186

Atmospheric Pressure on Laquids.

[Book II. within it. In this experiment the paper merely preserves the liquid surface level it remains perfectly free and loose; and so far from being close to the edge of the glass, it may, while the latter is held in a horizontal position, be withdrawn several lines from it without the water escaping; and it may be pierced full of small holes with the same effect.

If an inverted vessel be filled with any material that excludes the air, and whose specific gravity is greater than that of water, when lowered into the latter, the contents will descend and be replaced by the water. A bottle filled with sand, shot, &c. and inverted in water, will have its contents exchanged for the latter. As these substances, however, do not perfectly fill the vessel, and of course do not exclude all the air, the experiment succeeds better when the vessel contains heavy liquids, as mercury, sulphuric acid, &c. It is said that negroes in the West Indies often insert the long neck of a bottle filled with water, into the bung-holes of rum puncheons, when the superior gravity of the water (in this case) descends, and is gradually replaced with the lighter spirit.

In the preceding examples and those in subsequent chapters, it will be found that wherever a vacuity or partial vacuum is formed, the adjacent air, by the pressure above, rushes in and drives before it the object that intervenes, until the void is filled. If the nozzle of a pair of bellows be closed, either by the finger or by a small valve opening outwards; and a short pipe, the lower end of which is placed in water, be secured to the opening in the under board which is covered by the clapper; then if the bellows be opened, the pressure of the atmosphere will drive the water up the pipe to fill the enlarged cavity, and by then closing the boards, the liquid will be expelled through the nozzle. Bellows thus arranged become sucking or atmospheric, and forcing pumps. When the orifice of a syringe is inserted into a vessel of water and the piston drawn up, the air having no way to enter the vacuity thus formed than by the small orifice under the surface of the liquid, presses the water before it into the body of the syringe.

As every machine described in this book, and most of those in the next one, both proves and illustrates atmospheric pressure on liquids, we need not enlarge further upon it here. There are however some other particulars relating to it, which are necessary to be known: first, that its pressure is limited; and secondly, that it varies in intensity at different parts of the earth, according to their elevation above the surface of the sea. These important facts are clearly established in the accounts given of the discovery of the air's pressure, a sketch of which can scarcely be out of place here, since it was a pump that first drew the attention of modern philosophers to the subject, and which thereby became the proximate cause of a revolution in philosophical research, that will ever be considered an epoch in the history of science.

Chap. 2.]

Discovery of Atmospheric Pressure.

187

CHAPTER II.

Discovery of atmospheric pressure-Circumstances which led to it-Galileo-Torricelli-Beautiful experiment of the latter-Controversy respecting the results-Pascal-his demonstration of the cause of the ascent of water in pumps-Invention of the air-pump-Barometer and its various applicationsIntensity of atmospheric pressure different at different parts of the earth—A knowledge of this necessary to pump-makers-The limits to which water may be raised in atmospheric pumps known to ancient

pump-makers.

In the year 1641, a pump-maker of Florence made an atmospheric, or what was called a sucking pump, the pipe of which extended from 50 to 60 feet above the water. When put in operation, it was of course incapable of raising any over 32 or 33 feet. Supposing this to have been occasioned by some defect in the construction, the pump was carefully examined, and being found perfect, the operation was repeated, but with the same results. After numerous trials, the superintendent of the Grand Duke's water works, according to whose directions it had been made, consulted Galileo, who was a native of the city, and then resided in it. Previous to this occurrence, it was universally supposed that water was raised in pumps by an occult power in nature, which resisted with considerable force all attempts to make a void, but which, when one was made, used the same force to fill it, by urging the next adjoining substance, if a fluid, into the vacant space. Thus in pumps, when the air was withdrawn from their upper part by the 'sucker,' nature, being thus violated, instantly forced water up the pipes. No idea was entertained by philosophers at this or any preceding period, that we know of, that this force was limited; that it would not as readily force water up a perpendicular tube, from which the air was withdrawn, 100 feet high as well as 20-to the top of a high building as well as to that of a low one.

When the circumstances attending the trial of the pump at Florence were placed before Galileo, (his attention having probably never before been so closely directed to the subject) he could only reply, that nature's abhorrence to a vacuum was limited, and that it " ceased to operate above the height of 32 feet." This opinion given at the moment, it is believed was not satisfactory to himself; and his attention having now been roused, there can be no doubt that he would have discovered the real cause, had he lived, especially as he was then aware that the atmosphere did exert a definite pressure on objects on the surface of the earth. But at that period this illustrious man was totally blind, nearly 80 years of age, and within a few months of his death. The discovery is however, in some measure, due to him. It has also been supposed that he communicated his ideas on the subject to Torricelli, who lived in his family and acted as his amanuensis during the last three months of his life.

