The Physics of War (12 page)

He had to make his ships larger so that they could carry more, and at the same time he had to arm them. His shipbuilders, however, warned him of the problem of stability with large, heavy cannons on the deck. The major problem was the ship's center of gravity, or center of mass. The center of mass is basically the balance point of the ship; in short, it's the point that has an equal mass in all directions around it. In two-dimensional objects the center of mass can easily be determined by finding the point where the rotational force in one direction (clockwise) around the point is equal to the rotational force in the opposite direction (counterclockwise). Although it is simple and relatively easy to find in two dimensions, determining it in three dimensions can be complicated, and a ship is, of course, a three-dimensional object.

Basically, if a ship is to remain as stable as possible in water, its center of mass must be as low as possible in the water. This means that most of the weight has to be below sea level. Putting heavy guns on the deck would raise the center of mass. The only solution was to put the guns below the deck. But how was this possible? Henry and his shipbuilders decided that holes would have to be built in the sides of the ship. When not in use they would have to be covered with waterproof covers. These holes became known as gun ports.

In addition, however, there was the problem of recoil. If the guns were large and heavy, and there were very many of them, the recoil could lead to overall instability of the ship. Henry decided to put the cannons on wheels and leave sufficient space behind them for the recoil.

Over the years Henry built many large gunships, but his favorite was the
Mary Rose
, named for his sister, Mary Tudor. It was built between 1509 and 1511. It was a six-hundred-ton ship, the second largest in his navy, and it was designed for close-up fighting, with fifteen large bronze guns, twenty-four
smaller iron guns, and fifty-two antipersonnel guns. She was designed to sail into the enemy with guns blazing, then turn broadside and shoot all her broadside guns at it, and finally maneuver around so she could shoot all her guns from the other side.

In July 1545, however, the
Mary Rose
led the English fleet into battle against an advancing French fleet. Faster than the rest of her fleet, she met the French with all her frontal guns blazing, and then turned so her broadside guns could be used. Suddenly, however, a gust of wind caught her while her lower gun ports were open, and water poured into them so quickly she sank and took most of her crew with her. In 1545, however, Henry went on to build another, even bigger, ship called the
Great Henry
.

But as large and powerful as his ships were, they still had a problem: safe navigation on open seas.

WILLIAM GILBERT

The compass was the major navigational instrument of ship navigators. Normally the compass needle pointed north at the Pole Star, Polaris, but at sea compasses were not reliable. Sometimes the needle pointed directly at Polaris, but at other times it deviated considerably from it, and there didn't appear to be a consistent reason for the deviation. What was wrong? The problem was presented to William Gilbert, a prominent physician in London.

Gilbert was born in 1544 and educated at St. John's College, Cambridge. He obtained his medical degree in 1569 and left to practice medicine in London. When the compass problem was presented to him he knew almost nothing about magnetism or physics, in general. Furthermore, very little about magnetism or the “amber effect,” as it was called, was understood at the time. Some idea of the understanding of magnetism at the time can be gleaned from the commonplace contemporary idea that assumed that garlic affected a magnetic field. Gilbert showed that it had no effect.
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When Gilbert started his work, which consisted of both research and experimentation, the basic properties of loadstone (magnetic iron ore) were generally known, the amber effect (amber could acquire a charge when rubbed) was known, and the compass had been used by navigators for years. He soon showed that the “electric” field associated with amber was not the same thing as the field associated with magnetism. He did this by showing that amber's electrical effect disappeared with heat whereas a magnetic field did not. We now know, of course, that a magnetic field does change when severe heat is applied.
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Gilbert also invented the versorium, which consisted of a freely supported metal needle and a round loadstone. Its needle moved in response to either an electrical or magnetic field, so it was similar to our present-day electroscope in which two small metal “sheets” repel one another when given the same electrical charge. As a result of his studies with the instrument, Gilbert suggested, correctly, that the earth was a giant magnet. And it was the field of the earth that influenced the compass. In particular, he showed that the polarity of the earth was similar to that of a loadstone magnet. Up to that time most people believed that Polaris somehow attracted the compass needle. He went on to argue that the center of the earth was composed of iron, and, of course, we now know that it is. Another important property of magnetism that he discovered was that if a magnet was cut in half, the two halves both have a north and a south pole. In other words, they form new and completely similar, smaller magnets.

The magnetic field of the earth, as seen by Gilbert.

It was also believed at the time that the stars were on a fixed sphere that rotated around the earth. Gilbert suggested that it was the earth that rotated, not the stars. Furthermore, he didn't believe the stars were on a fixed sphere. And indeed, it was true that the earth rotated, and it was now clear why navigators sometimes had problems with their compasses. As Gilbert pointed out, the axis of magnetic earth didn't line up perfectly with the axis of its spin. In effect, there was a true north and a magnetic north, and the compass always pointed at magnetic north while Polaris was only approximately lined up with true north. The biggest problem was that the deviation, or apparent distance between the two poles, was different at different points on the earth. There was soon hope that tables could eventually be made to help navigators compensate for this. Several people worked on the problem, but early navigators were never able to use the tables effectively. However, at least they finally knew why compass needles acted up.

