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35-} AIR-GAUGE. 59
above the level of that in the cistern. Such a tube, forming, in fact, a water barometer, was placed by the late Professor Daniell in the hall of the Royal Society. It is very sensitive to changes in the pressure of the atmosphere, the column during a gale of wind rising and falling visibly in the tube.
(35) Pressure Gauge and Barometer.—If the tube were filled with a liquid denser than water, a proportionately shorter column of it would be sustained by the pressure of the air, the length of the column being inversely proportional to the density of the two liquids. Now as mercury is rather more than 13 times denser than water, this liquid metal will rise to a height only about -rV as great as that of water, or to a height of about 30 inches (762 millimetres) instead of 10*33 metres. This result is easily verified; for if a glass tube about one metre in length, and closed at one extremity, be completely filled with mercury, the aperture closed with the finger, and it be placed mouth downwards in a basin of mercury,—on removing the finger, the column of fluid metal will partially descend, and leave a void space of 240 or 250 millimetres (about 10 inches) in length in the upper part of the tube. But the most complete demonstration that the mercury is sustained solely by the pressure of the air upon that in the basin, is furnished by placing the whole apparatus under the receiver connected with the air-pump: as the air is exhausted, and consequently the pressure is diminished, the column sinks; but it recovers its former level on readmitting the air from without. A tube, or air-gauge, acting on this principle, is usually attached to every air-pump, as a convenient means of judging of the perfection of the vacuum. If it were possible wholly to exhaust the air from the receiver, the mercury would rise in such a gauge (which is simply a tube open at top into the receiver, and dipping below into a basin of mercury; until it stood at the same level as in the barometer at the time of the experiment: but this result is never attained in practice; the pressure of the portion of air remaining in the receiver always depresses the mercury nearly a millimetre in the gauge below this point. By means of the gauge, the density of the air still remaining in the receiver is readily ascertained, for the density is always proportional to the pressure. Suppose, then, the gauge showed a residual pressure of 2 millimetres, the remaining air would have only ^T of the density that it possessed at the commencement, if the atmospheric pressure shown by the barometer at the time were equal to that due to a column of 760 millimetres in height.
Regnault employs a gauge, at the side of which an ordinary
Fig. 15. mercurial barometer plunging into the same cistern is placed (fig. 15), so that the difference in height between the two columns of mercury may be read off with great accuracy by means of a graduated scale and verniers, v, c*
A simple pressure gauge or manometer (from fiavog, rare) for estimating the rarity or condensation of air in a confined space, is made by bending a tube into the form shown in fig. 16, and pouring water into the bend; the apparatus is attached at a to the air vessel, the other limb, b, being open to the atmosphere; by the difference of level, the pressure of the gas under experiment can be accurately estimated by a scale placed between the tubes. When the pressures are considerable, mercury is used instead of water. A pressure gauge of this simple description is in constant requisition in coal-gas works for estimating the pressure in the gasometer, in the street mains, or at any part of the services. A simple inverted tube when filled with mercury, with due precautions to exclude every particle of air, and furnished with accurate means of measuring the height of the
D column above the level of the mercury in the cistern, constitutes one of the most indispensable philosophical instruments—the barometer (from fiapoc, a weight, and pirpov, a measure). The diameter of the tube is of little consequence; but a tube of from 8 to 13 millimetres (J or £ an inch) wide, or wider, is preferable to one of smaller bore. A slight fixed correction for capillarity, varying with the diameter of the tube, is required for each instrument. In the best instruments of this de
I scription the whole scale is moveable by a rack and pinion, p (fig. 17), and can be adjusted so that its lower extremity, which for convenience of observation is made to terminate in a fine
* The difference in height may be still more accurately read off, without the
steel point e, can be brought to coincide ex- Fig. 17.
actly with the surface of the mercury in the cistern: unless this contrivance were adopted, it would not be possible to measure accurately the height of the column of metal, because the level of the mercury in the cistern is continually undergoing slight variations: as the metal rises in the tube it falls in the cistern, and vice versd: part of the cistern is constructed of glass, to allow the point of the scale to be seen. The height of the mercurial column above the level of the mercury in the cistern, when the instrument has been placed in a truly vertical position, is read off at the top by a vernier v, which estimates differences of a tenth of a millimetre. The barometer has been constructed in a great variety of forms, but the simple inverted tube is the best for ordinary purposes.''
