his finger, what he had expected and calculated took place: the mercury fell in the tube till it stood still at a height of about thirty inches above the mercury in the vessel. This happened in the year 1643. Ն FIG. I. This clever man was not long in disa covering what was the force which sustained the mercury in his tube to the height of thirty inches, or the water in the pump to the height of thirty-three feet-viz., the downward pressure of the air on the surface of the mercury or water in the vessel below. Air is a very light substance, but it has weight, and a large quantity of even a light substance will weigh very heavy; therefore, as the air extends above the earth to the height of at least forty-five miles (as is supposed), its weight must be something considerable. Torricelli's explanation of this was so new and strange, and people generally so cling to what they have believed for a long time, that nearly everybody still thought and taught in the old way,—and Torricelli had no proof to offer. But M. Pascal, of Paris, another famous philosopher, took great interest in Torricelli's discovery, tried the same experiment with water and other liquids, and always found that they stood at heights which might be pretty nearly calculated beforehand-the weights of the various liquids being known. And now he, in his turn, had a happy idea. "If this column of mercury or water," thought he, "which always seems to be the measure of some unknown force, is really supported by the weight of the air, as Torricelli thinks, then where the air is lightest the column will be least. On the top of a mountain, where there is less pressure of air, the mercury ought to stand much lower. And if this turns out to be so, I shall have proved Torricelli's theory to be the right one." He therefore wrote to a friend, asking him to ascend the Puy de Dôme, a mountain of Auvergne, 5000 feet high, and repeat Torricelli's experiment there. His friend did so; and reported that the column of mercury supported in the tube at the top of the mountain was three-and-a-half inches less than at the foot. This was in 1648. Thus was invented the barometer -an instrument for measuring the weight or pressure of the atmosphere. To make a barometer you require a glass tube about a yard long, closed at one end, and a vessel of pure mercury. Put a little mercury into your tube, and heat it well over a spirit-lamp; add a little more warm mercury, and heat again; and thus by degrees fill your tube. All this care is to ensure that the tube is perfectly dry and free from air. MUCH RAIN RAIN STORMY CHANGE VERY DRY FIG. 2.1 FAIR SETFAIR When it is quite full, stop the open end with your finger, invert it, and dip the open end under the surface of the mercury in your vessel. Now remove your finger, and if your mercury is quite pure and dry, and if you have been careful to allow no air to enter, the mercury will stand in the tube to such a height as will be exactly equal in weight to the pressure of the atmosphere at that time and place. When a barometer is used as a weather-glass, it is provided with a face like that of a clock, and with a hand which turns to the right or left as the mercury rises or falls, and points to a number which corresponds to the height of the mercury. The height of this column of mercury will vary with the state of the atmosphere. In England it will very rarely fall below twenty-eight inches, or rise above thirty-one inches. The following table shows what kind of weather may generally be expected according to the height of the barometer: It must not be supposed, however, that the weather will of necessity follow the barometer-for that instrument only measures the pressure of the air, not the state of the weather; but the same causes which render the atmosphere dense or light also generally bring fair or foul weather respectively. And so the chief practical use made of the barometer is to foretell the state of the weather. In hot weather a sudden fall of the mercury is almost always followed by thunder. In frosty weather a fall presages a thaw, and a rise will most probably be followed by snow. In general, when the mercury rises or falls, and the consequent sort of weather is a long time in making its appearance, it will most likely continue, when it does come, for some length of time; but if the expected change follows quickly, it may be expected to last for a short time only. It should not be forgotten that the weight of the air lessens as we ascend a height, so that (as Pascal was the first to discover) the barometer falls as you ascend a mountain. Speaking roughly, at small elevations, for every ninety feet above the level of the sea the barometer will fall one-tenth of an inch. An ascent of one mile in perpendicular height will cause the mercury to fall about six inches. The barometer may therefore be used to measure the approximate height of mountains. Figure 1 gives you a view of the real barometer. The tube and cup are all in one ; and in the cup you may see a small ball of iron or glass floating,-for either of these substances is lighter than mercury. To this ball is attached a cord, which passes round a small wheel and terminates in another ball (c), which is lighter than the ball b. Let us suppose that the barometer in the figure is standing at thirty inches. When a change takes place in the atmosphere which would cause the mercury to fall an inch, the mercury at a falls, and that at brises, each half an inch, making altogether the right difference of one inch. As the mercury at b rises it lifts the ball with it, and then the weight c falls half an inch, drawing the wheel and the hand round with it as far as d. In like manner a rise of half an inch would bring the hand round to e. This simple machinery in the weather-glass is all hidden, and the instrument which hangs upon the wall shows you only a face and two hands, one of which moves to the right or to the left as the mercury rises or falls (see fig. 2). The second hand is moved by a person's fingers, and is designed to remind one how the barometer stood when it was last consulted. Thus the difference in the position of the two hands shows how much the barometer has risen or fallen in a certain time; and this is important to know, as the weather to be expected depends much upon the rate at which the change in the barometer occurs. VERSES BY A PRINCESS. UNTHINKING, idle, wild, and young, But when the days of trial came, When sickness shook this trembling frame, PRINCESS AMELIA, d. of George III. THE GATHERING FOR WAR. [SIR WALTER SCOTT, though chiefly celebrated as a novelist, has also gained fair fame as a poet. His chief poems are The Lay of the Last Minstrel, Marmion, and The Lady of the Lake. The following stanzas are from the last-mentioned poem, and describe the old Highland method by which a chieftain summoned his clansmen to war. Sir Roderick Dhu was the chief of the clan that occupied the wild romantic country near Loch Katrine, in the Western Highlands. The signal for gathering for war was a yew cross, the ends of which were charred or blackened with fire, and stained with blood.] THEN Roderick, with impatient.look, From Brian's hand the symbol' took : The bubbles, where they launched their boat, Dancing in foam and ripple still, When it had near'd the mainland hill; 3 Still was the prow three fathoms wide, Speed, Malise, speed! the dun deer's hide Speed, Malise, speed! such cause of haste 6 5 |