44 A SHORT HISTORY OF SCIENCE 



Herodotus says that there was a war between the Lydians and 

 the Medes, and after various turns of fortune 

 in the sixth year a conflict took place, and on the battle being joined, 

 it happened that the day suddenly became night. And this change, 

 Thales of Miletus had predicted to them, definitely naming this year, 

 in which the event really took place. The Lydians and the Medes, 

 when they saw the day turned into night, ceased from fighting, and 

 both sides were desirous of peace. 



This eclipse is supposed to have taken place in 585 B.C. The 

 prediction of the year of an eclipse gained Thales a great repu- 

 tation with his contemporaries, though his designation with six 

 others as "wise men of Greece" appears to have had a primarily 

 political significance. None of the other six at any rate had any 

 scientific standing. He taught that the year has 365 days ; that 

 the equinoxes divide the year unequally; that the moon is illu- 

 minated by the sun. The mathematical attainments attributed 

 to Thales include the following theorems of elementary geometry ; 

 the angles at the base of an isosceles triangle are equal ; when two 

 straight lines cut each other the opposite angles are equal; the 

 first proof that the circle is bisected by its diameter; the in- 

 scription of the right triangle in the semicircle ; the measurement 

 of height by shadow, involving the principle of similar triangles. 



Plutarch relates that Niloxenus, conversing with Thaies con- 

 cerning King Amasis, says : 



Although he also admires you on account of other things, he 

 prizes above everything the measurement of the pyramids, in that 

 you have without any trouble and without needing an instrument, 

 merely placed your staff at the end of the shadow cast by the pyramid, 

 showing from the two triangles formed by the contact of the solar 

 rays that one shadow lias the same relation to the other as the pyra- 

 mid to the staff. 



Some writers even attribute to Thales a knowledge that the 

 sum of the angles of a triangle is two right angles, also of the 

 idea of a circle as a locus of a point having a certain property, 

 but conclusive evidence can hardly be adduced. Even the im- 