(36) The Syphon, which is another instrument in frequent use in the laboratory, depends for its operation partly upon the principle of atmospheric pressure. The syphon is a bent tube, by means of which liquids may be lifted above the level at which they stand, provided that they are ultimately transferred to a lower level. Suppose that it be desired to draw off a liquid without disturbing a powder which has settled down to the bottom of a vessel; a bent tube or syphon (s, fig. 18), one limb of which is longer than the other, is filled with water, and closed by placing the finger at the end of the longer limb; the instrument is then inverted, and the short limb is plunged rapidly into the liquid to be decanted. On removing the finger
employment of verniers, by using an instrument called a cathetometer (from KaOfTos, a perpendicular), which consists of a telescope of short focus, furnished with B spirit level, by which its horizontally may be secured. Upon a graduated scale, which is placed truly vertical, the telescope slides up and down, and is raised or lowered until its cross-wires coincide with the level of the mercury. With an instrument upon this principle properly constructed, differences in vertical height in the mercurial columns may be determined with minute precision. It is the method uniformly adopted by Uegnault.
* A table, giving the values of the barometric pressure in millimetres, expressed in the corresponding number of English inches, will be found at the end of this volume.
Fig- 18. from the longer limb, the liquid
flows, and will continue to do so as long as the shorter limb remains below the surface of the liquid in the vessel. If the vessel v, however, be raised until the longer limb of the syphon is immersed in the liquid that has run over, and the liquid stands at the same level in both vessels, no further flow will take place; if v be again depressed, the flow through the syphon will again be renewed. When, as was effected by the expedient of raising the lower vessel till the liquid stood at the same level in both, the acting limbs of the syphon are of equal length, the column of liquid in each has the same perpendicular height, and the downward pressure of each column will be the same: neither column will preponderate over the other: but if the vertical column of liquid be longer on one side than on the other, this longer column will necessarily press downwards with more force on that side than the column in the shorter limb presses in the opposite direction; the atmospheric pressure, however, is equal on both sides; the heavier column therefore runs out of the tube, drawing with it the liquid in the shorter limb, and the place of this liquid is supplied by a fresh portion from the vessel, owing to the pressure of the atmosphere which drives it up into the space that would otherwise become empty.
(37) Pressure of the Atmosphere.—From what has been already stated, it must be obvious that we are living at the bottom of a vast aerial ocean, and subject to the pressure due to the superincumbent mass,—a pressure which amounts to about 15 pounds upon every square inch of surface, and as has been estimated, to about 14 or 15 tons weight upon the surface of the body of a man of average stature.'
The existence of this pressure of the air is a matter of the highest importance to us. It admits of proof by experiment in a variety of ways. The receiver of the air-pump may at first be
* Every square centimetre supports an atmospheric pressure amounting to a little more than the weight of I kilogramme, or more esactly i'033 kilog., when the barometer stands at 76omm', and the surface of a man's body would sustain an average pressure of about 15000 kilogrammes.
38.] PRESSURE OF THE ATMOSPHERE. 63
lifted from the brass plate without difficulty, but after a few strokes of the pump in the ordinary process of exhausting, it becomes fixed by the pressure of the air, uncompensated by that within the vessel. It is for this reason that an arched form is given to the external surface of vessels designed to bear exhaustion. If the hand be placed over the mouth of a receiver having at the top an opening of 2 or 3 inches (6 or 8 centsmetres) in diameter, a very partial removal of the air will make this pressure painfully sensible; and if a piece of bladder be moistened and securely tied over the opening and then left to dry, its surface will, when a portion of the enclosed air is removed, become very tense and concave, and if the exhaustion be carried far enough, it will suddenly burst with a loud report.
But the question will naturally arise, how is it, that if our bodies are subjected to the pressure above indicated, we are not only able to support it without being crushed or rooted to the earth, but are even insensible of its existence. The reason is, that the pressure is equal in all directions. A very simple experiment will suffice to demonstrate the upward pressure. Take a glass jar with a smooth edge (a common wine-glass will do), fill it with water, close the mouth with a card or with a bit of paper, retain the card in its place with the hand, and turn the jar mouth downwards; the hand may be removed, the card will remain supported, and the water will not escape. Indeed we might thus support a column of water 10 metres long (but not longer), as that would just balance the pressure of a column of air of equal diameter. It is the pressure, exerted in all directions on the blood, pervading every tissue of our frame, which renders us unconscious of the atmospheric pressure. If the pressure upon the surface of the body be decreased, as by ascending in a balloon, considerable inconvenience is often experienced; bleeding at the nose, and other unpleasant symptoms sometimes arising, from the diminution of the atmospheric pressure on the surface of the body. Blood flows in the operation of cupping, because the atmospheric pressure is partially removed over the wounds inflicted by the lancets.
(38) Pneumatic Trough.—Among the many useful contrivances depending on the pressure of the air, is a simple but valuable apparatus of Priestley's, called the pneumatic trough, which enables us to confine air and gases in vessels, and to decant them from one to another with as much ease as liquids may be managed and poured.