Thus, if experiments on twilight give forty miles as the height, this implies that the air above this elevation reflects no appreciable ;:mount of light ; while, if we define the height to be to the point where the friction will not et light to a meteor, we have about seventy miles; but, of course, there is no reason why there : hould not be some air at much greater heights. It follows, from several considera- lions of other kinds, that the thickness of the crust is in all probability not less than miles.
These observations have been made both on the Atlantic, during a voyage from France to Eio Janeiro, and in the bay upon the shores of which the last-named city stands.
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They give, as a minimum, miles, and, as a probable height, miles. By observing, from the summit of the Faulhorn, the course of the crepuscular arcs, Bravais obtained a height of seventy-one and a half miles. The height, however, varies according to the temperature of the seasons, and remains always greatest at the equator. Another method, different from the preceding, consists in measuring the thickness of the penumbra which surrounds the earth's shadow on the moon during lu- nar eclipses, as well as the phenomena of refraction produced.
This measurement gives from fifty to sixty miles as the thickness of the terrestrial atmosphere, the influence of which is felt under this special aspect. The observations which accord the atmosphere a height far greater than the theoretical thirty-eight miles have been for many years the object of special discussion. Quetelet, director of the Brussels Observa- tory, has, after much research on this head, arrived at the conclusion that it does indeed extend much higher than had been supposed, but that the upper strata are not quite of the same nature as those nearer the earth.
This addition is supposed to be due to an ethereal atmosphere, very rarefied and differing from the lower atmosphere in which we live.
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It is the region where are mostly seen the shooting stars, which afterward disappear when they reach the terrestrial atmosphere. The special movements caused by the action of the winds and tempests are limited in their height by the effect of the seasons. Thus, as regards our climate, the agitated portion, in the vicinity of the earth, would not be more than from seven to ten miles high during the winter, while its height must be almost double in summer.
All that part of the atmos- phere which is above the latter would only experience a very slight and scarcely sensible movement, arising from the movable basis upon which it reposes. No difference has been discovered at the various eleva- tions which it is possible to attain for the purpose of collecting air and submitting it to analysis.
In the upper atmosphere the phenomena, of which we are scarcely able to form an idea by judging them from the surface of our globe, take place.
There, also, appear the shooting stars; descending from a still greater height, the aurora borealis, and those mighty luminous phenomena which we often witness without having the power to sub- mit them directly to the test of experiment. All these facts do not es- cape us altogether, especially as regards the aurora borealis and the magnetic phenomena.
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If we can not determine the cause, we can at least feel the effect with sufficient force to be in a position to appre- ciate them. We can quite admit that, above our at- mosphere ot oxygen, nitrogen, and vapor of water, there exists an at- mosphere excessively light, which may extend two hundred miles in height, and which is naturally composed of the very lightest gases. The terrestrial globe being about miles in diameter, this total thickness represents the fortieth of the globe's diameter.
The simulta- neous existence of these two atmospheres is, therefore, the general con- clusion at which we will, momentarily at least, stop. As to the basis of the atmosphere, we may now inquire if it ceases at the surface of the ground, and does not descend into the interior of the globe itself. Pressing upon all bodies upon the surface of the earth, it tends to penetrate in all directions between the molecules of liquids as into the interstices of the rocks. It is to be found in water as in all vegetables and all organic structures ; the earth and the porous stones are impreg- nated with it, and that in proportion to the force with which it presses.
It will be seen, therefore, that the air is not limited to the part which is, so to speak, a gaseous envelope, and that a sensible fraction of its constituent elements penetrates the waters of the ocean and the inter- stices of the ground. Certain savants have imagined that the air of which the atmosphere is composed is but the continuation of an inte- rior atmosphere ; but the rise in the temperature, due to the central heat, would prevent the condensation of gases, and must limit the pres- ence of air in the under strata.
Under ordinary pressure, sea-water absorbs from two to three per cent, of its volume, only the proportion of oxygen is much greater than in the ordinary air. The result of the calculation is, that the quantity of air absorbed by the ocean is not above a three- hundredth part of the atmosphere. We thus have a tolerably complete determination both as to the height and shape of this terrestrial atmosphere. WHILE treating of the height of the atmosphere, we have already seen that the air is denser in the lower regions of the aerial ocean-that is to say, near the surface of the earth than in the higher regions.
