Turgor pressure

“ In an actual tornado one of these infinitely numerous funnel-shaped isobaric surfaces may become distinctly outlined and visible, if the air has sufficient moisture to start precipitation when it reaches a surface of suitably low pressure. This quite usually occurs in Nature, the funnel sometimes reaching to earth, sometimes only part way, according to the pressure at which precipitation begins, this pressure depending, of course, on the percentage of humidity of the uprushing air. ”

“ A stop is put to this new generation of air, when about a fifteenth of the whole is produced, by its increasing pressure; and a similar boundary is fixed to its absorption or destruction by the decrease of atmospheric pressure. As water requires more heat to convert it into vapour under a heavy atmosphere than under a light one, so in letting off the water from muddy fish-ponds great quantities of air-bubbles are seen to ascend from the bottom, which were previously confined there by the pressure of the water. ”

“ It is a principle in hydraulics, not easily understood at first by the layman, that the pressure upon a weir or barrage in a river depends entirely upon the difference in level between the water on the upstream and on the downstream side, and not on the mere volume of water in the river behind it. ”

“ This half-bulb serves likewise for another purpose. From the moment that the large bulb makes the lime-water turbid with an air containing two per cent. of CO2, it is clear that the small one can cause the same turbidity only with air twice richer in CO2, i.e., of four per cent.
This apparatus, although it makes no pretensions to extreme accuracy, is capable of giving valuable information. The table that accompanies it is arranged for a temperature of 17° and a pressure of 740 mm. But different meteorological conditions do not materially alter the results. ”

“ In this situation if one part of the atmosphere be hotter from some fortuitous circumstances, or, has less pressure over it, the lower stratum will begin to ascend at this part, and resemble water falling through a hole as mentioned above. If the lower region of air was going forwards with considerable velocity, it will gain an eddy by riling up this hole in the incumbent heavy air, so that the whirlpool or tornado has not only its progressive velocity, but its circular one also, which thus lifts up or overturns every thing within its spiral whirl. ”

“ This method of torsion has several advantages over the ordinary method. First, the petiole being supported by the loop of wire, the weight of the leaf has no effect on the curve of response. In the second place, the periodic variation of turgor of the stem, as suggested by Millardet, will not in any way affect the record. Variation of turgor can only cause a swing to and fro, in a direction perpendicular to the plane which divides the pulvinus into upper and lower halves; it can in no way induce a torsional movement, or a variation of the rate of that movement. ”

“ Flowing peacefully on, secure in its mightiness, yet all the while somewhere along its course was being assembled the power that wrought this change, the terrific force in the nature of gases generated far in the depths of the earth. It might be thousands of miles away, conducted through unknown channels and crevices, seeking the point of least resistance, forced hither and thither by the ever increasing pressure, until a subterranean cavity is formed by a slight upheaval or displacement of the stratification. ”

“ In our own city, and in other maritime cities, and upon the sea-coast generally, the action of the tides in obstructing the outflow, and thus driving the contents of the sewers back toward the houses where the pipes terminate, is often enormous, and such as, by the most ample provision for escape through ventilators, can not always prevent a sudden pressure upon the water-traps sufficient to displace their contents, or to force the gas through the water in the form of bubbles, or, finally, to increase the capacity of the water to receive the sewer-gas by absorption. ”

“ From the nature of the case, the best values concerning the dynamic force of the tornado are only approximate, but even these are instructive. Normal air pressure is at the rate of 2117 pounds per square foot. Now, if the air pressure within the funnel is only three-fourths normal when the funnel involves a building, the air pressure inside the building will be 530 pounds per square foot greater than on the outside. Such a difference in pressure is sufficient to burst the walls of almost any building. ”

“ If on the other hand the bodies be laid in sand or gravel, decomposition will readily take place, the gases will easily permeate the superjacent soil and escape into the atmosphere, and this with a facility which may be judged of when the fact is stated, that under a pressure of only three-fourths of an inch of water, coal gas will escape by any leakage in the conduit pipes through a stratum of sand or gravel of three feet in thickness in an exceedingly short space of time. The three feet of soil seems to oppose scarcely any resistance to its passage to the surface ”

“ Natural gas is found at pronounced depths in porous strata usually sand rock-constituting a natural reservoir, and is brought to the surface and reduced to possession through wells drilled into the containing strata. When a surface owner thus reduces it to possession he becomes its owner, and it becomes a subject of commerce, like any product of the forest, field, or mine. In the inclosing strata it is under great pressure, called rock pressure, which causes it to flow out rapidly when the strata are penetrated. ”

“ They show that as
the sediments grow in depth there is a rising of the geotherm of
plasticity—whatever its true temperature may be—gradually
reducing the thickness of that part
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of the upper crust which is bearing the simultaneously increasing
compressive stresses. Below this geotherm long-continued stress
resolves itself into hydrostatic pressure; above it (there is, of
course, no sharp line of demarcation) the crust accumulates
elastic energy. The final yielding and flexure occur when the
resistant cross-section has been sufficiently diminished. ”

“ By reason of the constantly changing temperatures and the frequent filling and emptying of the reservoir, expansion and contraction, which are always at work tending to produce relative movements wherever one portion of a structure is weaker than another, must have assisted the water-pressure in the extension of the horizontal cracks, which, growing slowly, during the fifteen years, provided at last the area required to enable the intrusive water to overbalance the little remaining stability of the dam. ”

