Low pressure

“ The Jacob Strader, like nearly all the western river boats, is high pressure, because the grit of the water would rapidly wear out the more costly and complicated low pressure engines. She has two powerful inclined engines, not working together upon one shaft, but upon each side wheel, separately. ”

“ Thus, at Mount Washington, and at other stations of considerable altitude, expansion due to temperature-increase raises the center of mass in summer; mean pressure, therefore, is raised. In winter, low temperature causes contraction, lowering the center of mass and therefore the pressure. In other words, while sea level and also the observer’s station are at fixed altitudes, the center of mass is a varying altitude; it is raised by increasing temperature and lowered by decreasing temperature. ”

“ I remember the vulnerable and scattered character of the empire, the universality of our commerce, the peaceful tendencies of our democratic electorate, and the hard times, the pressure of competition, and the high taxation now imposed ; and with these factors vividly before me I decline to be a party to encouraging the military and militant circle of the war office and admiralty to join in the high and desperate stakes which other nations seem to be forced to risk. ”

“ If, then, the vertical depth of the earthy particles or sand above the bottom is so small that the arching effect against the sides is negligible, the full weight of the particles in contact, directly or vicariously, with the bottom acts as pressure on the bottom, while the full pressure of the water acts through the voids or on them, or is transmitted through material in contact with the bottom. ”

“ 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.” ”

“ Water or other fluid can pass freely upwards through the grating, but when it attempts to return the elasticity of the india-rubber, and the pressure of the water upon it, cause it to lie close on the grating, and thus prevent the return of the water. The india-rubber is prevented from rising too high by a perforated guard. In fig. 54 is shown an example of an india-rubber disc valve. A is the grating, B the india-rubber, C the guard secured to the grating or seat by the stud D and nut E. The grating is held in position by bolts and nuts F. The grating and guard are generally of brass. ”

“ 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 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. ”

“ As to the amount of pressure to be used, the practice is to allow a force of 10 tons for every inch of diameter of the pin, axle, or shaft.
Instead of being riveted in, the crank pin may be prolonged and screwed, and fitted with a nut. Another plan is to put a cotter through the crank and the crank-pin.
The distance from the centre of the crank-shaft to the centre of the crank-pin is called the radius of the crank. The throw of the crank is twice the radius. In a direct-acting [Pg 45] engine the throw of the crank is equal to the stroke of the piston. ”

“ Dimensions of Keys.
V. SHAFTING.
Shafting is nearly always cylindrical and made of wrought iron or steel. Cast iron is rarely used for shafting.
Axles are shafts which are subjected to bending without twisting.
The parts of a shaft or axle which rest upon the bearings or supports are called journals, pivots, or collars.
In journals the supporting pressure is at right angles to the axis of the shaft, while in pivots and collars the pressure is parallel to that axis.
Shafts may be solid or hollow. Hollow shafts are stronger than solid shafts for the same weight of material. ”

“ It presses with a weight of about fifteen pounds per square inch of surface, and its pressure is exerted in all directions, upward as well as downward. An ordinary man sustains a pressure of over one ton on each square foot of his surface, but as the air penetrates all portions of his body and exercises a pressure outward as well as inward he feels no inconvenience. If his body could be so tightly sealed that no air could enter and if then the air of the interior should be removed with a pump, his body instantly would be crushed to a shapeless pulp. ”

“ Of course you know the dangers of diving and the view now accepted regarding the rapid effervescence of the gases which are absorbed in the body fluids during exposure to pressure. I think you know that experiment has proved that when the pressure is suddenly relieved the gas is liberated in bubbles within the body. ”

“ It is therefore evident that if there be just enough pressure to give a good supply to the nearest lamp, there will be too little a short distance beyond it, and none at all at the extreme point; so that it is necessary to put on enough pressure to overcome the friction on all these miles of pipe, and give just enough gas at the extreme end. It follows that at all intermediate points the pressure is excessive. ”

“ In the caisson a man works under high air pressure. When he comes out, the pressure on the fluids of the body is reduced, and this sometimes causes the formation of a gas bubble in the vascular system. ”

“ If the gas should flow in too great a quantity, the pressure increases on the posterior face of the piston N, the spring is overcome, and the movable system advances a little toward the left; but then the orifices are partly covered and the flow diminishes. If the pressure then becomes too weak at the exit, the spring in its turn prevails, pushes the sheath toward the right, uncovers the orifices, and consequently a greater quantity of air may enter. ”

“ The fact that the heating effect of compressing air from an initial pressure of 10 pounds absolute to 90 pounds gauge pressure is theoretically equivalent to that of compression to 132 pounds at sea level, makes a two-stage arrangement more imperative in high level work than under ordinary conditions. ”

“ We see from these curves that for very short arcs the potential difference increases continuously with the pressure, but for longer ones there is a critical pressure at which the potential difference is a minimum, and that critical pressure seems to increase with the length of arc. ”

