If you take a blown-up bladder under water and let go of it, it will fly up to the surface of the water and will swim on it. Just so, when water is boiled in a pot, it becomes light at the bottom, over the fire, – it is turned into a gas; and when a little of that water-gas is collected it goes up as a bubble. First comes up one bubble, then another, and when the whole water is heated, the bubbles come up without stopping. Then the water boils.

Just as the bubbles leap to the surface, full of vapoury water, because they are lighter than water, just so will a bladder which is filled with hydrogen, or with hot air, rise, because hot air is lighter than cold air, and hydrogen is lighter than any other gases.

Balloons are made with hydrogen or with hot air. With hydrogen they are made as follows: They make a large bladder, attach it by ropes to posts, and fill it with hydrogen. The moment the ropes are untied, the balloon flies up in the air, and keeps flying up until it gets beyond the air which is heavier than hydrogen. When it gets up into the light air, it begins to swim in it like a bladder on the surface of the water.

With hot air balloons are made like this: They make a large empty ball, with a neck below, like an upturned pitcher, and to the mouth of it they attach a bunch of cotton, and that cotton is soaked with spirits, and lighted. The fire heats the air in the balloon, and makes it lighter than the cold air, and the balloon is drawn upward, like the bladder in the water. And the balloon will fly up until it comes to the air which is lighter than the hot air in the balloon.

Nearly one hundred years ago two Frenchmen, the brothers Montgolfier, invented the air balloons. They made a balloon of canvas and paper and filled it with hot air, – the balloon flew. Then they made another, a larger balloon, and tied under the balloon a sheep, a cock, and a duck, and let it off. The balloon rose and came down safely. Then they attached a little basket under the balloon, and a man seated himself in it. The balloon flew so high that it disappeared from view; it flew away, and came down safely. Then they thought of filling a balloon with hydrogen, and began to fly higher and faster.

In order to fly with a balloon, they attach a basket under the balloon, and in this basket two, three, and even eight persons are seated, and they take with them food and drink.

In order to rise and come down as one pleases, there is a valve in the balloon, and the man who is flying with it can pull a rope and open or close the valve. If the balloon rises too high, and the man who is flying wants to come down, he opens the valve, – the gas escapes, the balloon is compressed, and begins to come down. Then there are always bags with sand in the balloon. When a bag with sand is thrown out, the balloon gets lighter, and it flies up. If the one who is flying wants to get down, but sees that it is not what he wants below him, – either a river or a forest, – he throws out the sand from the bags, and the balloon grows lighter and rises again.

GALVANISM

There was once a learned Italian, Galvani. He had an electric machine, and he showed his students what electricity was. He rubbed the glass hard with silk with something smeared over it, and then he approached to the glass a brass knob which was attached to the glass, and a spark flew across from the glass to the brass knob. He explained to them that the same kind of a spark came from sealing-wax and amber. He showed them that feathers and bits of paper were now attracted, and now repelled, by electricity, and explained to them the reason of it. He did all kinds of experiments with electricity, and showed them all to his students.

Once his wife grew ill. He called a doctor and asked him how to cure her. The doctor told him to prepare a frog soup for her. Galvani gave order to have edible frogs caught. They caught them for him, killed them, and left them on his table.

Before the cook came after the frogs, Galvani kept on showing the electric machine to his students, and sending sparks through it.

Suddenly he saw the dead frogs jerk their legs on the table. He watched them, and saw that every time when he sent a spark through the machine, the frogs jerked their legs. Galvani collected more frogs, and began to experiment with them. And every time he sent a spark through the machine, the dead frogs moved their legs as though they were alive.

It occurred to Galvani that live frogs moved their legs because electricity passed through them. Galvani knew that there was electricity in the air; that it was more noticeable in the amber and glass, but that it was also in the air, and that thunder and lightning came from the electricity in the air.

