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pulling the lever N towards f. A small sluice which upon the side of the drum, Fig. 94, is then opened, and the barley is allowed to run off into the trough Q. When the drum is thus emptied, the sluice is shut, and the lever N is moved back, by which means the wheel м engages with the spindle R by the catch 2 5, and the sluice x being opened, the drum is filled with fresh barley. When this machine is set in motion, the barley is so thoroughly tossed and beaten between the stone and the case, owing to the double motion, that it is entirely deprived of its skin, and, if the process be continued, it also loses a further portion of its substance, and becomes pearl barley. By means of this machine three bushels of barley may be converted into pot barley in an hour, and into pearl barley in two hours.

different specific gravities; he found that the lighter the liquid, the greater the height of the column in the tube. But, as the effect of an invisible fluid like the air in supporting a column of so ponderous a fluid as mercury was still doubted, Pascal suggested a complete experimentum crucis by observing the height of the column of mercury supported at the bottom, and at the top, of a considerable elevation. Two tubes filled with mercury were conveyed to the mountain Puy de Dome, near Clermont, in Auvergne. At the base they both stood at 28 inches; one was left there, and the other taken to the summit. As the observers ascended, the mercury in the tube sank gradually to 24.7 inches, and as they descended, the mercury as gradually rose again to its former elevation. This result was exactly what Pascal had anticipated; for,

BARM. Another name for YEAST, probably from if the cause which sustained the column in the tube bæren, to raise. See BEER.

BAROMETER. The application of this word, from Bapos, weight, and μérpov, a measure, is not very appropriate, the common balance being rather a measurer of weight than this instrument, which merely indicates or measures the variations in the pressure of the atmosphere. The barometer is one of the most valuable instruments ever contrived for assisting the philosopher in searching out the laws of the wonderful ocean of air in which we live, and its invention belongs to a highly interesting period in the history of science. Some time previous to the year 1643, a pump was sunk at Florence for the purpose of raising water from an unusually great depth, when, upon working it, it was found that the water would rise no higher than about 32 feet. Galileo was consulted, but he did not explain the phenomenon satisfactorily. After his death, his pupil, Torricelli, who had long meditated on the subject, devised the following experiment, in the year above mentioned. He took a glass tube, about 4 feet in length, closed at one end and open at the other, and, having filled it with mercury, closed the open end with his finger, inverted it, and placed the open end below the surface of mercury contained in a basin. Holding the tube in a vertical position, he withdrew his finger, and observed that the mercury sank in the tube a certain distance, and, after a few oscillations, settled at the height of about 27 inches above the surface of the mercury in the basin. On comparing the height of the column of mercury with the height of the column of water raised by the pump, he found these heights to be in an inverse ratio of the specific gravities of the water and mercury. Observing also that the columns of water and mercury had no communication with the atmosphere at their upper extremities, but that they did communicate therewith at their lower extremities, he concluded that the two columns were suspended by the same cause, namely, the weight or pressure of the atmosphere.

In 1646, Pascal at Rouen verified Torricelli's experiment, and also varied it by employing liquids of

(1) The vacant space thus formed between the top of the mercury and the top of the tube is called the Torricellian vacuum.

were the weight of the atmosphere acting upon the external surface of the mercury in the basin, then it was to be expected that on taking the tube to the top of a mountain, a gradually diminishing quantity of atmosphere would be above it, and the column of mercury sustained by the weight of this incumbent atmosphere must undergo a corresponding diminution in height. The experiment was repeated by Pascal himself, with similar success, by taking one of the tubes to the top of a high tower in Paris. It was further observed that by keeping the tube for a length of time in a fixed position, the height of the column was found to vary from day to day within certain small limits: an effect which could only be ascribed to fluctuations in the weight of the atmosphere itself. The simple apparatus devised by Torricelli, consisting of a tube of mercury dipping into a cistern of the same metal, is in fact the barometer. Although this instrument has been distorted into endless varieties of shape, and with numerous additions, yet they have been abandoned one after the other, and the simple form in which Torricelli first observed the effect of atmospheric pressure in sustaining a column of mercury, has long been admitted to be the best form of apparatus for noting the daily and hourly fluctuations in that pressure. There are, however, certain precautions necessary to be attended to, both in the filling of the tube and in making observations.

