The Problem of the Sirex 



netic needle swung out of its position and 

 tending to return to it while moving with a 

 uniform speed through a resisting medium 

 in which a sheath of a diameter slightly 

 greater than the needle's ,opens bit by bit. 

 The Sirex behaves more or less in the same 

 fashion. His magnetic pole is the light out- 

 side. He makes for that direction by imper- 

 ceptible deviations as his tooth digs. 



The problem of the Sirex is now solved. 

 The trajectory is composed of equal ele- 

 ments, with an invariable angle between 

 them ; it is the curve whose tangents, divided 

 by infinitely small distances, retain the same 

 inclination between each one and the next; 

 the curve, in a word, with a constant angle 

 of contingence. This characteristic betrays 

 the circumference of the circle. 



It remains to discover whether the facts 

 confirm the logical argument. I take accu- 

 rate tracings of a score of galleries, selecting 

 those whose length best lends itself to the 

 test of the compasses. Well, logic agrees 

 with reality : over lengths which sometimes 

 exceed four inches, the track of the com- 

 passes is identical with that of the insect. 

 The most pronounced deviations do not ex- 

 ceed the small variations which we must 

 227 



