The Home of Mankind Read online

  Genghis Khan, a little slant-eyed Mongolian who during the first half of the thirteenth century ruled an empire slightly larger than any other that ever existed (it reached from the Yellow Sea to the Baltic and maintained itself in Russia until 1480), seems to have carried some sort of compass with him when he crossed the vast central Asiatic deserts. But it is impossible to say when the sailors of the Mediterranean had their first glimpse of this “blasphemous invention of the Devil,” as the Church people called it, which soon afterwards was to carry their vessels to the ends of the earth.

  Inventions of that sort, which are of world-wide importance, all seem to start in the same vague way. Some one returning from Jaffa or Famagusta probably brought a compass with him which he had bought from a merchant in Persia, who had told him that he had got it from some one who had just returned from India or China. The rumour spread through the ale-houses of the ports. Others wanted to see the funny little needle that had been bewitched by Satan and that would tell you where the north was, no matter where you happened to be. Of course, they did not believe such a thing could be true. Nevertheless, they asked their friend to bring them one too the next time he came back from the East. They even gave him the money, and six months later they had a little compass of their own. The darned thing worked! Then everybody must have a compass. Merchants in Damascus and Smyrna received urgent calls for more compasses. Instrument-makers in Venice and Genoa began to manufacture compasses on their own account. Suddenly we hear of compasses in every part of Europe. And within a few years the little glass-covered metal box had become such a commonplace sight that no one thought it worth while to write about an instrument that everybody had long since taken for granted.

  So much for its career which must for ever remain shrouded in mystery. But as for the compass itself, our knowledge about it has made great progress since first the sensitive needle guided the Venetians from their lagoons to the delta of the Nile. For example, we have discovered that the needle of the compass does not point to the true north except on a few spots on the globe, while at all other places it points either a little to the east or a little to the west—a difference which is technically known as the ‘variation of the compass.’ This is due to the fact that the magnetic north and south poles do not coincide with the North and South Poles of our planet but are several hundred miles towards the south and the north of the geographic Poles. The northern magnetic pole is situated in the island of Boothia Felix, an island to the north of Canada, where Sir James Ross first located it in 1831, and the southern magnetic pole is situated at 73° S. Lat. and 156° E. Long.

  It follows therefore that it is not enough for a captain to have merely a compass on board. He must also have charts which show him the variations of his compass in different parts of the world. That however has to do with the science of navigation and the present volume is not a handbook on navigation. Navigation is an exceedingly difficult and complicated branch of learning which refuses very positively to let itself be reduced to simple little words of one syllable. For our present purpose it is enough if you will kindly remember that the compass was used in Europe during the thirteenth and fourteenth centuries find that it was of tremendous help in making navigation a reliable science, and not merely a matter of fortunate guessing and hopelessly complicated calculations, which were far beyond the mental reach of most people.

  But that was only a beginning.

  One could now tell whether one were sailing north or north-by-east or north-north-east or north-east-by-north or north-east or north-east-by-east or in any of the other thirty-two ‘general directions’ indicated by the compass. But for the rest, the medieval skipper had only two other instruments to help him to find out in what part of the ocean he might be.

  In the first place, there was the lead-line. The lead-line was almost as old as the ships themselves. It would show the depth of the sea at any given point, and if one had a chart indicating the different depths of the sea through which one was slowly wending one’s way the lead-line would give some indication of the approximate neighbourhood in which one found oneself.

  And then there was the log. The log originally was a small log of wood which was thrown overboard from the bow and which was then closely watched to see how long it took to pass the stern. As the length of the ship from stern to bow was of course known, one could then figure out how much time the vessel needed to pass a given point and that would show (more or less) how many miles the ship was making per hour.

  The log of wood was gradually given up for the log-line, a long and thin but very strong piece of rope with a triangular piece of wood at the end. This rope had been beforehand divided into so many pieces by means of knots made at regular intervals and the moment it was heaved overboard another sailor started a sand-glass running. When all the sand had run through the glass (one knew of course beforehand how long that would take—two or three minutes) one pulled in the line and counted the knots that had run through one’s hands while the sand-glass was emptying its contents from one bulb into the other. After that, a very simple calculation would show how fast the vessel was going, or as sailors say, “how many knots.”

  But even if the captain knew the speed of his ship and the general direction it was following, there were the currents and the tides and the winds to upset even the most careful of his calculations. As a result, an ordinary ocean voyage, even long after the introduction of the compass, remained a most hazardous undertaking. The people who worked on the theoretical end of the problem realized that in order to make it something else, they would have to find a substitute for the old church-tower.

