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