The essential parts of a good barometer are a well-formed glass tube, Fig. 95, 33 or 34 inches long, of equal bore, containing pure mercury; a cistern of mercury, for receiving the open end of the tube; and an accurately-adjusted scale, for ascertaining the exact height of the column.

Fig. 95.

The mercury must be purified from all baser metals, such as tin, lead, zinc, and bismuth, all of which it dissolves with facility, and which are commonly used as adulterants; for, as the specific gravity of each of these metals is much less than that of mercury, their presence would cause the height of the barometer to

be greater than that of an instrument containing pure metal only. Mercury appears to be incapable of retaining either air or moisture, and the air-bubbles which rise from it when heated or relieved of atmospheric pressure are merely retained between the mercury and the glass vessel in consequence of the attraction existing between glass and air.

On pouring mercury into the barometer tube and inverting it, as in the Torricellian experiment, the film of air confined between the mercury and the inner surface of the tube, being relieved from pressure, will escape into the Torricellian vacuum, where it will by its elasticity oppose the pressure of the external air, and constantly maintain the mercurial column at a lower level than if the vacuum were perfect; so that the observed height of the column would indicate only the excess of the pressure above that within the tube. In order to get rid of this air, and any moisture within the tube, the mercury is introduced in small portions at a time, and boiled over a charcoal fire between each introduction, holding that part of the tube over the fire which contains the last portion of mercury introduced. The filling of a barometer tube requires many precautions, which are usually noticed in works devoted to the subject.1 When the tube is properly filled, it is inverted into a cistern of pure mercury, and when the column sinks to the proper level, its length above the surface of the mercury in the cistern exactly counterbalances the atmospheric pressure, unless, indeed, we take into account the minute quantity of vapour of mercury which, above the temperature of 60°, rises into the Torricellian vacuum; but this is so slight a cause of deterioration that it may be neglected.

The precautions taken to ensure contact between the mercury and the interior of the tube cannot of course be used for the exterior, where the tube is surrounded by the mercury in the cistern. A film of air is always retained at this part of the tube, and also at its under edges, which film creeps by small portions at a time into the interior, and rises up in minute bubbles into the vacuum, the film being constantly renewed by the descent of more air between the outside of the tube and the mercury in the cistern. In this way the most carefully constructed barometers deteriorate in the course of years, as was shown by Professor Daniell, on a comparison of the registers of the celebrated Meteorological Society of the Palatinate.

At Buda the difference is .035 inch; at Brussels .044 inch; at Munich .026 inch; from the summit of Peisenberg, a mountain in Bavaria, .026 inch; and from the summit of Mount St. Gothard the depression is also .026 inch.2

This irregular and uncertain deterioration of barometers cannot be too greatly deplored, because it vitiates the observations of all those earnest and competent observers who during many years have devoted daily portions of their valuable time to a record of the oscillations of the barometer at various stations. Indeed, until the defect complained of was remedied by Professor Daniell, the great mass of barometrical observations can scarcely be said to be of any scientific value. The remedy consists in welding a ring of platinum to the open end of the barometer tube, so as to bring it in contact with the mercury by this simple means the ingress of air into the tube has been effectually prevented.

The same excellent man and distinguished philosopher also invented a new mode of filling barometer tubes, by screwing the tube into the under surface of the table of an air-pump, and making such arrangements as enabled him to fill the tube and also to boil the mercury in vacuo. For the details of this contrivance we must refer to the original essay; but it may be noticed, as a striking proof of the absence of air and the perfect contact of the mercury with the glass, that, although the bore of the tube was more than half an inch, yet, on inverting it, the mercury did not fall at once to its usual height, but remained suspended to the top of the tube, until detached therefrom by a few concussions.

