A long time ago inventors and people like that had great fun with horses and hemispheres and making new discoveries. Miss Tumble said they were just ordinary people, like the children and herself, who thought about the world around them. It’s a pity that Freddie and Eve didn’t have horses and hemispheres in their classroom. Steam and electric kettles were all very well, but horses and hemispheres would have been mega fun!

Miss Tumble drew a large circle in the middle of the blackboard and divided it in two with a vertical line. She said the diagram represented a sphere divided into two hemispheres. “Who could write hemisphere on the blackboard?” she asked. Coleman and Eve were the first two to put up their hands. Miss Tumble gave each of them a piece of chalk and asked them to write hemisphere on opposite sides of the diagram representing the sphere. Eve had hemisphere written first. Coleman needed a little reminder from Miss Tumble that there was an ‘e’ at the end. Miss Tumble gave them each a prize of a lollipop, which was in the shape of a sphere. Freddie thought this was most unfair, as hemisphere had only ten letters in it. He said “that’s not fair” to Coleman when Coleman returned to his seat. Coleman didn’t mind. He was perfectly happy with his spherical lollipop. He knew it would last a long time if he sucked it slowly. Eve was delighted with her prize too, even though she didn’t care much for lollipops.

Long ago there was a man who lived in Magdeburg who had horses, ropes and plenty of helpers. He also happened to have a sphere made up of two hemispheres that fitted together. The hemispheres came apart very easily. The man's name was Otto von Guericke. He invented a pump that was able to pump the air out of the sphere when the two hemispheres had been placed together. The seal between the hemispheres was good enough to prevent any air from leaking back in from the surroundings. Otto von Guericke was able to show that when the air was removed from within the sphere the hemispheres remained tightly pressed together. He attached a strong rope to each hemisphere and arranged for horses to pull the ropes in an attempt to separate the hemispheres.  All the people who came to watch were very impressed when the horses were unable to pull the hemispheres apart. As soon as von Guericke allowed air into the sphere again, the hemispheres separated. The audience thought von Guericke was wonderful. News of the experiment spread and it gave many people an appreciation of the great force with which the air of the atmosphere pressed on things.

The hemispheres were known as Magdeburg hemispheres, after the town that von Guericke came from.

Miss Tumble explained that when the air had been pumped out of the sphere there was no longer anything pushing the hemispheres apart from the inside, yet on the outside the air of the atmosphere was still pushing the hemispheres together, so they stayed together. It would have taken more horses to separate them by pulling. However, once there was air pushing equally from inside and from outside, the hemispheres separated easily.

Some of the children did not understand what pressure was. Miss Tumble said that pressure was the effect of a gas (such as air or steam) or a liquid (such as water or mercury) pressing on something. A solid like rubber could also exert pressure, as when the sole of someone’s shoe pressed on the ground.

It wasn’t always necessary to have equipment in order to carry out an experiment. Miss Tumble asked all the children to keep their mouths closed and blow into their cheeks. They all laughed when she showed them how because it made her face look very fat and rounded. The children all enjoyed puffing out their cheeks. Miss Tumble said their cheeks moved out because the pressure of the air inside their mouths was greater than the pressure of the air outside. The air inside was pressing harder than the air outside.

Next Miss Tumble asked the children to keep their mouths closed, allow their chins to move downwards a little and then suck in their cheeks. They all did this after Miss Tumble had shown them how. In this case the pressure of the air inside their mouths was lower than the pressure of the air outside, so their cheeks moved inwards.

Miss Tumble asked the children if they knew what a vacuum was. They all knew what a vacuum cleaner was: a machine for cleaning the house by sucking up dirt. Miss Tumble said that a vacuum cleaner worked by creating a partial vacuum. A powerful pump inside it pumped air away very fast. The pressure inside the vacuum cleaner was then lower than the pressure of the surrounding air. Because of this the atmosphere pushed air into the suction tube of the vacuum cleaner. This air rushing into the tube was like a strong wind that carried any loose dust or dirt into the vacuum cleaner.

Freddie asked why the vacuum cleaner only created a partial vacuum. Miss Tumble said that in a perfect vacuum there would be no gas whatsoever present within a given space. If there was no gas at all present there would be no pressure, but if there was some gas present then it would be able to exert some pressure. The pump of a vacuum cleaner was not able to remove all the air from within it. Therefore, the space where the pressure had been reduced was said to be under a partial vacuum. When Otto von Guericke carried out his famous Magdeburg hemispheres experiment there would have been a partial vacuum inside the sphere, rather than a perfect vacuum.

Miss Tumble said the pressure of the atmosphere was due to the weight of all the air above us. As another story, Miss Tumble told the class that a man called Torricelli had invented a device called a barometer that could measure how hard the air of the atmosphere pressed on things. In the barometer the weight of some liquid in a tube was balanced against the weight of the air in the atmosphere. The pressure of the atmosphere was enough to support a height of 10 metres of water – the height of a two-storey house. Miss Tumble said that Torricelli could have used water in his barometer, but that it would then have been too tall to be practical. Mercury was a very heavy liquid metal and the pressure of the atmosphere was only able to support mercury in a tube to a height of about 760 millimetres. It was a very clever idea of Torricelli’s to use mercury in his barometer, because it made the barometer small enough for practical use.

It had taken a lot of concentration on the part of the children to try to understand Torricelli’s barometer and it was still not easy for them to see how the pressure of the atmosphere could support the liquid mercury in the tube of the barometer. They were very pleased when Miss Tumble said they could all do an experiment that would help them to understand this. She had a large basin of water and a lemonade bottle on her table. Each child had a chance to submerge the bottle in the water and allow all the air to escape from it. They then turned the bottle so that it was upside-down with the neck opening still below the water surface. They found that as long as the opening of the bottle was under the water, no air could enter it and it remained full.

Miss Tumble explained that the pressure of the atmosphere was supporting the weight of the water inside the bottle. She told the children that if they had a bottle that was more than ten metres tall a vacuum would be created above the water in the bottle, as the pressure of the atmosphere was only sufficient to support about 10 metres of water. The children were fascinated at the thought of a lemonade bottle more than ten metres tall. They wished they had one to check out what Miss Tumble had said. Except for Jennifer: she just wished she had one full of lemonade – she was very fond of lemonade.

Damien whispered to Janice that a Magdeburg hemisphere was like a fat belly. Janice said it was like Torricelli’s jelly belly, but she was laughing so much that she forgot to whisper very softly. Miss Tumble heard what Janice said, but didn’t really get the joke. She had to get quite cross before everyone would stop laughing. She put Damien and Janice standing on opposite sides of the classroom for ten minutes. Miss Tumble must have forgotten about that when she promised the class that they would be going on an outing to the Steam Museum at Straffan.