MAGNETIC EXPERIMENTS.

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Magnetic attraction will not be destroyed by interposing obstacles between the magnet and iron. If you lay a small needle on a piece of paper, and put a magnet under the paper, the needle may be moved backwards and forwards; and with a piece of glass or board the effect will be the same. This property of the magnet has afforded the means of several amusing deceptions. A small figure of a man has been made to spell a person's name. The hand, in which was a piece of iron, rested on a board, under which a person, concealed from view, with a powerful magnet, contrived to carry it from letter to letter, until the word was made up. If the figure of a fish, with a small magnet concealed in its mouth, be thrown into the water, and a baited hook be suspended near it, the magnet and iron by mutual attraction will bring the fish to the bait.

If you lay a sheet of paper, covered with iron filings upon a table, with a small magnet among them, and then shake the table a little, at the two ends or the poles, the particles of iron will form themselves into lines, a little sideways, which bend, and then form complete arches, reaching from some point in the northern half of the magnet to some other point in the southern half. If you shake some iron-filings through a gauze sieve upon a paper that covers a bar magnet, they will be arranged in beautiful curves.

Soft iron is attracted by the magnet more forcibly than steel, but it is not capable of preserving the magnetic property so long. The gradual addition of weight to a magnet kept in its proper situation, increases the magnetic power, but heat weakens it, and great heat destroys it. Bars of iron that have stood long in a perpendicular situation, are generally found to be magnetical; this circumstance, toge ther with the phenomena of the compass and the dipping needle, leaves no room to doubt but that the cause exists within the earth.

QUESTIONS.-1. Where is the natural magnet found? 2. Why are artificial magnets used in preference to natural? 3. How may very powerful magnets be formed? 4. How do the poles of a magnet attract and repel each other? 5. How does it appear that the attraction between the magnet and iron is mutual? 6. How does it appear that magnetic attraction will not be destroyed by the interposition of bodies? 7. What amusing deceptions has the attractive property of the magnet afforded? 8. How may the magnetic power be weakened or destroyed? 9. From what is it concluded that the cause of magnetism exists in the earth?

Wick'er, made of small sticks. A'eronaut, one who sails through the air.

Meteorological, relating to the phenomena of the atmosphere, such as the alterations of its weight and temperature, changes produced by evaporation and rain, its excessive agitations, its electricity, &c.

AEROSTATION, in the modern application of the term, signifies the art of navigating through the air, both in its principles and practice. Hence also the machines which are employed for this purpose, are called aërostats, or aërostatic machines; and on account of their round figure, air-balloons. Air-balloons are of two kinds, those filled with rarefied air, and those filled with hydrogen gas. The best forms for balloons are globular or oval. Large balloons, for hydrogen gas, must be made of silk, and varnished over so as to be air-tight. The car, or boat, is made of wicker-work, covered with leather, well varnished or painted, and is suspended by ropes proceeding from the net which goes over the bal loon. The hydrogen gas for filling the balloon is procured by putting a quantity of iron-filings, or turnings, with some sulphuric acid diluted with water, into casks lined with lead. From the top of these casks tin tubes proceed, which unite into one that is connected with the silk tube of the balloon. Balloons of oiled silk cannot be made smaller than five or six feet in diameter, as the weight of the material is too great for the air to buoy it up.

In 1729, Bartholomew Gusman, a Jesuit of Lisbon, caused an aërostatic machine, in the form of a bird, to be constructed, and made it ascend, by means of a fire kindled under it, in the presence of the king, queen, and a great concourse of spectators. Unfortunately, in rising, it struck against a cornice, was torn, and fell to the ground. The inventor proposed renewing his experiment; but the people had denounced him to the inquisition as a sorcerer, and he withdrew into Spain, where he died in an hospital. In 1766, the Honourable Henry Cavendish discovered that hydrogen gas (then called inflammable air,) was at least seven times fighter than common air. It occurred soon afterwards to

the celebrated Dr. Black, that if a thin bag were filled with this gaseous substance, it would, according to the established laws of specific gravity, rise in the common atmosphere; but he did not pursue the inquiry. The same idea was conceived by Mr. Cavallo, to whom is generally ascribed the honour of commencing the experiments on this subject. He had made but little progress, however, in these experiments, when the discovery of Stephen and John Montgolfier, papermanufacturers of France, was announced in 1782, and engaged the attention of the philosophical world. Observing the natural ascent of smoke and clouds in the atmosphere, those artists were led to suppose that heated air, if enclosed in a suitable covering, would also prove buoyant. After several smaller experiments, by which this idea was fully confirmed, they inflated a large balloon with rarefied air, which immediately and rapidly rose to the height of six thousand feet, and answered their most sanguine expectations.