The excellence of a barometer may be tested by the brightness of the mercurial column, and the absence of any flaw, speck, or dulness of surface; secondly, by what is called the barometric light, or flashes of electric light in the Torricellian vacuum, produced by the friction of the mercury against the glass, when the column is made to oscillate through an inch or two in the dark; thirdly, by a clicking sound produced by the mercury striking the top of the tube when the column is made to oscillate. If air be present it will form a cushion at the top, and prevent or greatly modify this click.

The sectional area of the tube is of no consequence to the height of the column of mercury supported. If the sectional area be equal to 1 square inch, the column of mercury 30 inches high will be counterbalanced by a column of atmospheric

In the register kept at Mannheim for twelve years, from 1781 to 1792, inclusive, the mean height of the barometer for the second six years is .062 inch lower often be observed in old looking-glasses, which may probably be than that of the first six years.

In the register kept at Padua during the same period, the mean of the last six years is .044 inch lower than that of the first six.

In the register kept at Rome, the average of the last six years is lower than that of the first by .114 inch.

(1) The construction of the barometer and its use as a meteorological instrument are described in detail in the Editor's "Treatise on Pneumatics," published in Weale's Rudimentary Series.

(2) Professor Daniell remarks:-" There is a defect which may referred to the same cause as the deterioration of barometers. I allude to a dulness which takes place in large spots over their surface, and which generally seems to radiate from the centre. I have frequently remarked this in the very old mirrors in some of the palaces upon the Continent. I imagine that this arises from

the slow insinuation of air by the edges, or some accidental crack

in the metal at the back of the glass." A damp wall will also produce a similar effect upon looking-glasses, the moisture probably favouring the entrance of the air. See "Elements of Meteorology," vol. ii. London, 1845. The two Essays on the Construction and Deterioration of Barometers are admirable specimens of patient research, which ought to be studied by every one interested in the subject.

air 1 inch square, and extending from the surface of the mercury in the cup b, Fig. 95, to the top of the atmosphere; and as we know the pressure of the air to be about 15 lbs. on the square inch, so the column of mercury 1 inch square in the barometer tube weighs about 15 lbs. If instead of mercury we take 15 lbs. of water, and form it into a column 1 inch square, we get in such case a height of about 32 feet. If the sectional area of the tube of the mercurial barometer be only half an inch, the column of mercury will still retain the same height, for it is counterbalanced by the same height of atmosphere, only the column of atmosphere has in this case a base of only half a square inch instead of an inch. So long therefore as the atmosphere presses with the same intensity upon the surface of the mercury in the cup, the column suspended in the tube will be of the same height whatever be its internal diameter.

contact, the height indicated by the scale is correct. In other forms of the barometer, the mercury in the cistern is always maintained at the same level, for which purpose the cistern is formed partly of leather, so that by means of a screw at the bottom the surface of the mercury may always be adjusted to the neutral point before taking an observation. The cistern is also sometimes provided with a gauge or float, which indicates when the mercury in the cistern is too high or too low. By turning the screw one way or the other, the mercury in the cistern is adjusted to the proper level. When there is no gauge, the relative capacities of the cistern and tube are ascertained and marked on the instrument together with the neutral point. In an example given by Mr. Belville,' the capacity for every inch of elevation of the mercury in the tube is supposed to be equal to th, which reduced to a decimal 0.025 inch for 1 inch; 0.013 inch for inch; 0.007 inch for inch.

Then if the observed height.
And the neutral point be

=

The difference above the neutral point will be..
Then add for capacity

The correct height will be

Inches.

= 30.400

= 30.000

.400 + .010 30.410

In this case the observed height is above the neutral point. In the following example it is below it.

Observed height

Neutral point

Difference below neutral point
Subtract for capacity.

