Pressure

“ If the pressure at every point of a given closed surface be known, and the distribution of sources within the surface be also known, then only one distribution of pressures can exist within the surface. ”

“ But when we are so prepared, the truth appears to be manifest, even independent of experience, and is seen to be a rule to which experience must conform. What, then, is the leading idea which thus enables us to reason effectively upon mechanical subjects? By attention to the course of such reasonings, we perceive that it is the idea of Pressure; Pressure being conceived as a measurable effect of heavy bodies at rest, distinguishable from all other effects, such as motion, change of figure, and the like. ”

“ The gentle pressure of one sensitive body upon another produces counter-pressure, a resistance that is positively [59] pleasurable, whereby the impenetrability of matter becomes a delightful instead of a frightful revelation to the mind of the Immutable Reality of the loving Creator, as the complement of our own changeful individuality! ”

“ You see, over our Cavorite this ceased to be the case, the air there ceased to exert any pressure, and the air round it, and not over the Cavorite, was exerting a pressure of fourteen pounds and a half to the square inch upon this suddenly weightless air. Ah! you begin to see! The air all about the Cavorite crushed in upon the air above it with irresistible force. The air above the Cavorite was forced upwards violently, the air that rushed in to replace it immediately lost weight, ceased to exert any pressure, followed suit, blew the ceiling through and the roof off. ”

“ Stress.—Every body that we know anything about is always under the action of forces. Every body upon which we can experiment is subject to the force of gravity, and must, for the purpose of experiment, be supported by other forces. Such forces are usually applied by way of pressure upon a portion of the surface of the body; and such pressure is exerted by another body in contact with the first. The supported body exerts an equal and opposite pressure upon the supporting body across the portion of surface which is common to the two. ”

“ But, with sound legs under him, he may run away, and meet with a new pressure. He may continue running, each new pressure prodding him as he goes, until he dies and his final form will be that predestined of the many pressures. ”

“ Moreover, each attitude—standing, sitting, lying down, turning over—entails a different set of pressures, both of the parts on one another and on the ground; and those partial arrests of motion which result from sitting down the feet alternately when running, send jolts or waves of varying pressure through the body. ”

“ What impulse can account for the expansive tendency of gases, and for their continuous pressure against the recipients in which they are contained? What impulse, above all, can account for universal attraction? ”

“ Once it gets going at a certain pressure, well, it is almost impossible to turn it off. And, my hat! what odd things come up with the water! ”

“ Impelled by a resistless force I still rose, although now against a crushing pressure and an active resistance which seemed to beat me back, and I fought my upward way in an agony which resembled nothing so much as the terrible moment when, from strangling or suffocation, all the forces of life struggle against death, and wrestle madly for another breath. ”

“ Different degrees of pressure necessarily impart different amounts of feeling. Perception of the different degrees of pressure made by an object betrays its inequalities of surface. The sense of feeling is also460 the sense for determining inequalities, for the Soft and Hard, for the Solid, Fluid, and Gaseous ”

“ Of these external and resisting substances, some yield easily, and change their figure, at least in some degree, in consequence of the pressure of our hand: others neither yield nor change their figure, in any respect, in consequence of the utmost pressure which our hand alone is capable of giving them. ”

“ We must, therefore, seek a satisfactory definition of matter elsewhere; and we find the clew to it in the consideration that the sense of effort we experience in antagonizing the downward pressure of a body is but a particular case of our more general cognition of resistance. When we project our hand against a hard and fixed solid body, our consciousness of its resistance to our pressure is exactly that which we experience when we try to raise a weight that we have not strength to lift ”

“ It was evident, nevertheless, that the air under a pressure which might be multiplied by thousands of atmospheres, would end by becoming perfectly solid, and that, then admitting our bodies resisted the pressure, we should have to stop, in spite of all the reasoning in the world. ”

“ But the causes which produce such pressure are capable of producing motion, and are generally seen producing motion, as in the above instance of the wrestlers, or in a pair of scales employed in weighing; and thus men come to consider pressure as the exception, and motion as the rule: or perhaps they image to themselves the motion which might or would take place; for instance, the motion which the arms of a lever would have if they did move. ”

“ The mutual action between two bodies is called stress. When the mutual action tends to bring the bodies nearer, or to prevent them from separating, it is called tension or attraction. When it tends to separate the bodies, or to prevent them from approaching, it is called pressure or repulsion. The names tension and pressure are used when the action is seen to take place through a medium. Attraction and repulsion are reserved for cases of action at a distance. The configuration of a material system can always be defined in terms of the mutual distances of the parts of the system. ”

“ And this pressure that one could not lift, this tension, this growing weight on the whole organism, would have become unbearable had it lasted even the smallest part of a second longer, if at its height there had not come a relief, a release—a little break somewhere, soundless, yet perceptible ”

“ The pressure at any point of the liquid which is above this level is negative unless another fluid as, for instance, the air, presses on the upper surface, but it is only the difference of pressures with which we have to do, because two equal pressures on opposite sides of the surface produce no effect. ”

“ Some fluids yield so very easily to the slightest pressure, that upon, ordinary occasions we are scarcely sensible of their resistance; and are upon that account little disposed to conceive them as bodies, or as things capable of pressure and resistance. There was a time, as we may learn from Aristotle and Lucretius, when it was supposed to require some degree of philosophy to demonstrate that air was a real solid body, or capable of pressure and resistance. ”