Gilbert gathered all his results into a book he titled
De Magnete
, which he published in 1600. For many years it was a standard work on electricity and magnetism. As a result, Gilbert became quite famous: he was elected president of the Royal College of Physicians, and in 1601 he became personal physician to Queen Elizabeth I. Much of the terminology now used in electricity and magnetism is, in fact, due to Gilbert: electricity, electrical force, electrical attraction, magnetic poles. Because of this, Gilbert is sometimes referred to as the father of electricity and magnetism.

We now know that if amber is rubbed with fur it generates what we call a negative charge; similarly, when a glass rod is rubbed with a silk cloth it accumulates a positive charge. Electrical field lines are assumed to point away from a positive charge and toward a negative charge. Also, there is a force between two charges: a repulsive force between like charges and an attractive force between dissimilar charges. And just as an electrical charge is surrounded by an electric field, a magnet is surrounded by a magnetic field, with the field lines assumed to be pointing out of the north pole and into the south pole. Again, similar poles repel and opposite poles attract.

Few, if any, weapons of war were devised using Gilbert's discoveries in the years soon after he made them, but we now know that his work is the basis of all our knowledge of electromagnetism, including electrical currents and circuits, and, as a result, most of the devices in our modern world are based on Gilbert's work. So Gilbert's discoveries eventually made a large contribution to warfare.

THE PROBLEM OF LONGITUDE

Gilbert had set the stage in showing that there were two norths: true north and magnetic north. And for years, scientists, astronomers, and others worked to see if this difference could be used to make navigation at sea safer. The English astronomer Edward Halley took several voyages to study the magnetic variance between the two. He even developed maps as the basis of a new method, but the problems seemed insurmountable.

For navigation, knowledge of both latitude and longitude was needed. The astronomer Eratosthenes had proposed many years earlier (3 BCE) that positions on the surface of the earth could be located by a grid of crossed lines similar to our present lines of latitude and longitude. Hipparchus took the idea a step further in 2 BCE; indeed, he even proposed that one of the series of lines (longitude) should be associated with time.

Lines of latitude and longitude.

Latitude was relatively easy to determine, both on land and at sea. It could be found during the day by measuring the altitude of the sun above the horizon at noon and comparing it to a prepared table. The problem was longitude; it was relatively easy to measure on land, but it was difficult to determine at sea, mostly because time was involved. At this time clocks depended on a pendulum, which worked great on land, but pendulums were very unreliable when set on the rocking and rolling deck of a ship.
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Because of this problem, ship captains would frequently ignore longitude and sail to the latitude of their destination, then follow it the rest of the way to their destination. But this was time consuming because it was not the shortest route, and it could be dangerous. Many shipwrecks resulted because of it. The problem became so serious that a prize of millions of dollars in today's money was offered in England to anyone who could come up with a solution. It was a great incentive. Other countries, including France, Spain, and Holland, soon offered similar prizes.

It was known at the time that the earth rotated three hundred sixty degrees on its axis in a day, or fifteen degrees an hour. At the equator this amounted to sixty miles every four minutes, and because of this, noon occurred four minutes later for every sixty miles of travel west. This told navigators that they could determine their distance from their home port if they knew the exact time at home and the exact time aboard the ship so that they could compare the two. Tables of latitude and longitude would have to be used in conjunction with this method, and they had already been set up. Determining the difference between the two clocks accurately, however, was not easy because the clock aboard the ship was so inaccurate. A better method was needed for determining the time on the ship and also the time of a distant reference point while on the ship.

Navigators eventually turned to astronomers. The moon and stars were, in a sense, very accurate clocks—particularly the moon as it moved past the stars. Its movement had been tracked very accurately for years, and the times at which it would eclipse various stars were well known. Because of the huge prize, observatories were set up in both England and France. The British Royal Observatory was located at Greenwich, and the French observatory was located at Paris, and soon a serious problem developed. Each of the observatories used their own zero meridian (origin of latitude).

The moon's apparent flight through the stars seemed to be an excellent clock. The moon moved 360° around the sky roughly every 27.3 solar days, or 13 degrees per day. Halley proposed using a telescope to observe the time when the moon occulted (eclipsed) various stars in its flight. He even prepared elaborate tables, but the method didn't work out well because few bright stars were in the path of the moon.

Other astronomers tried other methods. Even Galileo had suggested that the orbital period of Jupiter's four bright moons could be used as a clock. But they were difficult to identify for an observer at sea, particularly as the sea rolled beneath the observer.

In the end, the best solution was an accurate clock for the ship, and it came when the British clockmaker, John Harrison, realize that pendulums could not be used for clocks at sea. He devised a spring-driven clock, and it worked beautifully.