Correct height

Inches. = 29.500

= 30.000

.500

.013

29.487

It has been already stated that the height of the mercurial column a b, Fig. 95, must be measured from the surface of the mercury in the cistern. Now it will be obvious that the level of the surface must always change with the oscillations of the column. When the atmospheric pressure increases, and the mercury in the tube rises, a portion of the metal is drawn out of the cistern into the tube, and the level of the mercury in the cistern is depressed. So on the contrary, when the atmospheric pressure diminishes, a quantity of mercury is forced out of the tube into the cistern, and the level of the metal in the latter rises. If therefore the instrument be furnished with a fixed scale at the top of the column, graduated when the distance between the top of the column and the level of the mercury in the cistern is exactly 30 inches, it will be evident that when the top of the As the range of the barometer in this country is column sinks to 29 inches on the scale, the distance limited to about 3 inches, it is not necessary to between the two extreme points will not be 29 inches, commence the scale from the neutral point. The but somewhat less, depending on the capacity of the divisions usually begin at the 27th inch, and are cistern. So also, if the column rise to 31 inches, the continued to the 31st. But in instruments intended distance will be rather more than this, on account of to measure the height of mountains, or for accomthe additional quantity of mercury drawn into the panying balloons, the scale begins at the 12th or the tube. If the cistern be a section of a cylinder with 15th inch. Each inch is divided into 10 parts, and a flat bottom, bearing a certain known proportion to these are subdivided into hundredths by means of a the bore of the tube, such as 1: 100, and the mercury little sliding scale called a vernier or nonius2 attached rise 1 inch above the neutral point, then as much to the side of the large scale as in Fig. 96. mercury will be withdrawn from the cistern as fills measures exactly one inch and one-tenth in length, 1 inch of the tube; but as the base of the cistern is and is divided into 10 equal parts numbered from the 100 times greater than the bore of the tube, it is top downwards, while the divisions of the inches of obvious that this inch of mercury in the tube would the scale are numbered from the bottom upwards. cause a fall of only th of an inch in the level of the Now as 10 divisions on the vernier are equal to 11 mercury in the cistern; or in other words, the fall of on the scale, and as those 10 are all equal to each Tooth inch in the mercury in the cistern is accompanied other, it follows that each division of the former by a corresponding rise of ths in the tube. A must be equal to 1 division of the latter, or to similar effect is produced by any other change in theth inch. If therefore any division on the vernier height of the column, so that if the inches on the graduated scale be made each oth part less than an inch, the instrument will afford tolerably correct Observatory, Greenwich. London, 1849. This cheap but excellent results. In some instruments, however, the scale accurately divided into inches and parts of inches is made movable, and terminates in an ivory point which is brought down to the surface of the mercury. When this point and its reflection appear to be in

100

It

(1) A Manual of the Barometer. By J. H. Belville, of the Royal

little work ought to be in the hands of every one who uses a barometer.

(2) So called from Peter Vernier, a gentleman of Franche

Compté, who described it in a tract printed at Brussels in 1631.

The word nonius is derived from Peter Nunnez or Nonius, as his name has been Latinised, a Portuguese Mathematician born at Alcazar in 1497

Fig. 96.