“ The pressure of the air is a convenient unit to employ in practical work, where it is called an “atmosphere”; it is made the equivalent of a pressure of one kg/cm2; and one ton/inch2, employed as the unit with high pressure as in artillery, may be taken as 150 atmospheres.
12. Theorem.—A body immersed in a fluid is buoyed up by a force equal to the weight of the liquid displaced, acting vertically upward through the centre of gravity of the displaced liquid. ”

“ We must suppose that there is a pressure in the direction of the lines of force, combined with a tension in all directions at right angles to the lines of force. Such a state of stress would, no doubt, account for the observed effects of gravitation. ”

“ We might see any number of bodies falling to the ground, and might frame a correct law of their motion, without having the remotest conception of their possessing that downward pressure which we at once recognize when we take a lump of lead or iron into our hands; and it is obviously on our cognition of this pressure, that our idea of weight or ponderosity is based. ”

“ The word "force" is used of two phenomena: the pressure, which when two bodies are in contact connects the motion of each with the position of the other; and attraction or repulsion; that is to say, a change of velocity in one body depending on the position of some other body which is not in contact with it. ”

“ It is often understood as a sensation of contact as distinguished from pressure, but it is evident that, however gentle be the contact, a certain amount of pressure always exists between the sensitive surface and the body touched. Mere contact in such circumstances is gentle pressure; a greater amount of force causes a feeling of resistance or of pressure referred to the skin; a still greater amount causes a feeling of muscular resistance, as when a weight is supported on the palm of the hand ”

“ When I exert pressure on the skin of my body, I experience it as a pressure sensation. This same pressure can be sensed as light by the eye, as sound by the ear. I experience an electrical shock by the eye as light, by the ear as sound, by the nerves of the skin as touch, and by the nose as a smell of phosphorus. ”

“ Pressure and tension being taken as of opposite meanings, and coexistent in a mass of rock, we may admit as representing their directions three coordinate axes passing through a central point at right angles to each other. Along these axes the effects of pressure and of tension will be direct and total, so that a small plane situated at right angles to one of these lines will be subject to the pressures or tensions of that line only, and will be moved, if at all, along that line; but a small plane placed in some other position will be moved in a line not having the same direction. ”

“ The air exercises a pressure of some tons on the human body without such pressure being detected, how much more cause, therefore, is there for the perfect concealment of the ether pressure, which is exercised against the molecules of matter themselves. ”

“ It is from our dynamic Activity also that we derive our conception of Force. Force, though it is studied scientifically in the measurement of the great natural forces which operate constantly, is originally known to us in the stress or pressure to which muscular exertion in contact with a material body gives rise. Such a force if it could be correctly measured, would record the rate at which Energy was undergoing transmutation, and it is from such experience of pressure that our idea of Force is originally derived. ”

“ Emily. A body of water, in falling, does certainly less injury than a solid body of the same weight.
Mrs. B. The particles of fluids, acting thus independently, press against each other in every direction, not only downwards, but upwards, and laterally or sideways; and in consequence of this equality of pressure, every particle remains at rest, in the fluid. If you agitate the fluid, you disturb this [Pg 121] equality of pressure, and the fluid will not rest, till its equilibrium is restored.
Plate xiii.
Caroline. The pressure downwards is very natural; it is the effect of gravity ”

“ The reply is, that the foregoing illustrations are to be interpreted literally, and that the processes described are physical ones. The pressure of hunger is an actual force—a sensation implying some state of nervous tension; and the muscular action which the sensation prompts is really a discharge of it in the shape of bodily motion—a discharge which, on analyzing the mental acts involved, will be found to follow lines of least resistance. ”

“ If any part of a pressure, propagated through a fluid from a given point, be intercepted by any obstacle, the remaining part, which is not intercepted, will deflect into the spaces behind the obstacle. ”

“ Therefore, the air pressure pushes the soda water up into the straw and into your mouth where the pressure has been removed by sucking." Sometimes, when you have shown that you understand the principles very well, the teacher may let you take a short cut and just name the principle, but this will be done only after you have proved by a number of full answers that you thoroughly understand each principle named. ”

“ The pressure in the central portions of such a mass of gas would, of course, be greater than the pressure in the outer portions, just as the pressure rises as we penetrate from above downward into our terrestrial atmosphere. ”

“ In the first place, I shall take into consideration the general effects of attraction, or force; in which expression by the word force I mean pressure producing an effect on the motion of bodies that are free. Suppose we drop a stone from our hand, or from a high building, everybody knows that it begins to fall with a very small velocity, and that it gains velocity as it falls. ”

“ In the case of wind pressure on a sloping roof surface, R is the horizontal and L the vertical component of the normal pressure.
In experiments with the whirling machine it is the resistance to motion, R, which is directly measured. Let P be the pressure on a plane moved normally through a fluid. ”

“ Being unable to use his fists as was his wont, and being possessed of no other safety-valve, the pressure mounted steadily until it registered a point on his mental gauge that spoke eloquently of trouble to come. ”

“ I suppose you know—everybody knows nowadays—that, as a usual thing, the air has weight, that it presses on everything at the surface of the earth, presses in all directions, with a pressure of fourteen and a half pounds to the square inch? ”

“ It is a fundamental principle of dynamics that "force is wholly expended in the action it produces," [291] therefore, if all the forms of energy in the universe are the result of pressure, that pressure must be continuous ”

“ Q.--Then a force once called into existence cannot be destroyed?
A.--No; force is eternal, if by force you mean power, or in other words pressure acting though space. But if by force you mean mere pressure, then it furnishes no measure of power. Power is not measurable by force but by force and velocity combined. ”