“ Another theory which has at times been brought forward as an adequate cause of flowing wells is rock-pressure— i.e., pressure due to the weight of the overlying strata. The objections to this theory seem to me, however, so cogent that we may at once dismiss it from our minds. There is indeed little doubt that [164] the waters of the oasis are of meteoric origin, have travelled immense distances underground, and rise through bores placed in favourable localities by means of hydrostatic or hydraulic pressure, acting both through the pores of the rock and through open fissures. ”

“ When a tornado passes from land to water it becomes a waterspout only when the cloud-funnel [Pg 109] hangs low enough and the gyratory energy is sufficiently great. There is a great pressure on the water outside of the funnel and almost a perfect vacuum inside. This latter fact contributes largely to the destructive power of the tornado. When a funnel is central over a building a sudden vacuum is created outside of it and it bursts outwardly from the internal air pressure. ”

“ Small elevated tanks, like those of the windmill, frequently have heavy iron bands around their lower portion as a protection against the extra strain.
Before erecting a dam or reservoir, the maximum pressure to be exerted upon every square inch of surface should be accurately calculated, and the structure should then be built in such a way that the varying pressure of the water can be sustained. It is not sufficient that the bottom be strong ”

“ Thus the forces that are conspiring to increase the energy of the storm are powerfully assisted by the presence and condensation of aqueous vapor, and the increasing updraught and rarefaction are at once marked by the decreasing barometric pressure at the center. ”

“ It is obvious, however, that as a Fish rises or sinks it becomes exposed to an increase or a diminution of hydrostatic pressure, which will necessarily bring about the expansion or contraction of the volume of gas in the air-bladder, and, therefore, by decreasing or increasing the specific gravity of the animal, will tend to remove the Fish from its plane of least effort. To counteract this, and to restore the Fish to a plane of equilibrium at the new level, gas is either absorbed from the air-bladder, or more gas is secreted into the bladder, as the case may be. ”

“ THE DIVING-BELL.
Water is nearly 773 times as heavy as air. If we submerge a barometer a very little way below the surface of a water tank, we shall at once observe a rise of the mercury column. At a depth of 34 feet the pressure on any submerged object is 15 lbs. to the square inch, in addition to the atmospheric pressure of 15 lbs. per square inch—that is, there would be a 30-lb. absolute pressure. As a rule, when speaking of hydraulic pressures, we start with the normal atmospheric pressure as zero, and we will here observe the practice. ”

“ The amount of air contained in the well itself would not suffice to produce the violent effects observed and it is therefore assumed that the typical blowing well taps a subterranean reservoir of air, probably filling the interstices of sand and gravel beds. When the pressure of the external air is diminished, some of the imprisoned air escapes. For a given body of inclosed air, the smaller the channel or channels by which it emerges, the stronger the outdraft. When the barometric pressure outside increases, the current of air flows in the reverse direction. ”

“ In shallow water, bottom friction and the effects of local topography cause this elevation to persist and may even intensify it. The low atmospheric pressure that accompanies severe storms causes the third effect, which is sometimes referred to as the “inverted barometer” as the sea surface rises into the low pressure area. An inch of mercury is equivalent to about 13.6 inches of water, and the adjustment of the sea surface to the reduced pressure can amount to several feet at equilibrium. ”

“ Although the rate and direction of flow through the sandstones as a whole may remain more or less matters of conjecture, it seems probable that the water of any one bore is derived from all sides, rising as the result of the pressure exerted by the water held in the same bed situated at higher levels, whether in the immediate neighbourhood or at a considerable distance. ”

“ This sort of travel of underground water is common in many parts of the world. It should be clearly understood that such water does not flow freely as in a pipe along subterranean passageways, but rather it slowly works its way between the grains of porous rock. Where such water moves distinctly downward, and the porous layer has both above and below it an impervious rock layer like shale or clay, it gradually gets under greater and greater pressure. In some cases such pressure has actually been found by deep drilling to be equivalent to that of a column of water several thousand feet high. ”

“ On account of the small quantity of water which they contain, sudden fluctuations of pressure are caused by any irregularities in supplying the feed-water or in handling the fires, and the rapid and at times violent generation of steam causes it to accumulate in the contracted water-chambers, and leads to priming, with a consequent loss of water, and to overheated tubes. ”

“ It is obvious that in very permeable soils, such as sand, the hydrostatic pressure will be most felt, and should the range of tide be considerable the strain upon the earth will change and an active disturbing force be constantly present. This varying disturbing agent must be seriously considered in all dockwork or excavation near the sea or a tidal river, and also approximate to water, for there will always be danger of the water percolating, and when the foundations are not sufficiently deep of their being undermined. ”

“ That quantity of water which had been frozen by the mixture of snow and salt, was almost as much like snow as ice, such a quantity of air-bubbles were contained in it, by which it was prodigiously increased in bulk.
The pressure of the atmosphere assists very considerably in keeping fixed air confined in water; for in an exhausted receiver, Pyrmont water will absolutely boil, by the copious discharge of its air. ”

“ A laccolite of small volume will not exceed the limital area, but will grow by lifting its cover. If however the volume of intruded lava be great, its own weight becomes a factor in the equilibrium of forces and modifies the distribution of the pressures. As the rock bubble rises, the weight of the contained fluid is progressively subtracted from the pressure against its top, and this proceeds until the upward and lateral pressures become 91proportional to the resistances which severally oppose them. ”

“ This turbulent vaporization takes place at any temperature, even down to the freezing point, provided the pneumatic pressure be removed from the liquid by the air pump, or any other means. Ebullition always accelerates evaporation, as it serves to carry off the aqueous particles not simply from the surface, but from the whole body of the water. ”