in the tube about twenty-nine and a half inches above its surface in the vessel, sometimes more and sometimes less, according to the state of the atmosphere. It is evident, then, that the quicksilver is kept up in the tube to this elevation by the pressure of the atmosphere upon the surface of the mercury in the bason; for, if the bason and tube are put under a glass, and the air extracted, all the quicksilver in the tube will fall down into the bason; and, if the air be re-admitted, it will rise to the same height as before; or, if an opening be made in the top of the tube and the air admitted, the quicksilver will sink into the bason. The pressure, therefore, by the atmosphere on the earth, is the same as if a coating of quicksilver twenty-nine and a half inches thick were spread over every part of the earth's surface.

Now, it is proved that a square column of quicksilver twenty-nine and a half inches in height, and one inch thick, weighs just fifteen pounds, which counterpoise a column of air of the same thickness, extending to the top of the atmosphere; and, consequently, that air presses with this force upon every square inch of the earth's surface; and, of course, 2,160 pounds on every square foot, and 19,440 on every square yard. The experiment now described is, in fact, nothing else than the common barometer. The tube of the barometer is filled with quicksilver, or mercury; it then stands in a bason of quicksilver, is connected with a ball containing quicksilver, on the surface of which

the atmosphere presses, and, in most cases, stands at an elevation of about twenty-nine and a half inches, but subject to certain variations, according to the state of the atmosphere. When the weather is steady and serene, it rises to above thirty inches; when it is stormy and rainy, it frequently sinks to twenty-eight inches, or under, thus indicating the changes that take place in the weight of the air; and hence, it has obtained the name of the weather-glass.

Were the same experiment made with water, instead of mercury, a tube must be provided of about thirty-six feet long; and then it would be found, that the water in the tube would be supported by the atmospheric pressure to the height of thirty-two or thirty-three feet. This costly experiment, which has been seldom repeated, was first performed by the celebrated Pascal, at Rouen, in Normandy, in 1647. He exhibited the experiment both with water and with wine, in order to show the different heights to which these fluids would rise, according to their respective densities. He procured, at a glass-house, tubes of crystal glass forty feet long, which were fixed to the mast of a ship, that was contrived to be raised or depressed, as occasion required. He appointed a day for performing this experiment, and invited all the philosophers and others who doubted of the pressure of the atmosphere to attend, and to be witnesses of the wonderful nature of his experiment. The result was, according to the calculations he had previously made, that the

altitude of water in the tube was thirty-one and one-ninth Paris feet, equal to thirty-two feet two and a half inches English; and the altitude of the wine was somewhat greater, namely, thirty-one and two-thirds Paris feet, or thirty-two feet ten inches English; the wine, on account of its superior levity, rising about seven and a half inches higher than the water. He performed this experiment to convince the Aristotelian philosophers of those times of the folly of a notion which then prevailed, that the rise of the mercury in the Torricellian experiment and the rise of water in pumps were produced, not by the pressure of the atmosphere, but by an occult quality, which they denominated "Nature's abhorrence of a vacuum." They asserted that, in the upper part of the tube, deserted by the quicksilver, there were contained some spirits, evaporated from the quicksilver; which, being rarefied, filled up that space, thus assisting Nature, in a great emergency, against her mortal enemy, a vacuum. Well, then, gentlemen," says Pascal, "take your own way. Here are two tubes, the one I am to fill with water, and the other with wine. You will all readily admit that there is a greater quantity of spirits in wine than in water; and, consequently, that, if the empty space between the upper surface of the fluids and the top of the tube be filled with spirits, there will be a greater quantity of spirits in the upper part of the tube containing the wine, than in the tube containing the water; and, of course, the wine

will not rise so high as the water." To this they all readily assented. But, when the experiment was made, the wine was found to rise nearly eight inches higher than the water, as Pascal had previously calculated and predicted. This experiment was decisive; and, since that period, the figment of "Nature's abhorrence of a vacuum," along with many other absurdities, has been consigned to the slumber of the dark ages whence it originated.

A few years before the period now alluded to, the engineers of the grand duke of Florence, having received orders to raise a portion of water to the height of fifty or sixty feet, by means of a common pump, perceived, when they had made the attempt, that the pump refused its assistance, when the water was to be raised above thirty-two feet. They communicated the circumstance to Galileo, an eminent philosopher of that age, and asked him the reason of it. Galileo was not a little surprised, and was unprovided with an answer. He, however, put a good face on the affair, and gravely replied, that "Nature abhorred a vacuum only to the height of thirty-two feet." Torricelli, the disciple of Galileo, vexed at the water's refusing to ascend more than thirty-two feet in a tube void of air, made a new experiment with quicksilver, in the manner already described. He saw the quicksilver in the tube fall down, and leave an empty space at top, and remain suspended at the height of twenty-nine inches. "How," says he, "Nature abhors a vacuum only to the height

of thirty-two feet when it is water that ascends in a tube void of air, and only to twenty-nine inches when it is mercury! Vacuum does not frighten her beyond these measures! But why does she fear it to far more than twenty-nine inches when it is water that rises? Very likely this horror at vacuity is an idle fancy, a mere philosophical cant, which we take for good coin without understanding it." Reasoning in this way, and repeating a variety of similar experiments, he was, at length, led to the conclusion "that the diversity of the elevation of the two different fluids proceeded from the diversity of their weight, and that they were supported and counterpoised by a column of air, of the same diameter, reaching to the top of the atmosphere." Here the matter rests, and will rest throughout all succeeding generations.

This short sketch of the history of the experiments which relate to the pressure of the atmosphere will not be altogether out of place, if it has a tendency to guard us against the influence of preconceived notions, foolish prejudices, and of the authority of great names, which are some of the greatest obstructions to the expansion of the human mind, and the reception of useful knowledge. It was not before men began to emancipate themselves from such shackles that science commenced that brilliant career which has issued, in our times, in so many interesting and important discoveries. Similar prejudices are still prevalent in relation to the affairs of common life,