It was soon found that machines of this kind might be so contrived, as to convey small animals, and even human beings, through the air with ease. The first adventurer in this aerial navigation was Pilatre de Rozier, a daring Frenchman, who rose in a large balloon from a garden in the city of Paris, on the 15th of October, 1783, and remained a considerable time suspended in the air. He made several aerial voyages afterwards of greater extent, and in two of them was attended by other persons. In a short time, however, the use of rarefied air in aërostation was for the most part laid aside, as inconvenient and unsafe. On recurring once more to the discovery of Mr. Cavendish, the philosophers of Paris concluded that a balloon, inflated with hydrogen gas, would answer all the purposes of that contrived by the Montgolfiers, and would also possess several additional advantages. They made their first experiment in August, 1783, which was attended with complete success. Since that time, air-balloons filled with rarefied air have not been generally used.

The first aerial voyage in England was performed by Vincent Lunardi, a native of Italy. The diameter of his balloon was thirty-three feet. Soon after, Mr. Blanchard ascended, carrying up a pigeon, which flew away from the boat, laboured for some time with its wings to sustain itself in the air, and finally returned and rested on one side of the boat. He ascended so high as to experience great difficulty

of breathing, but perceiving the sea before him, he descended near Ramsey, about seventy-five miles from London, having travelled at the rate of nearly twenty miles an hour.

The singular experiment of ascending into the atmosphere with a balloon, and of descending with a machine, called a parachute, in the form of a large umbrella, was performed by Mr. Garnerin in 1802. The weather was clear and pleasant, and the wind was gentle. In about eight minutes the balloon and parachute had ascended to an immense height, and Mr. Garnerin in the basket, could scarcely be perceived. While the spectators were contemplating the grand sight before them, Mr. Garnerin cut the rope, and in an instant he was separated from the balloon, trusting his safety to the parachute. Before the parachute opened, he fell with great velocity; but as soon as the parachute was expanded, which took place a few moments after, the descent became very gentle and gradual. It was observed that the parachute, with the appendage of cords and basket soon began to vibrate like the pendulum of a clock, and the vibrations were so great, that more than once the parachute, and the basket with Mr. Garnerin, seemed to be on the same level, or quite horizontal; the extent of the vibrations, however, diminished as he descended. On coming to the earth, he experienced some strong shocks, but soon recovered, and remained without any material injury.

The fate of Rozier, the first aerial navigator, and of his companion Romain, has been much lamented. They ascended with an intention of crossing the channel to England. Their machine consisted of a spherical balloon, filled with hydrogen gas, and under this balloon, a smaller one filled with rarefied air, designed to diminish the specific gravity of the whole apparatus. For the first twenty minutes they seemed to pursue the proper course; but the balloon appeared to be much inflated, and the aëronauts appeared anxious to descend. Soon, however, when they were at the height of three quarters of a mile, the whole apparatus was in flames, and the unfortunate adventurers fell to the ground, and were killed.

The invention of balloons cannot be considered as having added much to the comfort or utility of man. The only practical purposes which it has been made to subserve, are those of aiding meteorological inquiries, and of inspecting

the fortifications and reconnoitring the camp of an enemy, which could not be approached by other means. The difficulties, under which this species of navigation labours, appear at present to be insurmountable; and the want of some means to control and regulate the movements of the aerial vessel is so essential, as to excite a fear that it cannot be supplied.

QUESTIONS.-1. What is aërostation? 2. What is the best form for a balloon? 3. What are the two kinds of balloons? 4. How is a balloon filled with hydrogen gas? 5. Who invented the first aërostatic machine, and what was the result? 6. What discovery did Cavendish make? (Hydrogen gas is 14 times lighter than common air, -see Lesson on water.) 7. What afterwards occurred to Dr. Black? 8. What idea did Cavallo conceive?-what is ascribed to him? 9. What discovery did the Montgolfiers make? 10. Who was the first aerial navigator? 11. What was the next discovery in this science? 12. What is said of the ascent of Mr. Blanchard? 13. Describe the experiment of Mr. Garnerin. 14. What was the fate of Rozier and Romain? 15. What is said of the advantages which have been derived from balloons? 16. Of the difficulties under which this spe cies of navigation labours? [NOTE. Small balloons may be made of thin strips of bladder, or other membrane, glued together.]

LESSON 75.

Natural History.

Pellu'cid, clear, transparent, not opaque.

THOSE Who with a philosophical eye have contemplated the productions of Nature, have all, by common consent, divided them into three great classes, called the Animal, the Vegetable, and the Mineral or Fossil kingdoms. These terms are still in general use, and the most superficial observer must be struck with their propriety. Animals have an organized structure which regularly unfolds itself, and is nourished and supported by air and food; they consequently possess life, and are subject to death; they are moreover endowed with sensation, and with spontaneous, as well as voluntary, motion. Vegetables are organized, supported by air and food, endowed with life, and subject to death as well as animals. They have in some instances spontaneous, though we know not that they have voluntary motion. They are sensible to the action of nourishment, air, and light, and