It was in 1643 that Torricelli announced the great discovery that water was raised in pumps by the pressure of the air. This he established by very satisfactory experiments. The apparatus in his first one, was made in imitation of the Florentine pump. He procured a tube 60 feet long, and secured it in a perpendicular position, with its lower end in water; then having by a syringe extracted the air at its upper end, he found the water rose only 32 or 33 feet, nor could he by any effort induce it to

188

Torricelli.

[Book II. ascend higher. He then reduced the length of the pipe to 40 feet, without any better success. It now occurred to him, that if it really was the atmosphere which supported this column of water in the pipe, then, if he employed some other liquid, the specific gravity of which, compared with that of water, was known, a column of such liquid would be sustained in the tube, of a length proportioned to its gravity. This beautiful thought he soon submitted to the test of experiment, and by a very neat and simple apparatus.

Quicksilver being 14 times heavier than water, he selected it as the most suitable, since the apparatus would be more manageable; and from the small dimensions of the requisite tube, a syringe to exhaust the air could be dispensed with. He therefore took a glass tube about four feet long, sealed at one end and open at the other. This he completely filled with quicksilver, which of course expelled the air; then placing his finger on the open end, he inverted the tube, and introduced the open end below the surface of a quantity of mercury in an open vessel; then moving the tube into a vertical position, he withdrew his finger, when part of the mercury descended into the basin, leaving a vacuum in the upper part of the tube, while the rest was supported in it at the height of about 28 inches, as he had suspected, being one-fourteenth of the height of the aqueous column. This simple and truly ingenious experiment was frequently varied and repeated, but always with the same result, and must have imparted to Torricelli the most exquisite gratification.a

Accounts of Torricelli's experiments were soon spread throughout Europe, and every where caused an unparalleled excitement among philosophers. This was natural, for his discovery prostrated the long cherished hypothesis of nature's abhorence of a vacuum; and at the same time, opened unexplored regions to scientific research. It met however with much opposition, particularly from the Jesuits; in many of whom it is said to have excited a degree of horror' similar to that experienced by them on the publication of Galileo's dialogues on the Ptolemaic and Copernican systems. They and others resisted the new doctrine with great perseverance, and even endeavored to reconcile the results of the experiments with the fuga vacui they so long had cherished. It was ingeniously contended that the experiment with quicksilver no more proved that the force which sustained it in the tube was the pressure of the atmosphere, than the column of water did in the first experiment; allowing this, it proved that this force, whatever it was, varied in its effects on different liquids, according to their specific gravity; a fact previously unknown, and apparently inconsistent with nature's antipathy to a void, which might be supposed to produce the same effects on all fluids-to have as great an abhorence to mercury as to water.

During the discussion great expectations were entertained by the advocates of the new doctrine from Torricelli; but unfortunately, this philosopher died suddenly in the midst of his pursuits and in the very vigor of manhood, viz. in his 39th year. This took place in 1647. The subject was however too interesting, and too important in its consequences, to be lost sight of. He had opened a new path into the fields of science, and philosophers in every part of Europe had rushed into it with too much ardor to be stopped by his decease. Among the most eminent of those

The apparatus employed in these experiments was not original with Torricelli. The air thermometer of C. Drebble, the famous alchemist, who died in 1634, was of the same construction, except that the upper end of the inverted tube was swelled into a bulb. It is frequently figured in Fludd's works.

Chap. 2.]

Pascal and Perrier.

189

was Pascal, a French mathematician and divine. In 1646 he undertook to verify the experiments of Torricelli, and still further to vary them. He used tubes of glass forty feet long, having one end closed to avoid the use of a syringe. He filled one with wine and another with water, and inverted them into basins containing the same liquids, after the manner of Torricelli's mercurial experiment. As the specific gravity of these liquids was not the same, he anticipated a difference in the length of the two columns; and such was the fact. The water remained suspended at the height of thirty-one feet one inch and four lines; while the lighter wine stood at thirty-three feet three inches. Pascal was attacked with great virulence by Father Noel, a Parisian jesuit, who resisted the new doctrine with infuriate zeal, as if it also was heresy, like Galileo's doctrine of the earth's motion round the sun.

