Miss Tumble told the children about one of the first steam engines, which was invented by a man called Thomas Newcomen. In this engine the pressure of steam and the pressure of the atmosphere were both used to cause a beam, which was like a seesaw, to rock.

This engine would have been too complicated for school kids to understand straight away, so Miss Tumble drew diagrams on the blackboard to explain how it worked. Also, Miss Tumble used a bicycle pump to explain the idea of a piston that moved up and down within a cylinder. She held the bicycle pump upright and blew into the bottom end. This caused the handle to move upwards. Then she took the bicycle pump apart and showed the children the piston inside. After that the children all knew what a piston was. Miss Tumble was a very organized person. She had a box of baby wipes ready to clean her hands after touching the inside of the pump. Many of the kids had bicycles and they enjoyed seeing Miss Tumble blowing into the bicycle pump.

Freddie asked whether Newcomen’s engine had been used to operate bicycle pumps. Miss Tumble didn’t think so, but said that many early steam engines had been connected to water pumps, which were somewhat similar in construction to a bicycle pump.

Of course, the seesaw was the easiest part of Newcomen’s engine to understand, as all the children had had experience of seesaws. Newcomen’s engine contained a heavy wooden beam, like a seesaw. One end of the beam was attached to a heavy pump rod that could pump water if it moved up and down. The other end of the beam was attached to a piston in a cylinder. The cylinder was attached to the ground by a sturdy frame so that it would not move. The centre of the beam was supported on a tower so that it could rock like a seesaw.

There were three pipes attached to the lower part of the cylinder of Newcomen’s engine: one to allow steam in from a boiler, one to allow cold water to be sprayed into the cylinder and one to allow water to drain out of the cylinder. Each of these pipes had a valve, much like an ordinary tap, that could be opened or closed. Miss Tumble drew the cylinder and labelled all the valves to help the children to understand what the cylinder of Newcomen’s engine was like.

Miss Tumble asked the children to imagine that Thomas Newcomen was about to start up his steam engine. Eve wanted to know if he was wearing overalls. Miss Tumble wasn’t sure if they had overalls in those days. However, she felt sure he would not have worn his good clothes. Early steam engines were rather dirty because of the coal for the fire, the smoke and the grease that was used for lubrication. Rose wanted to know if he had had a beard. Miss Tumble thought not, although she wasn’t sure of that either. So that everyone would have the same picture in their minds, she asked the children to suppose that Newcomen was standing beside his steam engine dressed in blue overalls with no beard. He had already fired up the boiler and the steam was ready to flow.

The beam of the steam engine was tilted so that the pump rod, which was very heavy, was down as low as it could go and the piston in the cylinder was up as high as it could go. The steam inlet valve and the cooling water valve were both closed and the drain valve was open. The cylinder contained only air at atmospheric pressure.

Newcomen opened the steam valve a little and allowed steam to flow into the cylinder for a while. It flowed out through the drainpipe and carried away the air that had originally filled the cylinder. The steam also had the effect of heating up the whole cylinder.

Freddie said that his mum sometimes heated up the tea pot at home by holding it over the steam coming out of the spout of the kettle. Then she threw the small amount of water that formed by condensation into the sink before making the tea.

Miss Tumble said that Freddie was very observant. She was very pleased that he had recognized another example of condensation.

So, Thomas Newcomen had heated up the cylinder of his steam engine just like the way that Freddie’s mum heated up the teapot at home. There had been a little condensation at first, which drained out through the drain pipe. Once the cylinder was heated up there was no need to waste any more steam by letting it flow out directly. Newcomen closed the drain valve and then closed the steam inlet valve so that the steam was trapped in the cylinder. Next came the magical moment: Newcomen opened the cooling water valve and sprayed cold water into the cylinder.

What happened? Well, Freddie and a few of the other children could just about picture it already.

The cold water caused the steam to condense. There was now a vacuum in the cylinder and there was no longer any steam or air pushing the piston upwards. Yet the pressure of the atmosphere was still pushing downwards on the piston so it moved

down into the cylinder dragging its end of the seesaw downwards. This had the effect of lifting the pump rod at the other end. Newcomen’s engine was big and heavy and this movement occurred slowly, but powerfully. When the piston was as low as it could go in the cylinder and the pump rod was as high as it could go Newcomen opened the drain valve for just enough time to let the water drain out of the cylinder again. Then he opened the steam inlet valve. The steam entered the cylinder at atmospheric pressure and the piston moved up the cylinder again owing to the weight of the pump rod at the other end of the beam.

In order to make the beam of the steam engine rock just once, Newcomen had to open and close the valve that allowed the steam in; he had to open and close the valve for the cooling water spray; and he had to open and close the drain valve to allow the water out of the cylinder. It didn't take Newcomen too long to find clever ways of using the movement of the beam to operate the steam valve, the water spray valve and the drain valve automatically. That way the beam could continue to rock with a seesaw motion without the need for a man to operate the valves by hand.

When Miss Tumble asked the children for their reactions to Newcomen’s steam engine, most of them thought it had been quite a good idea for its time. Rose said it would be great to attach Newcomen’s engine to the seesaw in the local playground. That was a popular idea and so was Roger’s suggestion of attaching Hero’s engine to the roundabout. Miss Tumble said there would also have to be a fire and a boiler to produce the steam to drive the two engines. An adult would be needed to light the fire and then mind it so that it would not go out. Eve was brave enough to say that she did not like the idea of having steam and smoke from a fire in the playground. On reflection, most of the kids felt that they quite liked operating the roundabout and the seesaw themselves, unaided by any engine.