to decrease as we ascend from the earth, so
that at the height of 3 5 miles, it is only one half its density on the surface of
the earth. From this it must be evident that if the mercury fall one-tenth of an
inch in rising through the height of 100 feet, we must rise through a greater
height to cause a fall of another tenth. The height of the surface of the atmo-

rsre above that of the earth is considered to be about 50 miles ; and we have
ady observed, that at the height of 3 miles the density is reduced to one-
half. Hence we should find, by ascending to the height of 3 miles in the
atmosphere, the mercury would stand at one-half the height of another
barometer at the surface of the earth. If, however, the decrease of density
were affected by the height alone, the determination of altitudes would be com-
paratively easy, as a simple formula may be given, which would immediately
show the relation between the height and density. The circumstance that

u



150 BARREL.

interferes with barometric observations, is temperature, which affects them in
two ways. 1. Increase of temperature expands the mercury in the barometer,
and thereby causes the column to be longer than at lower temperatures. 2.
The air itself becomes expanded by heat; and hence the column becomes
lengthened without any increase in its absolute weight. It might be thought
that these effects were too trivial to influence sensibly the results of our obser-
vations ; but it must be remembered, that as we ascend from the earth, the
temperature of the air rapidly decreases, so that at a certain height, dependent
on the latitude of the place, a freezing temperature constantly prevails.
Putting aside the effect of change of temperature, the simplest rule for deter-
mining heights is as follows: Observe the height of the mercury at the
bottom and top of the altitude to be ascertained ; take the logarithms of these
heights, and multiply their difference by 10,000, the product is the answer in
fathoms. Then suppose the mercury at the foot of a mountain to stand at 29.5
inches, and at its summit 26.4 inches, the calculation would be as follows :

Lower barometer. . . 29.5 log. .469822

Upper ditto 26.4 log. .421604

Difference .048218

10000

482,180,000 Fathoms,
or, 2893 feet,

which is the altitude, supposing the temperature to be at 31 Fahr. If the
temperature differ from this, it must be observed at the upper and lower station,
and a mean of the two taken, by adding them together, and dividing the sum
by 2. If the mean thus obtained exceed 31, the altitude before obtained must
be increased ~g for every degree of difference between them, and vice versa. If
we wish to correct the other error arising from the expansion or contraction of the
mercury in the barometer, we must observe the temperature of the mercury at the
upper and lower station ; then the altitude of the lower one must be increased, or
the higher one diminished, ^ part for each degree of difference between their
temperatures. To those who are unaccustomed to the use of logarithms, the fol-
lowing rule may be preferred : Take the sum and difference of the upper and
lower barometric heights, and divide one by the other ; multiply the quotient by
55000, and it will then answer in feet for a temperature of 55. Suppose, as
before, the height at the lower station to be 29.5, and at the upper, 26.4 then
29.526.4

= 0554

29.5 + 26.4

And .0554 X 5500 = 3047 feet.

This result, it will be seen, exceeds the other by 154 feet; it is not so exact,
but, in many cases, it may serve to furnish a tolerable approximation, when
logarithmic tables are not at hand. The corrections for temperature may be
applied as in the other formula.

BAROSCOPE. An instrument for shewing the weight of the atmosphere,
frequently confounded with the Barometer : the former, however, only proves
that the atmosphere has weight; while the latter determines its true quantity.

BARREL. This article, as made by coopers, is too well known to need a de-
scription; but the air and water-tight metallic barrels employed in the British navy
for preserving provisions, from their peculiarity of structure and utility, require a
place in this work. Those which we shall describe are of the most improved
kind, and were patented by the late Mr. Robert Dickenson, of the Eagle Foundry,
Southwark. These barrels are made of wrought iron, of a cylindrical form,
with a seam, soldered or rivetted, in the usual way. To strengthen the figure,
and adapt it for the reception of the heads, a strong iron hoop is rivetted to
eaeh end of the cylinder. These hoops are prepared at the iron works, by
rolling them into the form of a rebate, shown in section at , Fig.. 1. By this



Online LibraryLuke HebertThe engineer's and mechanic's encyclopædia, comprehending practical illustrations of the machinery and processes employed in every description of manufacuture of the British Empire .. (Volume 1) → online text (page 21 of 115)