So he tried to discover whether the dead frogs would not move their legs from the electricity in the air. For this purpose he took the frogs, skinned them, chopped off their heads, and hung them on brass hooks on the roof, beneath an iron gutter. He thought that as soon as there should be a storm, and the air should be filled with electricity, it would pass by the brass rod to the frogs, and they would begin to move.

But the storm passed several times, and the frogs did not move. Galvani was just taking them down, and as he did so a frog's leg touched the iron gutter, and it jerked. Galvani took down the frogs and made the following experiment: he tied to the brass hook an iron wire, and touched the leg with the wire, and it jerked.

So Galvani decided that the animals lived because there was electricity in them, and that the electricity jumped from the brain to the flesh, and that made the animals move. Nobody had at that time tried this matter and they did not know any better, and so they all believed Galvani. But at that time another learned man, Volta, experimented in his own way, and proved to everybody that Galvani was mistaken. He tried touching the frog differently from what Galvani had done, not with a copper hook with an iron wire, but either with a copper hook and a copper wire, or an iron hook and an iron wire, – and the frogs did not move. The frogs moved only when Volta touched them with an iron wire that was connected with a copper wire.

Volta thought that the electricity was not in the dead frog but in the iron and copper. He experimented and found it to be so: whenever he brought together the iron and the copper, there was electricity; and this electricity made the dead frogs jerk their legs. Volta tried to produce electricity differently from what it had been produced before. Before that they used to get electricity by rubbing glass or sealing-wax. But Volta got electricity by uniting iron and copper. He tried to connect iron and copper and other metals, and by the mere combination of metals, silver, platinum, zinc, lead, iron, he produced electric sparks.

After Volta they tried to increase electricity by pouring all kinds of liquids – water and acids – between the metals. These liquids made the electricity more powerful, so that it was no longer necessary, as before, to rub in order to produce it; it is enough to put pieces of several metals in a bowl and fill it with a liquid, and there will be electricity in that bowl, and the sparks will come from the wires.

When this kind of electricity was discovered, people began to apply it: they invented a way of gold and silver plating by means of electricity, and electric light, and a way to transmit signs from place to place over a long distance by means of electricity.

For this purpose pieces of different metals are placed in jars, and liquids are poured into them. Electricity is collected in these jars, and is transferred by means of wires to the place where it is wanted, and from that place the wire is put into the ground. The electricity runs through the ground back to the jars, and rises from the earth by means of the other wire; thus the electricity keeps going around and around, as in a ring, – from the wire into the ground, and along the ground, and up the wire, and again through the earth. Electricity can travel in either direction, just as one wants to send it: it can first go along the wire and return through the earth, or first go through the earth, and then return through the wire. Above the wire, in the place where the signs are given, there is attached a magnetic hand, and that hand turns in one direction, when the electricity is allowed to pass through the wire and back through the earth, and in another direction, when the electricity is sent through the earth and back through the wire. Along this hand there are certain signs, and by means of these signs they write from one place to another on the telegraph.

THE SUN'S HEAT

Go out in the winter on a calm, frosty day into the field, or into the woods, and look about you and listen: all around you is snow, the rivers are frozen, dry grass blades stick out of the grass, the trees are bare, – nothing is moving.

Look in the summer: the rivers are running and rippling, in every puddle the frogs croak and plunge in; the birds fly from place to place, and whistle, and sing; the flies and the gnats whirl around and buzz; the trees and the grass grow and wave to and fro.

Freeze a pot with water, and it will become as hard as a rock. Put the frozen pot on the fire: the ice will begin to break, and melt, and move; the water will begin to stir, and bubbles will rise; then, when it begins to boil, it whirls about and makes a noise. The same happens in the world from the heat. Without heat everything is dead; with the heat everything moves and lives. If there is little heat, there is little motion; with more heat, there is more motion; with much heat, there is much motion; with very much heat, there is also very much motion.

Where does the heat in the world come from? The heat comes from the sun.

