Hydrostatic pressure

“ Osmotic pressure will then appear to be due to the shock of the dissolved molecules against the membrane. It will come from one side of this partition to superpose itself on the hydrostatic pressure, which latter must have the same value on both sides.
The analogy with a perfect gas naturally becomes much greater as the solution becomes more diluted. It then imitates gas in some other properties; the internal work of the variation of volume is nil, and the specific heat is only a function of the temperature. ”

“ It is probable that osmotic pressure is not really of the same nature as gas-pressure, but depends on equilibrium of vapour-pressure. The vapour-molecules of the solvent are free to pass through the semi-permeable membrane, and will continue to condense in the solution until the hydrostatic pressure is so raised as to produce equality of vapour-pressure. ”

“ Carry this on until a flow is secured from the highest land on the earth, and then whence comes the flow to supply that? The mere statement of the case proves the existence of some force in nature other than hydrostatic pressure by which these vast bodies of water are driven to the surface. This hydrostatic pressure Professor Buckland thus illustrates and explains: "The portions of a water-logged, porous bed between two beds of clay may be illustrated by a tea-saucer placed within another tea-saucer, and having the narrow space between filled with sand and water. ”

“ Hence the conclusion is inevitable that some force not yet identified exists to which these great overflows are due. It should be remembered that up to this time it is generally held, to use the words of M. Garnier, that "unless there be a reservoir higher than the surface whence we intend to bore, we can not hope to obtain an overflowing fountain." And, as if conscious that there might be some mistake about this theory, he says further that gases may force water up, by which he means to suggest a cause other than hydrostatic pressure. ”

“ It is this principle which operates with such astonishing effect in hydrostatic presses, of which familiar examples are found in the hydraulic pumps, by the use of which boilers are tested. By the mere weight of a man’s body when leaning on the extremity of a lever, a pressure may be produced of upwards of 20 tons; it is the simplest and most easily applicable of all contrivances for increasing human power, and it is only limited by want of materials of sufficient strength to utilize it. ”

“ The old scientific principle known as the hydrostatic paradox, according to which a pressure applied at any point of an inclosed mass of liquid is transmitted unaltered to every other point, has been singularly fruitful in practical applications. ”

“ In general the space will remain filled with fluid if the pressure at every point remains positive. There will be a break of continuity, if at any point the pressure becomes negative, indicating that the stress at that point is tensile. In the case of ordinary water this statement requires modification. Water contains a variable amount of air in solution, often about one-twentieth of its volume. This air is disengaged and breaks the continuity of the liquid, if the pressure falls below a point corresponding to its tension. ”

“ But, further, in fluids such as water, air, steam, &c., to which the present division of the article relates, the tangential stresses that are called into action between contiguous portions during distortion or change of figure are always small compared with the weight, inertia, pressure, &c., which produce the visible motions it is the object of hydraulics to estimate. On the other hand, while a fluid passes easily from one form to another, it opposes considerable resistance to change of volume. ”

“ Then the whole velocity at any point will be inversely proportional to the radius of the point, and the fluid will describe stream lines having a constant inclination to the radius drawn to the axis of the current. That is, the stream lines will be logarithmic spirals. When water is delivered from the circumference of a centrifugal pump or turbine into a chamber, it forms a free vortex of this kind. The water flows spirally outwards, its velocity diminishing and its pressure increasing according to the law stated above, and the head along each spiral stream line is constant. ”

“ If the base of the material be sand and the fill be so-called quicksand in excess of the voids, pressure will cause the quicksand to set up hydraulic action, and the action of the piston will appear to be similar to that of a piston acting on purely aqueous material. ”

“ 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. ”

“ Now, if the piston should be loaded with a weight, the pressure on the outside would be that due to the atmosphere, plus the weight, while the pressure from the inside is simply equal to atmospheric pressure; thus the piston is forced to descend, but as the air inside of the cylinder has no means of escape, the volume it fills being diminished, its pressure rises until the pressure under the piston balances that above it. ”

“ In order to insure smooth and quiet running of the pump, it is necessary to keep the speed of the piston inside of the velocity with which the cylinder would fill under this pressure, reduced by the friction of the water, the pressure required to lift the valves, etc.
But this supposes that there is a perfect vacuum in A, and we cannot realize this in contact with hot water. Water at any temperature will boil unless it is under a pressure equal to or greater than that corresponding with the temperature. ”

“ The true doctrine of Hydrostatics, however, assuming the Idea of Pressure, which it involves, in common with the Mechanics of solid bodies, requires also a distinct Idea of a Fluid, as a body of which the parts are perfectly movable among each other by the slightest partial pressure, and in which all pressure exerted on one part is transferred to all other parts. ”

“ By having his pump attached to the well tube directly, the working of the pump creates a vacuum, and the atmospheric pressure on the earth's surface violently forces the liquid to fill the vacuum thus formed, thereby giving a much greater amount of water than can be otherwise obtained. It is a well established fact that more water can be obtained by the drive well than by any other. ”

“ The theorems of hydrostatics are thus true for all stationary fluids, however viscous they may be; it is only when we come to hydrodynamics, the science of the motion of a fluid, that viscosity will make itself felt and modify the theory; unless we begin by postulating the perfect fluid, devoid of viscosity, so that the principle of the normality of fluid pressure is taken to hold when the fluid is in movement.
5. The Measurement of Fluid Pressure.—The pressure at any point of a plane in the interior of a fluid is the intensity of the normal thrust estimated per unit area of the plane. ”

