Читать книгу Science: A History in 100 Experiments - John Gribbin - Страница 13
Оглавление| No. 8 | WEIGHING THE ATMOSPHERE |
In the early 1640s, the Italian Evangelista Torricelli investigated the problem that water could not be pumped up (by a suction pump) from a well more than about 30 feet (roughly 9 metres) deep. The way these pumps work is similar to the way it is possible to suck water into a bicycle pump if the open end is placed below the surface of the water. If you had a very long bicycle pump standing upright in a swimming pool, you would be able to suck water up to just over 30 feet, but no further, no matter how hard you pulled on the handle. Torricelli reasoned that the weight of the air pressing down on the surface of the water in the well could push the air in a pipe up this far, but no further. So he set out to test the idea using a denser liquid, mercury, instead of water. Mercury is roughly fourteen times denser than water, so Torricelli worked out that a column of water 30 feet high must exert the same pressure at its base as a column of mercury a bit more than 2 feet (more than 60 centimetres) high. He found that if a glass tube sealed at one end and full of mercury was stood upright with its open end in a dish of mercury, the level of the mercury in the tube would fall to 30 inches (76 centimetres), leaving a gap above the top of the mercury in the tube, and matching his calculation. The gap contained nothing at all, and became known as the Torricelli Vacuum.
Torricelli noticed that the exact height of the column of mercury in his tubes changed from day to day, and he realized that this was because the pressure of the atmosphere weighing down on the mercury in the dish was changing. He had invented the barometer. Torricelli died in 1647, but his discoveries were taken up and developed by the Frenchman Blaise Pascal, who studied the way the pressure of the air, measured by this kind of early barometer, varied with the weather. Another Frenchman, René Descartes, visited Pascal in 1647, and suggested that it would be interesting to take a barometer up a mountain, to find out how the pressure of the air changed with altitude. Pascal lived in Paris, but his brother-in-law Florin Périer lived near a mountain, the Puy-de-Dôme, and in 1648 Pascal persuaded him to do the experiment. Périer write to Pascal to describe what happened:
© Science Source/Science Photo Library
Imaginative illustration of Florin Périer measuring the air pressure as he ascends the Puy-de-Dôme, a volcanic mountain in France with a height of 1464 metres.
‘The weather was chancy last Saturday … [but] around five o’clock that morning … the Puy-de-Dôme was visible … I decided to give it a try. Several important people of the city of Clermont had asked me to let them know when I would make the ascent … I was delighted to have them with me in this great work.
… at eight o’clock we met in the gardens of the Minim Fathers [monastery], which has the lowest elevation in town … First I poured 16 pounds of quicksilver … into a vessel … then took several glass tubes … each four feet long and hermetically sealed at one end and opened at the other … then placed them in the vessel … the quick silver stood at 26" and 3½ lines above the quicksilver in the vessel … I repeated the experiment two more times while standing in the same spot … [it] produced the same result each time …
I attached one of the tubes to the vessel and marked the height of the quicksilver and … asked Father Chastin, one of the Minim Brothers … to watch if any changes should occur through the day … Taking the other tube and a portion of the quick silver … I walked to the top of Puy-de-Dôme, about 500 fathoms higher than the monastery, where upon experiment … found that the quicksilver reached a height of only 23" and 2 lines … I repeated the experiment five times with care … each at different points on the summit … found the same height of quicksilver … in each case …’5
Equally importantly, the priest at the bottom of the mountain reported that the reading on his barometer had not changed during the day. There was less weight of air pressing down at the top of the mountain than at the bottom. So the experiment revealed that the atmosphere gets thinner as you go higher, and suggests that if you go high enough it will thin out entirely, with a vacuum above it, like the vacuum above the mercury in Torricelli’s tubes. Pascal then carried out a mini-version of the experiment by carrying a barometer up about 50 metres to the top of the bell tower at the church of Saint-Jacques-de-la-Boucherie. The mercury dropped by two ‘lines’. Many people, including Descartes, refused to accept Pascal’s interpretation of the evidence, and insisted that there must be some invisible substance filling the ‘empty’ space in the tubes and (presumably) the space above the atmosphere. But further experiments eventually proved that Pascal was right (see here).
