t the upper extremity. When applied to the propulsion of
steam-vessels the screw is horizontal; and being put in motion by a
steam-engine, drives the water backward, when its reaction, or return,
propels the vessel.

The mechanical ingenuity of Archimedes was next displayed in the various
machines which he constructed for the defence of Syracuse during a three
years' siege by the Romans. Among these inventions were catapults for
throwing arrows, and ballistae for throwing masses of stone; and iron
hands or hooks attached to chains, thrown to catch the prows of the
enemy's vessels, and then overturn them. He is likewise stated to have
set their vessels on fire by burning-glasses; this, however, rests upon
modern authority, and Archimedes is rather believed to have set the
ships on fire by machines for throwing lighted materials.

[Illustration: Death of Archimedes.]

After the storming of Syracuse, Archimedes was killed by a Roman
soldier, who did not know who he was. The soldier inquired, but the
philosopher, being intent upon a problem, begged that his diagram might
not be disturbed; upon which the soldier put him to death. At his own
request, expressed during his life, a sphere inscribed in a cylinder was
sculptured on his tomb, in memory of his discovery that the solid
contents of a sphere is exactly two-thirds of that of the circumscribing
cylinder; and by this means the memorial was afterward identified. One
hundred and fifty years after the death of Archimedes, when Cicero was
residing in Sicily, he paid homage to his forgotten tomb. "During my
quaestorship," says this illustrious Roman, "I diligently sought to
discover the sepulchre of Archimedes, which the Syracusans had
totally neglected, and suffered to be grown over with thorns and briars.
Recollecting some verses, said to be inscribed on the tomb, which
mentioned that on the top was placed a sphere with a cylinder, I looked
round me upon every object at the Agragentine Gate, the common
receptacle of the dead. At last I observed a little column which just
rose above the thorns, upon which was placed the figure of a sphere and
cylinder. This, said I to the Syracusan nobles who were with me, this
must, I think, be what I am seeking. Several persons were immediately
employed to clear away the weeds and lay open the spot. As soon as a
passage was opened, we drew near, and found on the opposite base the
inscription, with nearly half the latter part of the verses