The Day the Universe Changed - James Burke [161]
However, some time later it was shown that the cause of the rotation was not radiation pressure at all. The vanes spun because the light heated the small amount of gas present in the near-perfect vacuum; along the edges of the vanes, unequal heating would result in gas creep towards the hotter parts of the vane, where the gas would condense, causing a rise in pressure. It was this inequality of gas pressure which caused the vanes to spin. In response to theoretical expectations the radiometer produced the right results for the wrong reasons.
Galileo’s drawing of his telescopic view of the moon, showing the mountains that according to Aristotle were not supposed to exist.
Guericke experimenting with the sulphur ball.
Galileo used the same technique on a different occasion. In Venice, in 1609, he made his first telescopic observations and came to the heretical and dangerous conclusion that Copernicus had been right and that the earth did indeed circle the sun. Through the telescope, which he predicted would show him whether or not what Aristotle had said about the universe was true, he saw what he took to be evidence that the earth was not the centre of the solar system. The quality of image which the telescope provided was extremely poor, full of aberrations and distortions. Galileo drew pictures of what he saw: the satellites of Jupiter, the phases of Venus and the surface of the moon with its mountains and ‘seas’. Of all these, the satellites best proved his case. The pictures he had drawn of the moon seen through the telescope looked inaccurate even to the naked eye.
When Galileo showed his critics what the telescope had revealed, he did so in a specific way. He first showed them how it magnified distant objects such as carved lettering on a building, or ships at sea. These were familiar sights and the telescope did indeed show them more clearly. Then Galileo pointed his telescope at the sky, where the detail it would show was entirely unfamiliar. However, was it not evident to all that the telescope magnified objects? It was a brilliant non sequitur and Galileo’s opponents said so. But there was no terrestrial standard which could be used to judge what the telescope showed in the sky. Knowing this, Galileo took advantage of the prior acceptance of telescopic powers by those who had been prepared to look through the telescope and who were, therefore, already predisposed to his view. He concentrated their attention on the entirely incomparable satellites, and played down the image of the moon, where the inadequacy of the instrument was clearly visible and would have undermined his argument.
One example of how data were regarded as extraneous occurred in 1663, when Otto von Guericke became interested in the way some substances were attractive when rubbed. One such material was sulphur. Guericke moulded a sulphur ball and rubbed it as it spun. His intention was to further the investigations carried out earlier by William Gilbert, the English doctor whose work on magnets, published at the beginning of the century, had stimulated experimental studies of attractiveness. According to Gilbert the earth was a giant magnet holding everything to its surface by magnetic attraction. Johannes Kepler had shown that this form of attraction kept the planets in orbit round the sun. Magnetism, according to the physical structure of nature at the time, was the basic phenomenon holding everything together.
Using the sulphur ball as his instrument, Guericke measured its attractiveness in all environments and under all conditions. He noticed that besides exhibiting attraction while it was being rubbed, the sulphur ball also made a crackling noise and gave off a spark. But the instrument had been designed only to investigate magnetism. It could not, therefore, according to the experimental structure, provide significant data on any other phenomena outside Guericke’s investigations, so he ignored the sparks, mentioning them only briefly at the end of a lengthy work. It was fortunate that he did so, because his observation