After making several experiments, one at length occured to Pascal, which he foresaw would, if successful, effectually silence all objectors. He reasoned thus: If it is really the weight or pressure of the atmosphere, that sustains water in pumps, and mercury in the tube, then, the intensity of this pressure will be less on the top of a mountain than at its foot, because there is a less portion of air over its summit than over its base; if therefore a column of mercury is sustained at 28 or any other number of inches at the base of a very high mountain, this column ought to diminish gradually as the tube is carried up to the top; whereas, if the atmosphere has no connection with the ascent of liquids, (as contended) then the mercury will remain the same at all elevations, at the base as at the summit. Being at Paris, he addressed a letter to his brother-in-law, M. Perrier, (in 1647) from which the following is an extract: "I have thought of an experiment, which, if it can be executed with accuracy, will alone be sufficient to elucidate this subject. It is to repeat the Torricellian experiment several times in the same day, with the same tube, and the same mercury; sometimes at the foot, sometimes at the summit of a mountain five or six hundred fathoms in height. By this means we shall ascertain whether the mercury in the tube will be at the same or a different height at each of these stations. You perceive without doubt that this experiment is decisive; for if the column of mercury be lower at the top of the hill than at the base, as I think it will, it clearly shows that the pressure of the air is the sole cause of the suspension of the mercury in the tube, and not the horror of a vacuum; as it is evident there is a longer column of air at the bottom of the hill than at the top; but it would be absurd to suppose that nature abhors a vacuum more at the base than at the summit of a hill. For if the suspension of the mercury in the tube is owing to the pressure of the air, it is plain it must be equal to a column of air, whose diameter is the same with that of the mercurial column, and whose height is equal to that of the atmosphere, from the surface of the mercury in the basin. Now the base remaining the same, it is evident the pressure will be in proportion to the height of the column, and that the higher the column of air is, the longer will be the column of mercury that will be sustained." This experimentum crucis, was made on the 19th September, 1648, the year after Torricelli's death, on the Puy de Dome, near Clermont, the highest mountain in France; and the result was just as Pascal had anticipated. The mercury

fell in the tube as M. Perrier ascended with it up the mountain, and when he reached the summit it was three inches lower than when at the base. The experiment was repeated on different sides of the mountain, and continued by Perrier till 1651, but always with the same results. Pascal made others on the top of some of the steeples in Paris; and all

190

Limits of Atmospheric Pressure

[Book II. proved the same important truth, viz. that the pressure of the atmosphere was that mysterious power, which under the name of nature's abhorrence to a vacuum had so long eluded the researches of philosophers. The subject was taken up in England by Boyle, who pursued it with unremitted ardor, and whose labors have immortalized his name; but it was Germany that bore off the most valuable of the prizes which the discovery offered to philosophers. The Torricellian experiment gave rise to the AIR PUMP ; and in 1654, a Prussian philosopher, a mathematician and a magistrate, Otto Guerricke, of Magdeburgh, made public experiments with it at Ratisbon, before the emperor of Germany and several electors. Some authors ascribe the invention of the pump to Candido del Buono, one of the members of the Academie del Cimento at Florence, and intimate that the first essays with it were only made by Guerricke.

The apparatus of Torricelli, i. e. the glass tube and basin of mercury, was named a baroscope, and afterwards a barometer, because it measured the pressure of the atmosphere at all elevations; hence to it, engineers in all parts of the earth may have recourse, to determine the perpendicular length of the pipes of atmospheric pumps.

Another application of the barometer was the natural result of Perrier's first experiment on the Puy de Dome. As he ascended that mountain with it, the mercury kept falling in exact proportion to the elevation to which the instrument was carried; hence it is obvious, that when the tube is properly graduated, it will measure the height of mountains, and all other elevations to which it can be carried. By it, aeronauts determine the height to which they ascend in balloons. The observations of Perrier were continued daily from 1649 to 1651, during which he perceived that the height of the column slightly varied with the temperature, wind, rain, and other circumstances of the atmosphere; and hence the instrument indicated changes of weather, and became known and is still used as a weather glass." The extent of these variations is about three inches, generally ranging from twenty-eight to thirty-one, and are principally confined to the temperate zones. In tropical regions, the pressure is nearly uniform, the mercury standing at about thirty inches throughout the year. These facts have an important bearing on our subject; for an atmospheric pump or siphon, with a perpendicular pipe thirtyfour or thirty-five feet long, might operate during certain states of the atmosphere, while in others it could not; and in some parts of the earth it would be altogether useless.

66

It will appear in the sequel, that the physical properties of the atmosphere which we have enumerated, must necessarily be understood, in order perfectly to comprehend the action of the machines we have to describe. As regards the aërial pressure, its limits and variation at different altitudes, we need only remark, that a sucking pump or a siphon, which raises water thirty-three feet in New-York and Buenos Ayres, London and Calcutta, St. Petersburgh and Port Jackson in New Holland, could not, in the city of Mexico, elevate it over twenty-two feet; and at Quito, and Santa Fe de Bogota in South America, and Gondar the capital of Abyssinia twenty feet, on account of the great elevation of these cities; (from the same cause, the pressure of the atmosphere on Mont Blanc is only about half that on the plains) and if Condamine and Humboldt, when on the summit of Pinchincha, had applied one to raise water there, or on the side of Antisana, at the spot where, from the great rarity or tenuity of the air, the face of the latter philosopher was streaming with blood, his attendant fainted, and the whole party exhausted, it would not have raised water over twelve or fourteen feet; (the mercury in the barometer fell