The Lesson of the Bath.—One of the most valuable discoveries made by Archimedes, the famous scholar of Syracuse, in Sicily, relates to the weight of bodies immersed in water. Hiero, King of Syracuse, had given a lump of gold to be made into a crown, and when it came back he suspected that the workmen had kept back some of the gold, and had made up the weight by adding more than the right quantity of silver; but he had no means of proving this, because they had made it weigh as much as the gold which had been sent. Archimedes, puzzling over this problem, went to his bath. As he stepped in he saw the water, which his body displaced, rise to a higher level in the bath, and to the astonishment of his servants he sprang out of the water, and ran home through the streets of Syracuse almost naked, crying, "Eureka! Eureka!" ("I have found it! I have found it!").
What had he found? He had discovered that any solid body put into a vessel of water displaces a quantity of water equal to its own bulk, and therefore that equal weights of two substances, one light and bulky, and the other heavy and small, will displace different quantities of water. This discovery enabled him to solve his problem. He procured one lump of gold and another of silver, each weighing exactly the same as the crown. Of course the lumps were not the same size, because silver is lighter than gold, and so it takes more of it to make up the same weight. He first put the gold into a basin of water, and marked on the side of the vessel the height to which the water rose.
Next, taking out the gold, he put in the silver, which, though it weighed the same, yet, being larger, made the water rise higher; and this height he also marked. Lastly, he took out the silver and put in the crown. Now if the crown had been pure gold, the water would have risen only up to the mark of the gold, but it rose higher, and stood between the gold and silver marks, showing that silver had been mixed with it, making it more bulky; and by calculating how much water was displaced, Archimedes could estimate roughly how much silver had been added. This was the first attempt to measure the specific gravity of different substances; that is, the weight of any particular substance compared to an equal bulk of some other substance taken as a standard. In weighing solids or liquids, water is the usual standard.
How this Solid Earth keeps Changing.—The student of history reads of the great sea-fight which King Edward III. fought with the French off Sluys; how in those days the merchant vessels came up to the walls of that flourishing sea-port by every tide; and how, a century later, a Portuguese fleet conveyed Isabella from Lisbon, and an English fleet brought Margaret of York from the Thames, to marry successive Dukes of Burgundy at the port of Sluys. In our time, if a modern traveller drives twelve miles out of Bruges, across the Dutch frontier, he will find a small agricultural town, surrounded by corn fields and meadows and clumps of trees, whence the sea is not in sight from the top of the town-hall steeple. This is Sluys.
Once more. We turn to the great Baie du Mont Saint Michel, between Normandy and Brittany. In Roman authors we read of the vast forest called "Setiacum Nemus," in the centre of which an isolated rock arose, surmounted by a temple of Jupiter, once a college of Druidesses. Now the same rock, with its glorious pile dedicated to St. Michael, is surrounded by the sea at high tides. The story of this transformation is even more striking than that of Sluys, and its adequate narration justly earned for M. Manet the gold medal of the French Geographical Society in 1828.
Once again. Let us turn for a moment to the Mediterranean shores of Spain, and the mountains of Murcia. Those rocky heights, whose peaks stand out against the deep blue sky, scarcely support a blade of vegetation. The algarobas and olives at their bases are artificially supplied with soil. It is scarcely credible that these are the same mountains which, according to the forest-book of King Alfonso el Sabio, were once clothed to their summits with pines and other forest trees, while soft clouds and mist hung over a rounded, shaggy outline of wood where now the naked rocks make a hard line against the burnished sky. But Arab and Spanish chroniclers alike record the facts, and geographical science explains the cause. There is scarcely a district in the whole range of the civilized world where some equally interesting geographical story has not been recorded, and where the same valuable lessons may not be taught. This is comparative geography.