31

30

29

28

coincide, or is in a line or 'Fair;' and, more particularly in winter, a fine with a division on the bright day will succeed a stormy night, the mercury scale, the two lines im- ranging as low as 29 inches, or opposite to 'Rain.' mediately above or below It is not so much the absolute height as the actual those which coincide will rising and falling of the mercury which determines be separated by a distance the kind of weather likely to follow." Instrumentexactly equal to 8th inch; makers still continue to engrave these words on the the pair, two divisions re-scale, apparently for no other reason than old-estamoved from the first, has a blished usage; their customers would probably think deviation ofths of an the instrument imperfect without them, just as the inch, and so on. Thus in readers of Moore's Almanack insist upon having the Fig. 96, the line marked 6 supposed influence of the planets upon the different on the vernier coincides members of the body entered for every day in the with the line 28.9 on the year. Indeed, the defects of the common barometer, scale, but the two lines as it leaves the hand of the instrument-maker, are so immediately above them, serious as to render this instrument almost worthless marked 5 and 29, do not to science. "In the shops of the best manufacturers exactly coincide; and this and opticians," says Professor Daniell, "I have obwant of coincidence must served that no two barometers agree; and the differamount toth of th of ence between the extremes will often amount to a an inch, orth of an quarter of an inch; and this with all the deceptive inch. In the next two lines, appearance of accuracy which a nonius, to read off to marked 4 and 29.1, it will the five-hundredth part of an inch, can give. The be seen that they fail to common instruments are mere playthings, and are by coincide byth of ths no means applicable to observations in the present of an inch, or ths of an state of natural philosophy. The height of the merinch. In like manner the cury is never actually measured in them, but they are lines marked 3 and 29.2, graduated from one to another, and their errors are 2 and 29.3, 1 and 29.4, and 0 and 29.5, deviate from thus unavoidably perpetuated. Few of them have each other respectively by 18, 18, andths any adjustment for the change of level in the mercury of an inch. The same reasoning will also apply to of the cistern, and in still fewer is the adjustment the lines situated below the coincident lines marked perfect. No neutral point is marked upon them, nor 6 and 28.9. Thus 7 and 28.8 immediately below is the diameter of the bore of the tube ascertained; them fail to coincide byth of an inch, and so on and in some the capacity of the cistern is perpetually with respect to the others. The point to be attended changing, from the stretching of a leathern bag, or to is that a division on the vernier is th of an inch from its hygrometric properties. Nor would I quarrel larger than a division on the scale. with the manufacture of such playthings: they are calculated to afford much amusement and instruction; but all I contend for is, that a person who is disposed to devote his time, his fortune, and oftentimes his health, to the enlargement of the bounds of science, should not be liable to the disappointment of finding that he has wasted all from the imperfection of those instruments upon the goodness of which he conceived he had good grounds to rely. The questions now of interest in the science of meteorology require the measurement of the five-hundredth part of an inch in the mercurial column; and, notwithstanding the number of meteorological journals, which monthly and weekly contribute their expletive powers to the numerous magazines, journals, and gazettes, there are few places indeed of which it can be said that the mean height of the barometer for the year has been ascertained to the tenth part of an inch."

In applying the vernier to measure off small fractions of an inch in the height of the barometer, we first notice the height of the column by the fixed scale, which in Fig. 96 is more than 29.5 inches, but less than 29.6. In order to measure the hundredths of an inch, we place the zero or top of the vernier scale exactly level with the top of the mercury; we next observe that of all the lines on the vernier only one can coincide with a line on the scale. In the figure the line marked 6 on the vernier coincides with a line on the scale; and as from the top of the mercury to these coincident lines there are six pairs which do not coincide, and as each pair deviates byth of an inch more than the pair below it, the uppermost pair must evidently differ byths of an inch. We thus get the height of the mercury in our figure, which is 29 inches andths of an inch, or, expressed decimally, 29.56.

The words "Change," "Fair," and "Rain," engraved on the plate of the barometer, are calculated to mislead the observer; for, as Mr. Belville observes, "from the observations of two centuries we find that heavy rains and of long continuance take place with the mercury at 29.5 inches, or 'Change;' that rain frequently falls when it stands as high as 30 inches,

The barometer ought to be fixed in a truly vertical position, and, if possible, with a northern aspect, in order that it may be subject to as few changes of temperature as possible. It is usual, for the sake of comparison, to reduce the observations to 32°, for which purpose tables for correction for temperature are given in scientific works devoted to the subject of the barometer. "The height of the cistern of the

The causes of the oscillations of the barometer are too intricate to be discussed within the limits of a short article, but the reader will find them stated at some length in the treatise on Pneumatics, in Weale's Rudimentary Series. When the barometer is used alone, it has a far more direct application to the theory of the winds than to that of rain, and with this view Mr. Belville has constructed the best set of rules we have ever seen, connecting the phenomena of the barometer with the direction of the wind, and also with the appearance of the clouds, according to Howard's nomenclature. These rules occupy 11 pages of Mr. Belville's little manual, to which we must refer. This work also contains a table showing the mean height of the barometer at noon for Greenwich, for every day in the year, deduced from 30 consecutive years' observations, viz. from 1815 to 1844, and reduced to 32° Fahr. The following are the monthly means from this table:

barometer above the level of the sea, and, if possible, | equally upon the mercury, the glass of the tube, and the difference of the height of the mercury with some the metal of the scale, are also made in accurate standard, should be ascertained, in order that the observations, and tables are provided for all these observations made with it should be comparative with purposes. others made in different parts of the country. Before taking an observation, the instrument should be gently tapped, to prevent any adhesion of the mercury to the tube; the gauge should be adjusted to the surfaceline of the cistern, and the index of the vernier brought level with the top of the mercury." The best times of the day for observing the barometer are at 9 A.M. and 9 P.M., when it stands higher, and at 3 A.M. and 3 P.M., when it stands lower, on an average, than any other times during the twenty-four hours; as may be proved by examining a barometric register kept for a long period, and taking the average of each set of observations made at the same hour. In this climate, and in summer, the mean of a few weeks is sufficient to elicit this fact. As 3 A.M. is an inconvenient time for observations, a person with time at his disposal should select the other three hours for making his entries. If he can make only two observations, the proper periods are the convenient hours of 9 A.M. and 9 P.M. If he can make only one observation, then noon is the time. Professor Daniell remarks that those who merely consult the barometer as a weatherglass would find it an advantage to attend to the three above-mentioned periods, for he has noticed that by much the safest prognostications from this instrument may be formed from observing them when the mercury is inclined to move contrary to its periodical course. If the column rise between 9 A.M. and 3 P.M., it indicates fine weather; if it fall from 3 to 9, rain may be expected.

In the application of rules for prognosticating the weather, by observations made on the barometer, the most important fact to be remembered is, that the state of the weather to be expected is not so much connected with the absolute height of the column as with its motion, whether rising or falling. A fall in the mercury generally indicates approaching rain, high winds, or a thunder-storm; but snow is more frequently preceded by a rise than by a fall. With this exception, a rising state of the barometer commonly indicates the approach of fine weather. The lowest depressions occur with the wind at S. and S.E., when much rain falls, and frequently short and severe gales blow from these points. A N.E. wind is more conducive to a high state of the barometer than any other. When the mercury rises or falls steadily for two or three days together, it is generally found that rather a long continuance of settled weather will follow rainy in the latter case, and fine and dry in the former. By the same rule, frequent fluctuations in the height of the column are found to coincide with unsettled weather.

:

In a tube of small bore, when the mercury is rising its surface is convex; when it is falling it is concave, as if the centre of the fluid column always preceded the sides in all its motions. In such a tube, in nice observations, a correction has to be made for capillarity. Corrections for temperature, which acts un

January 29.909 second maximum
February
29.859

29.857 second minimum

March
April

29.856

29.884

29.910 maximum

29.894

May

June

July

August 29.890
September 29.872
October 29.851

November 29.801 minimum
December 29.884

The mean annual pressure for noon at Greenwich is 29.872 inches.

It appears from Mr. Belville's table, that "at certain seasons of the year, great periodic maxima and minima take place. The greatest daily mean pressure for the year occurs about the 9th January, and the minimum daily mean depression towards the end of November. It is a remarkable coincidence, that the lowest daily mean temperature for 30 years, occurs on the 8th and 9th of January, and the daily mean temperature for November rises suddenly 4° in the last few days in November."

From a table of the greatest and least observed heights of the barometer at Greenwich, between 1811 and 1848, it appears that the maximum elevation for the 38 years, occurred in 1825, when the mercury stood at 30.89 inches; in 1821 it reached 30.82 inches; in 1835, 30.84 inches, and in February 1849, 30.86 inches. In the extreme depressions those of 1821 and 1843 differ only by 21 hundredths; the first occurred on the 25th December, when a Troughton's mountain barometer sank as low as 27.89 inches. "A heavy rain of some hours' duration with the wind at south-east, had preceded the minimum pressure; a gale from the north-west followed, in which the mercury rose a few tenths. The depression of 1814, 28.21 inches, happened at

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