The air, light and unsubstantial as it may appear to us to be, has consequently a positive weight. Each square foot of the earth's surface sustains a con- siderable pressure, the amount of which we shall presently attempt to estimate, corresponding to the height and density of the column of air above it. Our ancestors were not able to measure the atmospheric pressure; but we must not conclude from this that they were ignorant of the effects which it exercised, especially when the wind was violent.
Yet this force, which every one felt without being able to measure, was not ren- dered determinate until the middle of the seventeenth century. In , the Grand Duke of Tuscany having ordered the construc- tion of fountains upon the terrace of the palace, if was found impossible to make the water rise more than thirty-two feet. The duke wrote to Galileo in reference to this strange refusal of the water to obey the pumps.
Torricelli, the pupil and friend of Galileo, gave the true ex- planation of the fact, and proved, as we shall see, that this column of water of thirty-two feet was in equilibrium with the weight of the at- mosphere. The celebrated invention of Torricelli has sometimes been erroneous- ly attributed to Pascal. The French philosopher himself alludes to the mistake, and shows how much of the merit is due to him in the follow- ing terms : " The report of my experiments having been spread abroad in Paris, they have been confounded with those made in Italy ; and, thanks to this misunderstanding, some, according me an honor to which I can lay no claim, attributed the Italian experiment to me, while oth- ers unjustly deprived me of the credit of those to which I was really en- titled.
Not content with giving it these distinctive marks, I have stated in so many words that I am not the inventor of the barom- eter ; that it was made in Italy four years previously, and was the cause of my making similar experiments. Let us then examine for a moment the mechanism and action of the pump. Every one knows that these simple and old-fashioned contrivances serve to raise water either by suction or pressure, or by both combined.
Hence their classification as suction-pumps, forcing-pumps, and suction and forcing pumps. Before Galileo's day, the ascension of water in the suction-pump was ascribed to the fact of nature abhorring a vacuum ; but it is, in reality, merely an effect of atmospheric pressure. Take a tube, at the lower extremity of which is a piston, and place this lower end in water.
If the piston is drawn up, a vacuum is created below, and the atmospheric pressure, acting upon the surface of the liquid external to the pump, makes it rise in the tube and follow the movement of the piston. Herein lies the principle of the suction- pump, which is essentially composed of the body of the pump, in which a piston moves, communicating by a tube with a reservoir of water see Fig.
At the point where the body of the pump and the suction-tube join is placed a valve, opening upward, and in the body of the piston there is an opening formed by a similar valve. For water to reach the body of the pump, the suction-valve must be less than thirty-two or thirty-three feet above the level of the wa- ter in the well, otherwise the water would cease to rise at a certain point in the tube, Fig. In addition, to insure raising at each ascent of the piston a volume of water equal to the volume of the body of the pump, the spout must be placed at a less height than thirty-two feet above the reservoir.
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Thus the suction-pump will not raise water to a height of more than thirty-two feet ; but the water having once passed above the pis- ton, the height to which it can then be raised depends solely upon the force which drives the piston. The suction and force pump see Fig. At the base of the body of the pump, over the orifice of the suction-pipe, is, as before, a valve opening upward. Another valve, also open- ing upward, closes the aperture of the bent tube, which runs into a receptacle called the air-vessel.
Finally, the farce- pump only acts mechanically, and does not atmospheric pressura ft differs on] y from the other in that it has no suction-pipe, its body going right into the water which is to be drawn up. By the aid of this principle he was led to invent the barometer. To exer- cise equal pressures, the liquid columns must be of heights inversely proportional to their density. Thus, a liquid twice as heavy as water would, with a column of sixteen feet, be in equilibrium with the atmos- phere; and quicksilver, which is nearly thirteen and a half times as heavy as water, would be in equilibrium if the height of the column were diminished in this proportion that is, to about twenty-nine inches.
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Torricelli inventing the Barometer. Take a glass tube, three feet in length, and open only at one end ; fill it with quicksilver, and then, placing the finger on the open end see Fig. The tube full of quicksilver.
The tube in the basin. Immediately the finger is removed, the quicksilver inside will descend several inches and then stop see Fig.
