7 Cai Lun 7

Cai Lun was a eunuch who entered the service of the

imperial palace in 75 CE and was made chief eunuch under

the emperor Hedi (reigned 88–105/106) of the Dong

(Eastern) Han dynasty in the year 89. About the year 105

Cai conceived the idea of forming sheets of paper from the

macerated bark of trees, hemp waste, old rags, and fish-

nets. The paper thus obtained was found to be superior

in writing quality to cloth made of pure silk (the principal

writing surface of the time), as well as being much less

expensive to produce and having more abundant sources.

Cai reported his discovery to the emperor, who com-

mended him for it. Important improvements were

subsequently made to Cai’s papermaking process by his

apprentice, Zuo Bo, and the process was rapidly adopted

throughout China, from which it eventually spread to the

rest of the world. Cai himself was named a marquess in 114.

Heron of Alexandria

(fl. c. 62 CE, Alexandria, Egypt)

H

eron (or Hero) of Alexandria was a Greek geometer

and inventor whose writings preserved for posterity

a knowledge of the mathematics and engineering of

Babylonia, ancient Egypt, and the Greco-Roman world.

Heron’s most important geometric work, Metrica,

was lost until 1896. It is a compendium, in three books, of

geometric rules and formulas that Heron gathered from a

variety of sources, some of them going back to ancient

Babylon, on areas and volumes of plane and solid figures.

Book I enumerates means of finding the area of various

plane figures and the surface areas of common solids.

Included is a derivation of Heron’s formula (actually,

Archimedes’ formula) for the area A of a triangle,

A = √(s(s−a)(s−b)(s−c))

in which a, b, and c are the lengths of the sides of the