KINETIC OR MEC'HANKAl- VIKW OK NA'iritK. 61 



rotary motion acquire i)roperties which they do not 

 possess otherwise — viz., rigidity — i.r., reaction against 

 change of shape (the stifiness of a travelhng rope thrcjwn 

 off' a pulley is a familiar illustration); stability — i.r., re- 

 action against change of position and motion, as in a sjiin- 

 ning-top or a bicycle ; elasticity — i.e., tendency to revert to 

 the same position, if violently disturbed. 'I'he gyroscope^ 

 had been invented in 1852 by Foucault, and used by 

 him and other physicists in France and Germany to 

 illustrate the rotation of the earth. It was nr)w shown 

 that portions of a perfect Huid- — i.e., of a liody which 

 possesses neither rigidity, nor stal)ility, nor elasticity 

 — when in a state of rapid rotational motion, acquire 

 these gyrostatic properties ; that whirling portions can- 

 not be naturalh' created, but that if once in existence 

 they preserve their identity, being permanently differ- 

 entiated from the surrounding ffuid, which may be at rest 

 or in the state of ffow. These differentiated portions of 

 the liquid were called by Helmholtz vortex filaments ; 

 he showed that in a lit^uid without a boundary they 

 must run back into themselves, foniiing rings which 

 might be knotted and linked together in many ways. 



^ A much older invention was j of this work, and through the 



that of Bohnenberger (1817), known inexhaustible wealth of esj)eri- 



by his name. The name "gyro- mental illustrations contained in 



scope " was introduced by Fou- I many of Lord Kelvin's addresses 



cault ; and that of " gyrostat," as j (see ' Popular Lectures and Ad- 



defining an apparatus which ac- 

 quires stability through rotational 

 (whirling or gyrating) motion, was 

 used first by Lord Kelvin. An 

 extensive treatment of the subject 

 is to be found in the first part 

 of Thomson and Tait's ' Natural 

 Pliilosophy' C^nd ed.), i)p. SH-il.'i. 

 It is mainly through tlie influence 



dresses,' vol. i. i)p. 143 ■'«/'/., 218 

 S(jq. ; iii. 165 .^'jq., 245), that gj'ro- 

 static and vortex motion has l>ecome 

 in this countrj" a favourite study of 

 matiiematicians and natural j>hil- 

 o.sophers, and forms an iuiiH)rtant 

 feature in almost every recent 

 attempt t<> de.'icril>e the proi>ertie» 

 of matter and ether. 