In winter the sun travels low, to one side, and its beams do not fall straight upon the earth, and nothing moves. The sun begins to travel higher above our heads, and begins to shine straight down upon the earth, and everything is warmed up in the world, and begins to stir.

The snow settles down; the ice begins to melt on the rivers; the water comes down from the mountains; the vapours rise from the water to the clouds, and rain begins to fall. Who does it all? – The sun. The seeds swell, and let out rootlets; the rootlets take hold of the ground; old roots send up new shoots, and the trees and the grass begin to grow. Who has done that? – The sun.

The bears and moles get up; the flies and bees awaken; the gnats are hatched, and the fish come out from their eggs, when it is warm. Who has done it all? – The sun.

The air gets warmed up in one place, and rises, and in its place comes colder air, – and there is a wind. Who has done that? – The sun.

The clouds rise and begin to gather and to scatter, – and the lightning flashes. Who has made that fire? – The sun.

The grass, the grain, the fruits, the trees grow up; animals find their food, men eat their fill, and gather food and fuel for the winter; they build themselves houses, railways, cities. Who has prepared it all? – The sun.

A man has built himself a house. What has he made it of? Of timbers. The timbers were cut out of trees, but the trees are made to grow by the sun.

The stove is heated with wood. Who has made the wood to grow? – The sun.

Man eats bread, or potatoes. Who has made them grow? – The sun. Man eats meat. Who has made the animals, the birds to grow? – The grass. But the grass is made to grow by the sun.

A man builds himself a house from brick and lime. The bricks and the lime are burnt by wood. The wood has been prepared by the sun.

Everything that men need, that is for their use, – all that is prepared by the sun, and on all that goes much sun's heat. The reason that men need bread is because the sun has produced it, and because there is much sun's heat in it. Bread warms him who eats it.

The reason that wood and logs are needed is because there is much heat in them. He who buys wood for the winter, buys sun's heat; and in the winter he burns the wood whenever he wants it, and lets the sun's heat into his room.

When there is heat, there is motion. No matter what motion it may be, – it all comes from heat, either directly from the sun's heat, or from the heat which the sun has prepared in the coal, the wood, the bread, and the grass.

Horses and oxen pull, men work, – who moves them? – Heat. Where does the heat come from? – From the food. And the food has been prepared by the sun.

Watermills and windmills turn around and grind. Who moves them? – Wind and water. And who drives the wind? – Heat. And who drives the water? – Again heat. Heat raises the water in the shape of vapour, and without this the water would not be falling down. A machine works, – it is moved by steam. And who makes steam? – Wood. And in the wood is the sun's heat.

Heat makes motion, and motion makes heat. And both heat and motion are from the sun.

STORIES FROM ZOOLOGY

THE OWL AND THE HARE

It was dusk. The owls began to fly through the forest to find some prey.

A large hare leaped out on a clearing and began to smooth out his fur. An old owl looked at the hare, and seated himself on a branch; but a young owl said to him:

"Why do you not catch the hare?"

The old owl said:

"He is too much for me: if I get caught in him, he will drag me into the woods."

But the young owl said:

"I will stick one claw into his body, and with the other I will clutch a tree."

The young owl made for the hare, and stuck one claw into his back so that all his talons entered the flesh, and the other claw it got ready to push into the tree. The hare yanked the owl, while the owl held on to the tree, and thought, "He will not get away." The hare darted forward and tore the owl. One claw was left in the tree, and the other in the hare's back.

The next year a hunter killed that hare, and wondered how the owl's talons had grown into the hare's back.

HOW THE WOLVES TEACH THEIR WHELPS

I was walking along the road, and heard a shout behind me. It was the shepherd boy who was shouting. He was running through the field, and pointing to something.

I looked, and saw two wolves running through the field: one was full-grown, and the other a whelp. The whelp was carrying a dead lamb on his shoulders, and holding on to one of its legs with its teeth. The old wolf was running behind. When I saw the wolves, I ran after them with the shepherd, and we began to shout. In response to our cries came peasants with dogs.

