54 HEROES OF INVENTION AND DISCO VER V. 



of light and colour, show that the internal constitution of matter 

 and its chemical properties had also much occupied his thoughts. 

 Thus, too, in other departments, genius has found its sufficient 

 materials and instruments in the humblest and most common 

 articles, and the simplest contrivances. Fergusson observed 

 the places of the stars by means of a thread with a few beads 

 strung on it, and Tycho Brahe did the same thing with a pair 

 of compasses. The self-taught American philosopher, Ritten- 

 house, being, when a young man, employed as an agricultural 

 labourer, used to draw geometrical diagrams on his plough, 

 and study them as he turned up the furrow. Pascal, when a 

 mere boy, made himself master of many of the elementary 

 propositions of geometiy, without the assistance of any master, 

 by tracing the figures on the floor of his room with a bit of coal. 

 This, or a stick burned at the end, has often been the young 

 painter's first pencil, while the smoothest and whitest wall he 

 could find supplied the place of a canvas. Such, for example, 

 were the commencing essays of the early Tuscan artist, Andrea 

 del Castagno, who employed his leisure in this manner when 

 he was a little boy tending cattle, till his performances at last 

 attracted the notice of one of the Medici family, who placed 

 him under a proper master. The famous Salvator Rosa first 

 displayed his genius for design in the same manner. To these 

 instances may be added that of the English musical com- 

 poser Mr. John Davy, who is said, when only six years old, 

 to have begun the study and practice of his art by imitating 

 the chimes of a neighbouring church with eight horse-shoes, 

 which he suspended by strings from the ceiling of a room in 

 such a manner as to form an octave. 



But to return to the subject of our notice. Davy first pursued 

 his chemical studies without teacher or guide, in the manner 