  CHURCH-TOWER NAVIGATION

  I am not trying to be funny when I say this. The church-tower or the tree on top of the high dune or the windmill on the dike or the barking of a watch-dog had been of such tremendous importance in the realm of navigation because it was a fixed point, something that would not change its position, no matter what happened. And given one such ‘fixed point,’ the sailor could then make his own deductions. “I must go further towards the east,” he would say to himself remembering the last time he had been in that part of the world, or “further towards the west or south or north to arrive where I want to be.” And the mathematicians of that day (brilliant men, by the way, who, considering the scanty information and the faulty instruments at their disposal, did as good work as was ever done in their particular field) knew perfectly well where the crux of the situation lay. They must find a fixed point in nature to act as a substitute for the fixed point established by man.

  They began their search about two centuries before Columbus (I am mentioning his name because 1492 is a date every man, woman, and child seems to know) and they have not finished it even in this day of wireless time-signals and under-water signals and mechanical steering-gears.

  Suppose you find yourself at the foot of a tower on top of which there waves a flag. That flag will then be right straight over your head and as long as you remain at the foot of your tower it will be right straight over your head. But if you move away from it and try to look at it, you must lift your eyes at an angle, and that angle will depend upon the distance you are away from the tower.

  And once this ‘fixed spot’ had been discovered the rest would be comparatively easy, for it would all be a matter of angles, and even the Greeks had known how to measure angles, for they had laid the foundation for the science of trigonometry which deals with the relationship between the sides and the angles of the triangle.

  This brings us to the most difficult part of this chapter, indeed, I might say of the entire book—the search for what we now call latitude and longitude. The true method to establish one’s latitude was discovered hundreds of years before longitude. Longitude (now that we know how to find it) looks much simpler than latitude. But it offered certain almost insurmountable difficulties to our clockless ancestors. Whereas latitude, being merely a matter of careful observation and even more careful figuring, was something they were able to solve
at a comparatively early date.

  Since navigation, depending as it does on measuring latitude and longitude, is entirely an affair of angles, no possible advance could be made in that science until trigonometry had once more been discovered by the people of Europe. The Greeks had laid the foundations for this science a thousand years before, but after the death of Ptolemy (the famous geographer of Alexandria) trigonometry had been forgotten or discarded as a superfluous luxury—something a little too clever to be quite safe. But the people of India, and after them the Arabs of northern Africa and Spain, had no such scruples and they had nobly carried on where the Greeks had left off. The word ‘Zenith’ (the astronomical name for the point in the visible heavens immediately above an observer) and ‘Nadir’ (the point in the invisible heavens immediately under the observer), both of which are pure Arabic, bear witness to the fact that when trigonometry was once more admitted to the curriculum of the European schools (which happened some time during the thirteenth century) it was a Mohammedan and not a Christian branch of learning. But during the next three hundred years the Europeans made up for lost time. For although they were once more able to work with angles and triangles, they still found themselves faced by the problem of discovering some definitely fixed point away from the earth to act as a substitute for their church-tower.

  The most reliable candidate for this sublime honour was the North Star. The North Star was so far away from us that it never seemed to change its position, and besides, it was so easy to locate that even the most stupid fisherman could find it once he had lost sight of land. All he had to do was to draw a straight line through the two stars that were farthest to the right in the Plough and he couldn’t miss it. And, of course, there was always the sun, but its course had never been scientifically mapped out and only the most intelligent mariners could avail themselves of its assistance.

  As long as people were forced to believe that the earth was flat, all calculations were bound to be hopelessly at odds with the true state of affairs. Early during the sixteenth century there came an end to these makeshift methods. The ‘disc’ theory was discarded for the ‘sphere’ theory and the geographers at last came into their own.

  The first thing they did was to cut the earth into two equal halves, which were divided by a plane running at right angles will) the line connecting the North and the South Poles. The dividing line was called the equator. The equator therefore was everywhere equally for removed from both the North and the South Pole. Next the distance between the Poles and the equator was divided into ninety equal parts. Next ninety parallel lines (circles, of course, for remember the earth was round at last) were drawn between the Poles and the equator, each one about sixty-nine miles away from the next, since sixty-nine miles represented one-ninetieth of the supposed distance between the Pole and the equator.

  Geographers gave these circles numbers, beginning from the equator and going up (or down) to the Poles. The equator itself became 0° and the Poles 90°. Those lines were called degrees of latitude (the elongated base of the capital L in my sketch will make you remember that latitude runs horizontally) and a little ° placed at the right of the number was used as a convenient symbol for the word ‘degree,’ which was too long to be used in mathematical calculations.

  All this meant an enormous step forward. But even so, the business of going to sea remained a very dangerous experience. A dozen generations of mathematicians and sailors had to devote themselves to compiling data about the sun, giving its exact position for every day of every year and every clime, before the average skipper was able to solve the latitude problem, and this is how it was done.

  Experience teaches us that as we approach the North Pole the angle between the lines drawn from the point at which we stand to the sun and to the horizon below the sun will gradually grow smaller. Thus in my figure the angle SA″ H″ is clearly less than the angle SA′ H′. This angle is therefore becoming smaller as the latitude grows higher. By using this fact the mathematicians were able to draw up tables giving the latitude of any place when its sun-horizon angle at mid-day has been measured.