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The equilibrium is established, and the liquid column which remains sus- pended in the pipe is a true balance, for the weight of the column of mercury is exactly in equilibrium with the atmospheric pressure. Torricelli gave to this tube of quicksilver, thus placed vertically in a basin of quicksilver, the name of Barometer; that is to say, a contriv- ance to indicate the weight of the air, from the Greek flapog, weight, and fjLirpov, measure. Its invention by Torricelli dates from Three years later, Pascal repeated the experiment in France with a wa- ter-barometer, and even a wine-barometer.
This was at Rouen. Then, by means of cords and pulleys, the tube was placed upright and the lower end put into a vessel full of water. As soon as the cork that kept it closed was removed, the whole liquid column in the tube fell, until its surface was about thirty-three feet above the level of the water in the vessel. The remaining sixteen feet above were destitute of air.
Consequently, the liquid column itself formed an equilibrium to the at- mospheric pressure, and from this he drew the conclusion that a column of water or of wine of the same density thirty-two feet high weighs as much as a column of air on the same base. The surface of the earth is pressed upon as if it was covered with a body of water thirty-two or thirty-three feet deep, and we who live upon the bed of this ocean of air undergo the same pressure.
If it is the pressure of the air which causes the elevation of the quick- silver or the water, as we ascend into the atmosphere, the weight of the column of quicksilver raised, and consequently the height of this col- umn, must gradually diminish in a manner dependent on the strata of air left beneath it. The experiment was made on the Puy-de-D6me, ac- cording to the instructions of Pascal, by his brother-in-law, Florin Pe- rier, upon the 19th of September, , and repeated by Pascal himself on the Tour St. Jacques at Paris.
The results were decisive, and the barometer became an easy and accurate means of measuring the total weight of the atmosphere, and the variations in the pressure which it exerts at different times and places upon the surface of the globe. We thus see that it was between and that the atmospheric press- ure was demonstrated by the construction of the barometer and the ex- periments which its discoverers at once entered upon. By a coincidence not at all unusual in the history of science, while the indications of the barometer were being studied in Italy and France, experiments were being made in Holland to ascertain the pre- cise weight of the air, but by quite a different process.
In , Otto de Guericke, burgomaster of Magdeburg, invented the air-pump, by which the air may be exhausted from any receptacle and a nearly absolute vacuum created. The ingenious inventor conceived in the same year the idea of weigh- ing a globe of glass, first leaving in it the air which it contained, and then weighing it again when the air had been removed by the air- pump. Aristotle had long before suspected that air had weight, and to make sure of the fact, he weighed a leather bottle, first empty and afterward when inflated with air; for, he remarked, if the air has weight, the leather bottle will be heavier when weighed the second time than it was the first time.
The experiment not confirming his supposition, he concluded that the air had no weight. Nevertheless, several of the ancient philosophers admitted the material nature of air as a fact. Thus the Epicureans compared the effects of the wind with those of water in motion, and considered the elements of the air as invisible bodies. During the reign of the peripatetic philosophy, however, it was assumed that air was without weight, and there were but few philosophers who did not share this erroneous opinion.
We have seen that, by repeating judiciously the experiment of Aris- totle, Otto de Guericke demonstrated the real weight of air. If Aris- totle's experiment led to a contrary result, it must be attributed to the change in the volume of the leather bottle during his two trials, for every body, when weighed in a fluid, loses in weight a quantity equal to the weight of the fluid displaced.
The leather bottle made use of by Aris- totle would have shown an increase of weight if weighed in a vacuum. Let us suppose that about cubic inch- es, of air were introduced into it by in- spiration; its weight would have in- creased by about grains, but at the same time the bottle would become in- flated, and its volume, being increased by cubic inches, would have dis- placed a volume of air of equal weight, so that its loss in weight would be also grains, and the weight of the air and bottle together would consequent- ly remain the same as before.
But in the experiment of Otto de Guericke the globe was always of the same size, Whether empty Or full of air, and its Fig. Otto de Guericke, at the same time, conceived the idea of the Magdeburg Hemispheres, so called from the town in which he invented them, and which consist of two hollow hemispheres of copper, with a diameter of from four to five inches. The hemispheres fit each other hermetically.
One of them has attached to it a cock that screws on to the plate of an air-pump, and the other a ring which acts as a handle to move it backward or forward.