44 



HISTORY OF SCIENCE. 



Apollonius treated also of involutes and evolutes, and of geometrical 

 maxima and minima. 



NICOMEDES, a .geometer who lived about two centuries before the 

 commeric'eme'rTt of the Christian era, is remembered as the inventor of 

 a peculiar curve called the conchoid, from its supposed resemblance 

 to the outline of a shell. We are provided with the means of me- 

 chanically tracing this curve by means of an arrangement of straight 

 rulers contrived by Nicomedes himself. As it may be interesting to 

 some readers to observe for themselves the curious way in which the 

 same movement of a single straight ruler may be made to trace out 

 an indefinite number of differently curved lines, we shall briefly de- 

 scribe the instrument of Nicomedes by help of Fig. 15. A B and c D 

 are flat rulers, of which A B has a groove or slit parallel to its length, 





and c D has a fixed peg at E, which passes through a slit in the 

 movable ruler F G, which is itself provided with a fixed peg at H, that 

 always slides in the groove A B. 'A pencil or tracing-point being fixed 

 on any part of the ruler F G, the movement will cause this point to 

 describe a conchoid, the form of which varies according to the point 

 of F G to which the pencil is attached. The conchoid affords a means 

 of solving some interesting geometrical problems. 



Pre-eminent among the many distinguished men belonging to the 

 school of Alexandria, we find the great astronomer HippARCHUsjB.c. 

 j6o 125). He has been called, not without justice, the father of 

 scientific astronomy, and the Newton of the Greeks. Only one oflfie 

 works of Hipparchus is now extant, and it is perhaps the least impor- 

 tant of the treatises he is known to have written. It is a Commentary 

 on an astronomical poem by an author named Aratus, and is chiefly 

 interesting as showing that Hipparchus was acquainted with spherical 