  Then at last any reasonably intelligent sailor, provided he could read and write, was able to determine within a couple of miles how far away he was from the North Pole and from the equator, or in technical terms, in what N. lat. (degree of latitude north of the equator) or S. lat. he might find himself. It was not quite so easy once he had crossed the equator, for then he was no longer able to fall back upon the Pole Star, which is not visible on the southern hemisphere. But that problem too was eventually solved by science, and after the end of the sixteenth century latitude ceased to be a matter of concern to those who went down to the sea in ships.

  There remained, however, the difficulty of determining one’s longitude (the word makes it easy for you to remember that the longitudinal degrees run in a vertical direction), and it took two whole centuries more before that puzzle had been successfully solved. In trying to establish the different latitudes the mathematicians had been able to start out with two fixed points—the North Pole and the South Pole. “Here,” so they could say, “stands my church-tower, the North Pole (or the South Pole) and it will remain there until the end of time.”

  But there was no East Pole and no West Pole either, because the axis of the earth did not happen to run that way. Of course one could draw an endless number of meridians, circles going round the earth and crossing both Poles. But which one of these millions of meridians was the one to choose as ‘the Meridian’ that was to divide the world into halves, so that thereafter the sailor could say, “I am a hundred miles east or west of ‘the Meridian’ ”? The old notion of Jerusalem as the centre of the earth was still strong enough to make many people demand that the meridian running through Jerusalem be recognized as long. 0°, or our vertical equator. But national pride prevented this plan. Every country wanted long. 0° to run through its own capital, and even to-day, when we are supposed to be a little more liberal-minded in this respect, there still are German, French, and American maps which show long. 0° running through Berlin, Paris, and Washington. And in the end, as England was the country which happened to do most for the advancement of nautical knowledge during the seventeenth century (when the problem of longitude was finally solved), and as all nautical affairs were then under the supervision of the Royal Observatory, built at Greenwich, near London, in 1675, the meridian of Greenwich was finally adopted as the particular meridian which was to divide the world into longitudinal halves.

  Then at last the sailor had his longitudinal church-tower, but he was still faced with another difficulty. How was he to discover how many miles he was either east or west of that Greenwich meridian once he was out on the high seas?

  In the eighteenth century much progress was made towards solving his hardest problems. A long step forward was made when the sextant was invented. Three men, two Britons and one American, share the credit for this invention; they were John Hadley, Sir Isaac Newton, and Thomas Godfrey of Philadelphia. It has often happened in the history of inventions that the same idea has struck several people almost at the same moment.

  The sextant is a complicated instrument (a sort of miniature nautical observatory which one can carry under one’s arm) which allows the sailor to measure all sorts of angular distances. It was direct heir to the clumsy medieval astrolabe and cross-staff and the quadrant of the sixteenth century.

  A group of merchants and naval officers petitioned Parliament in 1714 to take steps to encourage the invention of some method of determining the longitude of ships at sea. A committee was set up, known as the Board of Longitude, and its most distinguished member, Sir Isaac Newton, pointed out that if a ship could carry a clock which kept exact time, she would be able to find her longitude. But variations of climate and differences in the gravitational ‘pull’ of the earth had so far prevented the manufacture of such a clock. The British Government offered a prize of £20,000 to anyone who could overcome these difficulties, and it was gai
ned by a Yorkshireman who had begun life as a carpenter. His name was John Harrison. He made a clock with a pendulum like a gridiron of alternate brass and steel bars, so that the metals, expanding and contracting with heat or cold, should counteract each other and keep the clock accurate. His invention was tried out on board H.M.S. Centurion in 1735. The ship was able to reckon her course with absolute correctness, while her navigating officer, dispensing with Harrison’s chronometer, was ninety miles ‘out’ in his calculations.

  And to-day, no matter where a ship happens to be, provided she carries a chronometer, she will, thanks to Harrison, always know what time it is in Greenwich, And since the sun revolves round the earth in twenty-four hours (it is the other way round, but I am using the expression for the sake of convenience) and passes through fifteen degrees of longitude in one hour, all we need to do in order to determine how for we have travelled east or west of the Meridian is first to determine what time it is on the spot where we ourselves happen to be and then compare our local time with Greenwich time and note the difference.

  For example, if we find (after careful calculations which every ship’s officer can make) that it is twelve o’clock where we are, but two o’clock by our chronometer (which gives us the exact Greenwich time), then we know that since the sun travels through fifteen degrees in one hour (which means four minutes for every single degree) and since there is a difference of two hours between our time and that of Greenwich, that we must have travelled exactly 2 × 15° = 30°. And we write down in the log-book (so called because it was originally used to record the ship’s rate of progress by means of a log of wood attached to a knotted cord and dropped into the water) that on such and such a day at noon our ship found itself at long. 30° west.