agreeable odor accompanying it. It has even been asserted that an acrid, sticky substance was deposited by it.

This fog caused great alarm all over the world, especially in the cities of Paris, Stockholm, Geneva, Rome, and Copenhagen, where it continued for a month or longer. During its prevalence there were often thunderstorms and powerful lightning flashes. Another circumstance that caused it to differ from ordinary dry fogs was the fact that, during its prevalence, a phosphoric gleam appeared in the air of sufficient brightness to permit the smallest print to be read at midnight.

In the opinion of some, a poisonous substance accompanied the fog; for a severe epidemic of catarrh followed. This, however, might have been caused by the breathing of any ordinary dust particles.

A similar dry fog, possibly also due to the presence of some extremely tenuous material from realms of space far outside atmosphere, was the fog of 1831. This fog spread over the entire Northern Hemisphere, and was attended by the same phosphoric light as the preceding fog. It gave to the sun's disc a curious blue or green color.

Besides dry fogs due to the presence in the atmosphere of matter that comes from outside the earth itself, are the dry fogs that can be directly traced to fine dust particles, generally of mineral origin, in the air. For example, during the great volcanic eruption of Krakatoa, in 1883, an eruption noted for its great power, volcanic dust was shot with such force from the crater that it is said to have reached many, many miles above the level of the sea. This height was so great that, although the coarser particles soon fell to the earth, the finer particles, carried by the wind, were spread over the entire earth, and, remaining suspended in the air for a year or longer, produced the remarkable sunsets that created so great an excitement over the whole world, especially among scientists.

I imagine that many, if not all of my readers who live in

the temperate zone of the United States, know that beautiful time of the year called Indian summer, that occurs about the end of October and early in November. The air is then filled with dust particles, probably due to the presence of carbon obtained from the burning of forest or prairie regions in certain parts of the country. I do not mean that Indian summer is so pleasant because of the hazy condition of the atmosphere, but rather in spite of this hazy condition of the atmosphere.

The dry fog, known in Germany under the name of the Höhrauch (high smoke), is of a somewhat similar nature. The times of the year when it occurs, however, lack the pleasant climatic conditions that characterize Indian summer in North America; for, in Germany, it is regarded as a disagreeable phenomenon. The sky assumes a grayish tinge, which, at a certain height above the horizon, is attended by a brownish smoky haze. At the same time a peculiar smell of smoke makes the atmosphere as disagreeable to the sense of smell as its gray color is to the eye.

The cause of the Höhrauch is now generally thought to be the burning of peat areas in the heath districts of Northern Germany. In order, if possible, to win a somewhat larger harvest from the sterile peaty soil, the peat is set on fire in the spring. Since, as has been estimated, at least 60,000 acres are fired every year, consuming about 2,000,000,000 pounds of vegetable matter, you may form some idea of the amount of carbon particles thus thrown into the air, and, therefore, why the air acquires its disagreeable smoky color and odor.

CHAPTER XXII

NAVIGATION OF THE ATMOSPHERE

The desire to fly has existed among mankind from the earliest times. The gods, genii, and wizards of the ancient world were credited with the power of readily making their way through the air. Indeed, it was not so very long ago that, even in such a highly cultured centre as Boston, fairly intelligent people credited witches with the power of flying through the upper regions of the air with a no more complicated device than their far-famed broomsticks.

Some very marvellous stories have been told about people flying, or being carried through the air. As early as 625 B. C., it is said that a voyage through the air was actually accomplished by Kai Kaoos, King of Persia, the Cyaxares of the Greeks. An account of this aërial flight, taken from Persian and other Oriental tales found in a manuscript in the British Museum, is given by Bishop Wilkins of Chester as follows:

"To the king it became a matter of great concern how he might be enabled to ascend the heavens without wings; and for that purpose he consulted the astrologers, who presently suggested a way in which his desires might be successfully accomplished.

"They contrived to rob an eagle's nest of its young, which they reared with great care, supplying them with invigorating food.

"A frame of aloes-wood was then prepared, and at each of the four corners was fixed, perpendicularly, a javelin, surmounted on the points with the flesh of a goat. At each corner again one of the eagles was bound, and in the middle the king was seated with a goblet of wine before him. As

soon as the eagles became hungry they endeavoured to get at the goat's flesh upon the javelins, and by flapping their wings, and flying upwards they quickly raised the throne from the ground. Hunger still pressing on them, and still being distant from their prey, they ascended higher and higher in the clouds, conveying the astonished king far beyond his own country. But, after a long and fruitless exertion, their strength failed them, and, unable to keep their way, the whole fabric came tumbling down from the sky, and fell upon a dreary solitude in the kingdom of Chinawhere Kai Kaoos was left a prey to hunger, alone, and in utter despair."

No mention is made in this very improbable story as to why Kai Kaoos was not instantly killed.

The tale is almost rivalled by the story told in “Arabian Nights" of what happened to Sinbad the Sailor during his second voyage, in which he claimed to have been carried through the air by attaching himself to the foot of an immense bird called a roc.

It is stated by several writers that, some time about 365 B. C., Archytas of Tarentum invented a wonderful automaton in the shape of a pigeon that possessed the power of flying through the air.

But enough of these foolish stories. Let me now tell you something about what makes a balloon rise in the air. If you take a piece of cork in your hand and plunge it below the surface of water in a tub, and then open your hand, the cork will be pushed up through the water until it reaches the surface. Indeed, it will be pushed up so far that most of it will be raised above the level of the water.

Now, a balloon rises through the air for exactly the same reason that the cork rises through the water. When any solid body such as a cork is immersed in water, it displaces a volume of water exactly equal to its own volume. In doing this it loses a weight equal to the weight of the water it displaces. Since water is so much denser than cork, the

weight thus lost by the cork, by reason of the water it displaces, is much greater than its own weight. In other words, the cork loses an amount of weight greater than its own, so that while in the water it may properly be said to weigh less than nothing.

It may seem strange for a cork or any other thing to weigh less than nothing, but I believe I can explain it to you. The weight of a body is due to the force of gravity pulling it downwards towards the earth's centre. What is generally called the buoyancy of water is due to a force pushing the cork directly upwards, or away from the earth's centre. When, therefore, I speak of the cork weighing less than nothing, I simply mean that the buoyancy, or upward pressure caused by the displacement of the water, is greater than the weight, or the downward pull caused by the attraction of the earth. The cork in the water is in a position similar to that of the rope employed in the tug-of-war. The side representing the weight of the cork is less strong than the side represented by the buoyancy of the water. There is, therefore, only one thing that can happen. The cork is pushed upward and rises rapidly through the water and, reaching the surface, is pushed out of the water into the air until it displaces an amount of water exactly equal to its own weight, when, the two forces being equal, equilibrium results and the cork quietly floats on the surface.

This power which liquids possess of buoying up bodies immersed in them was discovered by Archimedes, of Syracuse, one of the most learned of the early philosophers.

Archimedes had been requested by his royal master to find out whether a golden crown that had been made for him by his goldsmiths really contained all the gold he had given them, or whether they had mixed with it a quantity of the baser metals. The king imposed this condition, however, on Archimedes, that, in making this investigation, the crown was not to be destroyed.

This happened in 230 B. C. The problem gave Archimedes