The moment the old wolf saw the dogs and the people, he ran up to the whelp, took the lamb away from him, threw it over his back, and both wolves ran as fast as they could, and disappeared from view.

Then the boy told what had happened: the large wolf had leaped out from the ravine, had seized the lamb, killed it, and carried it off.

The whelp ran up to him and grasped the lamb. The old wolf let the whelp carry the lamb, while he himself ran slowly beside him.

Only when there was danger, did the old wolf stop his teaching and himself take the lamb.

HARES AND WOLVES

The hares feed at night on tree bark; the field hares eat the winter rye and the grass, and the threshing-floor hares eat the grain in the granary. Through the night the hares make a deep, visible track through the snow. The hares are hunted by men, and dogs, and wolves, and foxes, and ravens, and eagles. If a hare walked straight ahead, he would be easily caught in the morning by his tracks; but God has made a hare timid, and his timidity saves him.

A hare goes at night fearlessly through the forests and fields, making straight tracks; but as soon as morning comes and his enemies wake up, and he hears the bark of dogs, or the squeak of sleighs, or the voice of peasants, or the crashing of a wolf through the forest, he begins to toss from side to side in his fear. He jumps forward, gets frightened at something, and runs back on his track. He hears something again, and he leaps at full speed to one side and runs away from his old track. Again something makes a noise, and the hare turns back, and again leaps to one side. When it is daylight, he lies down.

In the morning the hunters try to follow the hare tracks, and they get mixed up on the double tracks and long leaps, and marvel at the hare's cunning. But the hare did not mean to be cunning. He is merely afraid of everything.

THE SCENT

Man sees with his eyes, hears with his ears, smells with his nose, tastes with his mouth, and feels with his fingers. One man's eyes see better, another man's see worse. One hears from a distance, and another is deaf. One has keen senses and smells a thing from a distance, while another smells at a rotten egg and does not perceive it. One can tell a thing by the touch, and another cannot tell by touch what is wood and what paper. One will take a substance in his mouth and will find it sweet, while another will swallow it without making out whether it is bitter or sweet.

Just so the different senses differ in strength in the animals. But with all the animals the sense of smell is stronger than in man.

When a man wants to recognize a thing, he looks at it, listens to the noise that it makes, now and then smells at it, or tastes it; but, above all, a man has to feel a thing, to recognize it.

But nearly all animals more than anything else need to smell a thing. A horse, a wolf, a dog, a cow, a bear do not know a thing until they smell it.

When a horse is afraid of anything, it snorts, – it clears its nose so as to scent better, and does not stop being afraid until it has smelled the object well.

A dog frequently follows its master's track, but when it sees him, it does not recognize him and begins to bark, until it smells him and finds out that that which has looked so terrible is its master.

Oxen see other oxen stricken down, and hear them roar in the slaughter-house, but still do not understand what is going on. But an ox or a cow need only find a spot where there is ox blood, and smell it, and it will understand and will roar and strike with its feet, and cannot be driven off the spot.

An old man's wife had fallen ill; he went himself to milk the cow. The cow snorted, – she discovered that it was not her mistress, and would not give him any milk. The mistress told her husband to put on her fur coat and kerchief, – and the cow gave milk; but the old man threw open the coat, and the cow scented him, and stopped giving milk.

When hounds follow an animal's trail, they never run on the track itself, but to one side, about twenty paces from it. When an inexperienced hunter wants to show the dog the scent, and sticks its nose on the track, it will always jump to one side. The track itself smells so strong to the dog that it cannot make out on the track whether the animal has run ahead or backward. It runs to one side, and then only discovers in what direction the scent grows stronger, and so follows the animal. The dog does precisely what we do when somebody speaks very loud in our ears; we step a distance away, and only then do we make out what is being said. Or, if anything we are looking at is too close, we step back and only then make i