“ In ordinary hydraulics, liquids are treated as absolutely incompressible. In dealing with gases the changes of volume which accompany changes of pressure must be taken into account.
§ 3. Viscous fluids are those in which change of form under a continued stress proceeds gradually and increases indefinitely. A very viscous fluid opposes great resistance to change of form in a short time, and yet may be deformed considerably by a small stress acting for a long period. ”

“ Steam pressures stated above must be maintained at the pump, to give full speed and 100 lbs. water pressure. Pressure at boilers must be a little more, to allow for loss of steam pressure between boiler and pump. Pumps in poor order, or too tightly packed, will require more steam. ”

“ Water is nearly incompressible and if no means were employed to take care of the pressure changes, the sudden stopping and starting of the flow would cause serious jars and start leaks in the pipes. Two common devices for controlling sudden changes in the water pressure are the standpipe and the air cushion. ”

“ The values of the corrections for any given gas at different initial pressures are directly proportional to the pressure. Values of the Corrections.—If we take for the gas hydrogen the values c=1·5 c.c. at 0° C., b=8·0 c.c., with the index n=1·5, which satisfy the observations of Joule and Thomson on the cooling effect, and those of Regnault, Amagat and Chappuis on the compressibility, the values of the absolute zero θ0, calculated from Chappuis’s values of the pressure and expansion coefficients at 100 cms. ”

“ By screwing the inlet pipe in or out, as required, it can be so adjusted that the diaphragm does not occlude its inner aperture, and consequently the full volume of gas can pass through to the burners below. The temperature of the water in the water-tank therefore begins to rise, and in consequence the volume of the water to increase. This results in the water rising up into the tube (T) , and therefore the dynamical pressure which is exercised by the water upon every part of the two cylinders of the incubator and consequently also upon the india-rubber diaphragm of the regulator is increased. ”

“ When pressure is reduced too much too suddenly, some of the gases in your blood form bubbles—just like a kettle boiling. And those bubbles in your blood can cause plenty of trouble.”
Stan turned to March. “Sure,” he said. “Remember those experiments everybody has in first-year chemistry? Making water boil when you put it on a cake of ice? The water’s under pressure in a closed container, and cooling it condenses the steam vapor so that pressure is reduced. So the air forms bubbles which escape when pressure goes down.” ”

“ As an air-ship rises from the ground, the density, and therefore pressure, of the air decreases, which causes the hydrogen in the envelope to expand proportionately. Rise in temperature has the same effect. When an air-ship ascends, the gas therefore expands, and at a certain point would burst the envelope were valves not provided to allow some of the gas to escape. It is important to realize that as the expansion occurs at a rate corresponding to the decrease in density no alteration in lift occurs so long as gas is not lost through the valves. ”

“ Now it is clear, that if this atmospheric resistance be subject to any variation in its intensity, from causes whether natural or artificial, the repulsive energy necessary to be introduced by the heat, will vary proportionally: if the atmospheric pressure be diminished, then less heat will be necessary to vaporize the liquid. If, on the other hand, this pressure be increased, a greater quantity of heat will be required to impart the necessary elasticity. ”

“ The stress considered in hydro-mechanics is always a pressure, as liquids are in general capable of sustaining only a slight tension without disruption: the intensity of the pressure is measured by the number of units of force per unit of area. Thus we say, one thousand pounds of pressure per square inch of piston—the pounds and the square inches are the units used in these calculations. ”

“ When all the fittings, burners excepted, have been connected, the whole system of pipes must be tested by putting it under a gas (or air) pressure of 9 or 12 inches of water, and observing on an attached pressure gauge whether any fall in pressure occurs within fifteen minutes after the main inlet tap has been shut. The pressure required for this purpose can be obtained by temporarily weighting the holder, or by the employment of a pump. If the gauge shows a fall of pressure of one quarter of an inch or more in these circumstances, the pipes must be examined until the leak is located. ”

“ As the wells are on one side eighteen and on the other about twenty-five miles distant, and as the consumption is variable, the pressure at the city can not be given. Greater pressure might be obtained at the wells, but this would increase the liability to leakage and bursting of pipes. For the prevention of such casualties safety-valves are provided at the wells, permitting the escape of all superfluous gas. The enormous force of this gas may be appreciated from a comparison of, say, 200 pounds pressure at the wells with a two-ounce pressure of common gas for ordinary lighting. ”

“ Hence, hydrogen, starting from zero to the highest pressures, exhibits a gradually, although only slightly, varying coefficient of expansion, whilst for air and other gases at the atmospheric and higher pressures, the coefficient of expansion increases with the increase of pressure, so long as their compressibility is greater than should follow from Mariotte's law. But when at considerable pressures, this kind of discrepancy passes into the normal (see Note 25) , then the coefficient of expansion of all gases decreases with an increase of pressure, as is seen from the researches of Amagat. ”

“ This sometimes occurs when water gets into a narrow chink between buildings, and, finding its way down, opens finally into some cavity under the floor. The pressure exerted here is immense, and there are few bodies able to resist it. It is owing to this that the pipes for conveying water are burst, on account of the pressure exerted on the insides of the pipes; and this occurs the more frequently, the higher the source from which they are filled. In practice, every vessel containing liquid should increase in strength in proportion to its depth. ”