KINETIC OR MECHANICAL VIEW OF NATURE. 61 



41. 

 Earlier 



rotary motion acquire properties which they do uot 

 possess otherwise viz., rigidity i.e., reaction against ^ 

 change of shape (the stiffness of a travelling rope thrown motion 

 off a pulley is a familiar illustration) ; stability i.e., re- 

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

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

 the same position, if violently disturbed. The gyroscope 1 

 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 now shown 

 that portions of a perfect fluid i.e., of a body which 

 possesses neither rigidity, nor stability, nor elasticity 

 when in a state of rapid rotational motion, acquire 

 these gyrostatic properties ; that whirling portions can- 

 not be naturally created, but that if once in existence 

 they preserve their identity, being permanently differ- 

 entiated from the surrounding fluid, which may be at rest 

 or in the state of flow. These differentiated portions of 

 the liquid were called by Helmholtz vortex filaments ; 

 he showed that in a liquid without a boundary they 

 must run back into themselves, forming rings which 

 might be knotted and linked together in many ways. 



1 A much older invention was 

 that of Bohnenberger (1817), known 

 by his name. The name "gyro- 

 scope " was introduced by Fou- 

 cault ; and that of " gyrostat," as 

 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 

 Philosophy' (2nd ed.), pp. 314-415. 

 It is mainly through the influence 



of this work, and through the 

 inexhaustible wealth of experi- 

 mental illustrations contained in 

 many of Lord Kelvin's addresses 

 (see 'Popular Lectures and Ad- 

 dresses,' vol. i. pp. 143 sqq., 218 

 sqq. ; iii. 165 sqq., 245), that gyro- 

 static and vortex motion has become 

 in this country a favourite study of 

 mathematicians and natural phil- 

 osophers, and forms an important 

 feature in almost every recent 

 attempt to describe the properties 

 of matter and ether. 



