less and its pressure 'diminishes. The mercury, therefore, in the barometer sinks, because as the air becomes rarer or lighter, it cannot hold up so heavy a volume of mercury. The difference of a hundred feet in height in the atmosphere will occasion a difference of one-tenth of an inch in the length of the column of mercury. By thus reckoning one-tenth of an inch for the first hundred feet of elevation, the height of places from the level of the sea may be easily ascertained. The straight tube 3 barometer is considered the most perfect form of the instrument. The wheel barometer is less perfect, yet as this is the ordinary form of the weather Upon glass, I will illustrate its arrangement. The As the surface of the mercury in the ball is very large, and that at the bottom of the tube comparatively small, the motion of the quicksilver, and consequently of the balance or float at bottom, will be very considerable; and as the weight moves up and down it turns the pulley, and to that is attached a hand or index, which passes over the divisions of a large 1graduated circular scale, D. 1. Vide Root. 2. Mercury being thirteen times heavier than water, or 13.598. 3. Barometers were invented by Torricelii, a. D. 1608. 4. If the mercury be high, the weight of the air is shown to be great, and it is inferred that the clouds will rise and the weather will be fair; if the mercury be low, that the clouds and vapour of the atmosphere will descend and cause rain. FRANCE. Early History of France. THE ancient name of 2France was Gaul, and the inhabitants were called Gauls. In the year 390, B. C., they invaded Rome under Brennus, and took that city, but were expelled by Camillus. In the time of Julius Cæsar, the Gauls had made some little progress towards civilization; but they were still a barbarous people, and retained many practices that belong only to savages. So brave and obstinate were the Gauls in the defence of their country, that it required all the genius of Julius Cæsar, one of the greatest leaders that ever lived, aided by the immense power of Rome, to subdue them. Cæsar was occupied no less than nine years in conquering the Gauls, and it is supposed a million of men were slain in the dreadful struggle. From the time of Cæsar's conquest, about 50 years B.C., Gaul was a Roman province, and the people gradually adopted the manners and customs of the Romans. Even their language became changed, and assumed a resemblance to the Latin. But between the third and fourth century, the Franks, a German tribe whom I have mentioned, got possession of the greater part of Gaul. It is said that the Franks who first established themselves in Gaul were led by Pharamond. He died in 428, and was succeeded by his son Clodian, who was celebrated for the beauty of his hair. Clodian died in 448, and was succeeded by Merovius; Merovius died in 458, and was succeeded by Childeric. Very little is known of these kings, except the last. 1. Vide Root. 2. From the word Franks, signifying free men, originally from between the Elbe and the Rhine. 3. Clovis, first Christian king of France, 496. 4. Among his four sons, 6. At Rome, Christmas, 800. 3 Pharamond, their leader, was the founder of the French name and monarchy. Clovis, the fourth king in succession from him, when only nineteen years of age, entirely freed it from the Romans, and made Paris his 'capital. 4 After the death of Clovis, France was divided among several petty kings. Pepin the Short thrust all the other kings from their thrones, and made himself 1 sole ruler of France. 1 King Pepin the Little had a son who was called Charlemagne, or Charles the Great. The epithet was given him because he was a mighty king and conqueror, but he also deserved it on account of his height, which was not an inch less than seven feet. This king was continually at war. He 'subjugated the Saxons, and other tribes who lived in Germany. He likewise made conquests in Spain and Italy. At length, ruling over France, Germany, and other countries, he was crowned 5 Emperor of the West. Charlemagne died in the year 814 when he was quite an old man. Some of the succeeding kings of France were Charles the Bald, Louis the Stammerer, Charles the Fat, Charles the Simple, Louis the Foreigner, and Hugh Capet. These sovereigns performed no actions that need be recorded. In the year 912 Normandy was yielded to Rollo the Norman leader. GEOGRAPHICAL.-Area, 205,000 square miles. Population, 33 millions. Write the names of its principal rivers. What places are situated on the banks of the rivers-Seine, Loire, Rhine, Rhone, Garonne, Meuse, and Moselle ? CHRONOLOGICAL.-The Gaël, Gauls, or Celts, began to enter Europe from Asia about 2000, B.C. General dispersion of the Celts through Europe about .... 390, B.C. France invaded by the Goths and other tribes from Germany 400, A.D. 418, A.D. 496, A.D. 751, A.D. 987, A.D. PNEUMATICS AND HYDROSTATICS. Balloons. THE art of1 navigation through the air is called 1aerostation. The machines which are employed for this purpose, are called aerostats, or more commonly, on account of their round figure-air balloons, or simply, balloons. Balloon means a spherical, hollow body. The person who sails in the balloon, is called the 'aeronaut, from two Greek words, which mean a sailor of the air. The balloon is constructed upon the principle of the expansion and specific gravity of the air. The weight of a body compared with that of another body of the same bulk, is called its specific gravity. Thus a portion of water weighs eight hundred times more than a portion of air of the same bulk, or occupying an equal space. The specific gravity of air is therefore eight hundred times less than that of water. In estimating the specific gravity of bodies, water is taken as the standard of comparison, and considered as unity or one. The specific gravity of gold is 19.640, or more than nineteen times greater than that of water. That of glass is 3, or three times greater than that of water. Of oil .920, or but little less than that of water. The specific gravity of the human body, when alive, is .891, or about one-ninth less than that of water. 2 Now you know that when a solid body is thrown into a fluid, if its specific gravity is greater, it will sink; and if it is less, it will be borne up by the fluid, as a stone will sink, and a cork float in water. So of two fluids, the lighter will be borne up by the one whose specific gravity is greatest. Thus oil floats in water. Hydrogen gas rises in the common air, because its specific gravity is seven times less than that of air. The experiment of the balloon consists in filling it with a substance whose specific gravity is less than that of the atmosphere. You remember that the specific gravity of the atmosphere decreases as it recedes from the earth; consequently the balloon will ascend until the specific gravity of the surrounding air is equal to that of the substance with which the balloon is filled. The highest point ever attained was by Mr. Green, of nearly five miles. Beyond this animal life can scarcely exist, both on account of the rarefaction of the air and the intense cold. The inventor of balloons was Montgolfier in 1783, who took his idea of them from seeing light clouds float in the atmosphere. He thought that by making a large bag, and filling it with heated air, he could make the whole lighter than the common air; he therefore burnt straw under a bag of vast size, open at the lower end, and the rarefied air passed in it, which rendered the whole so light that it bore up men from the earth. This was a fire balloon, that is, one inflated with heated atmospheric air. Balloons are now made of silk, prepared in a peculiar manner, and filled with coal gas from the gas works. Fire-balloons, those raised by heated air, may be made of tissue paper, with a wire round the bottom, to which a cup containing some spirits of wine on cotton may be attached, the flame of which will rarefy the air within, and raise the balloon. A slight difficulty of breathing is experienced as the balloon ascends high up, and the blood-vessels of the nose being less supported by the external pressure, will sometimes give way, and bleeding ensues. If the aeronaut wishes to ascend, he throws out bags of sand or other heavy matter; and if to descend, he opens a small valve which allows the gas slowly to escape; and on approaching the earth, by throwing out grapplingirons, he secures a safe descent. 1. Vide Root. 2. Persons sink in consequence of cold contracting the size of the body; from receiving water, whilst sinking and rising again; and from the effects of fear |