Galileo Galilei is famous for an experiment he did not carry out – but it was a real experiment, inspired by his work. He was Professor of Mathematics in Padua from 1592 until 1610, and during that time he worked in mechanics and astronomy as well as mathematics.
To put Galileo’s achievements in perspective, at the time he was working there were many people – educated people – who thought that a bullet fired horizontally from a gun, or a ball fired from a cannon, would fly a certain distance in a straight line, then stop and drop vertically to the ground. It was Galileo who first appreciated that the trajectory followed when a bullet is fired from a gun, or when an object such as a ball is thrown up in the air, is a parabola – and he carried out tests to prove this.
© New York Public Library/Science Photo Library
Nineteenth century illustration showing a glamourised version of Galileo’s experiment rolling balls down inclined planes. Although the scene depicted is fictional, Galileo really did carry out such experiments.
Among the many experiments he carried out in the years around the turn of the century there was a series of studies in which he rolled balls with different weights down inclined planes. He timed how quickly the balls moved using his pulse, and reached two important conclusions. The first was that the ‘natural’ state of a ball rolling off the slope was to continue horizontally (literally ‘towards the horizon’), unless it was stopped by friction. Without friction, he reasoned, the ball would roll on forever. This was an early insight into what Isaac Newton, following Robert Hooke, developed as his ‘First Law’ of mechanics, that an object stays at rest or moves in a straight line at a steady speed unless it is acted upon by an outside force.
Galileo’s second discovery was that the speed with which the balls rolled down the slope did not depend on their weight. For any particular slope, it took the same amount of time for any of the balls to get from the top to the bottom. This applied no matter how steep he made the slope. So he concluded – without actually dropping things vertically – that, apart from the effects of wind resistance, all falling objects would accelerate downwards at the same rate.
This infuriated some of his colleagues, philosophers of the old school who believed that Aristotle, who said that heavy objects fall faster than light objects, could not be wrong. So, in 1612, two years after Galileo moved from Padua to Pisa, one of them really did drop two weights from the leaning tower in a public demonstration intended to prove that Aristotle was right. The balls hit the ground very nearly at the same time, but not exactly. The Aristotelians said that this proved Galileo was wrong. But Galileo had an answer: ‘Aristotle says that a hundred-pound ball falling from a height of one hundred cubits hits the ground before a one-pound ball has fallen one cubit. I say they arrive at the same time. You find, on making the test, that the larger ball beats the smaller one by two inches. Now, behind those two inches you want to hide Aristotle’s ninety-nine cubits and, speaking only of my tiny error, remain silent about his enormous mistake.’4
Among other things, this true story highlights the power of the experimental method. If you carry out an experiment honestly, it will tell you the truth, regardless of what you want it to tell you. The Aristotelians wanted to prove Galileo was wrong, but the experiment proved he was right – within, as we would now say, the limits of possible experimental error.
By 1612 Galileo was nearly 50, and his days as an experimental physicist were essentially over. His famous clash with the Church authorities in Rome did not take place until the 1630s, and led to his spending his final years, from 1634, under house arrest at his own home (a relatively lenient sentence considering that he had been forced to confess to heresy). There, he summed up his life’s work on mechanics and promoted the scientific method pioneered by Gilbert in a great book, Discourses and Mathematical Demonstrations Concerning Two New Sciences, usually known as Two New Sciences, published in 1638 in Holland. The book was enormously influential, the first real scientific textbook, and an inspiration to scientists across Europe – except, of course, in Catholic Italy, where it was banned. As a direct result, from being a leading light in the scientific renaissance, Italy became a backwater, while the real progress was made elsewhere.
| No. 7 | CIRCULATION OF THE BLOOD |
Even after the publication of the Fabrica (see here), in the second half of the sixteenth century and early in the seventeenth century there was still strong opposition to the idea that classical teachers such as Galen could be wrong. So, although the English physician William Harvey was born in 1578 and studied the circulation of the blood in the early decades of the seventeenth century, he did not publish his discoveries until 1628, by which time he had gathered an overwhelming weight of evidence to support his ideas. (He did, however, give lectures on his work in 1616.) The result was a book, De Motu Cordus et Sanguinis in Animalibus (On the Motion of the Heart and Blood in Animals, known as De Motu), which presented an open-and-shut case based on a series of genuinely scientific experiments carried out over the previous two decades. All this was done in Harvey’s spare time as a successful physician who had studied in Cambridge and Padua and, like William Gilbert, became (in 1618) one of the Court Physicians to James I, and later personal physician (a much more important post) to Charles I. Both Williams were contemporaries of another William, Shakespeare, who died in 1616.
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Title page from William Harvey’s De Motu.
Before Gilbert, following the teaching of Galen, it was thought that veins and arteries carried two different kinds of blood. One kind, supposedly manufactured in the liver, was thought to be carried through the veins to nourish the tissues of the body, getting used up in the process and being replaced by new blood from the liver. The other kind of blood was thought to be carried in the arteries, conveying a mysterious ‘vital spirit’ from the lungs to the tissues of the body.
As with Gilbert (see here), the way in which Harvey worked and presented his results was as important as the discoveries themselves. He did not base his ideas on abstract philosophising, but on direct measurements and observations. The key insight came when he measured the capacity of the heart and, by taking a typical pulse rate, worked out how much blood it was pumping each minute. He found that, in modern units, a human heart pumps about 60 cubic centimetres with each beat, adding up to nearly 260 litres in an hour. That much blood would weigh three times as much as a human, so it was clearly impossible that it was all being manufactured in the liver (or anywhere else) every hour. The only alternative was that there was a lot less blood, and that it was continuously circulating around the body, out from the heart through the arteries and back through the veins. An equivalent system circulates blood between the lungs and the heart, carrying not ‘vital spirit’ but oxygen. All this was born out by Harvey’s observations of the tiny valves in veins (discovered by one of Harvey’s teachers in Padua, Hieronymous Fabricius) which allow venous blood to flow towards the heart, but not away from it.
Having reached this conclusion by observation, Harvey then established his case with a series of experiments, one of which stands out for its simplicity and clarity. If he was right, there must be a connection between arteries and veins. As arteries lie deeper below the surface of the skin than veins, he tested this by tying a cord (ligature) around his own arm, tight enough for it to cut off the flow of blood in his veins, but not in his arteries. As blood continued it flow from the arteries into the blocked-off veins, the veins behind the ligature swelled up dramatically. He also pointed out that arteries near the heart are thicker than those further away from it, because they have to be strong in order to cope with the