“ In the air lock the pressure must be built up slowly so that it will permeate a man’s whole system. When we realize that the pressure that men have to support in caisson work may amount to from fifty to one hundred tons on the whole body, it is difficult at first to understand why the body is not flattened out like a pancake. It is only by permitting the system to absorb the pressure so that there is as much internal pressure as that outside that a man is able to enter a compressed air chamber without harmful results. ”

“ 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. ”

“ The effects of this powerful agent begin to be distinctly felt at the pressure of about five atmospheres. Perhaps they might be noticed at a lower pressure, and I am inclined to attribute to this cause the unfavorable symptoms presented by workmen who have spent several months in compressed air; but this is a complex problem. In any case, if the necessities of industry subject men to pressures higher than six atmospheres, they will be in danger not only at the instant of decompression, but even from the effects of the compression. ”

“ Heat due to Compression. We saw in Section 89 that whenever the pressure exerted upon a gas is increased, the volume of the gas is decreased; and that whenever the pressure upon a gas is decreased, the volume of the gas is increased. If the pressure is changed very slowly, the change in the temperature of the gas is imperceptible; if, however, the pressure is removed suddenly, the temperature falls rapidly, or if the pressure is applied suddenly, the temperature rises rapidly. When bicycle tires are being inflated, the pump becomes hot because of the compression of the air. ”

“ We all know that air or any other gas placed in a vessel presses against the sides of the vessel, and against the surface of any body placed within it. On the kinetic theory this pressure is entirely due to the molecules striking against these surfaces, and thereby communicating to them a series of impulses which follow each other in such rapid succession that they produce an effect which cannot be distinguished from that of a continuous pressure. ”

“ The upward stroke of the piston continues, and the air expands without receiving heat, until its temperature falls to that of the refrigerator, that is, to the temperature that it had at the beginning of the first operation. The fall of pressure is represented by the adiabatic C D, and work represented by the area C D d c is done by the air. ”

“ If, however, the air pressure is increased above the normal by including the spark balls in a vessel in which air can be compressed, then the spark length, corresponding to a given potential difference, very rapidly decreases. ”

“ This pressure, it should be noted, is the net or effective pressure, the pressure with which the gas raises the liquid in a water-gauge glass out of the level while the opposite end of the water column is exposed to the atmosphere. The absolute pressure in a vessel containing gas at an effective pressure of 12 inches of water is 7 oz. plus the normal, insensible pressure of the atmosphere itself--say 15-1/4 lb. ”

“ The reason why a satisfactory quantitative experimental demonstration of the principle of the equality of pressure cannot be given is that the influence of the weight of the liquid and of the friction of the pistons cannot be altogether eliminated. ”

“ 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. ”

“ At the top of the mountain the expansion is quite sufficient to render the bladder tight, the pressure within being then actually greater than the pressure without. By means of an air-pump we can show the expansion of a balloon partly filled with air, when the external pressure has been in part removed.
But why do I dwell upon this? Simply to make plain to you that the unconfined air, heated at the earth's surface, and ascending by its lightness, must expand more and more the higher it rises in the atmosphere. ”

“ Effect of Pressure on the F.P.—The effect of pressure on the fusing-point depends on the change of volume during fusion. Substances which expand on freezing, like ice, have their freezing points lowered by increase of pressure; substances which expand on fusing, like wax, have their melting points raised by pressure. In each case the effect of pressure is to retard increase of volume. ”

“ 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. ”

“ Here again, since the intensity of the pressure on A becomes greater as it descends owing to the increased head, the apparent increase of weight of the lift-ram as it rises is automatically balanced; water from the high-pressure system is admitted down the hollow ram B and does the work of lifting the live load. ”

“ For this Elastical power, according to the Experiment, has no less force of pressure then an hundred pound weight or more: which pressure (as in all flexible bodies that have a Spring power in them) is perpetual and every where in the Aire, if it be there at all. And therefore any Cylinder of Aire in the same height from the ground, and of the same diameter with that of the Sucker of the Pump, will press as forcibly as an hundred pound weight. ”

“ His observations on mountain-sickness led him to conclude that it was caused by “diminution in atmospheric pressure, which operates in at least two ways—namely, (a) by lessening the value of the air that can be inspired in any given time, and, (b) by causing the air or gas within the body to expand, and to press upon the internal organs”; and that “the effects produced by (b) may be temporary and pass away when equilibrium has been restored between the internal and external pressure.” ”

“ If an ordinary pipe, connecting reservoirs open to the air, rises at any joint above the line of virtual slope, the pressure at that point is less than the atmospheric pressure transmitted through the pipe. At such a point there is a liability that air may be disengaged from the water, and the flow stopped or impeded by the accumulation of air. If the pipe rises more than 34 ft. above the line of virtual slope, the pressure is negative. ”