232 THE POPULAR SCIENCE MONTHLY 



minute details of the phenomena of motion which are involved in this 

 turbulence. Let one, for example, watch the movement of the water 

 at a point in a brook. There is indeed a fairly steady average velocity 

 of the water at the point and a certain mean rhythmic variation, but 

 superposed upon this average motion there is an erratic variation of 

 velocity which is infinitely manifold, the details of which are beyond 

 the scope of any descriptive science. A descriptive science like me- 

 chanics is concerned with how things progress as a phenomenon de- 

 velops itself; how the structural parts of a bridge stretch and shorten 

 as a car passes across the bridge; how the pressure and temperature 

 of steam vary during the successive stages of admission, expansion and 

 exhaust of a steam engine; how electro-motive force, current strength, 

 electro-magnetic force and all of the changing variables play in the 

 operation of a dynamo. But who could recite the story of the most 

 minute details of these phenomena? It can not be done, and if it 

 could be done, it would be of no avail, for these details can never be 

 twice alike and the very essence of a science lies in the discovery of 

 types which in their important features recur so that a knowledge of 

 these types may serve as a basis for anticipation and design. 



Fire is the most familiar example of a turbulent phenomenon, and 

 its most striking characteristic is that its progress is not dependent 

 upon any external driving cause; when once started it goes forward 

 of itself and with a rush. Tyndall in referring to this matter says 

 that to account for the propagation of fire was one of the philosophical 

 difficulties of the eighteenth century. A spark was found sufficient 

 to initiate a conflagration, and the effect seemed beyond all proportion 

 greater than the cause ; herein lay the philosophical difficulty. Indeed 

 the simple idea of cause and effect is not applicable to physical phe- 

 nomena involving turbulence. Every physical phenomenon involving 

 turbulence is to some extent self-sustaining, and every such phe- 

 nomenon has a certain impetuous quality which may carry it beyond 

 anything that is commensurate with the original initiating cause. 

 These remarkable characteristics of turbulence are now definitely for- 

 mulated as the second law of thermodynamics. 



The most important practical thing in connection with the turbulent 

 aspect of any physical phenomenon is its general result or consequence, 

 just as the important thing about the burning of a house is the loss. 

 How utterly useless and uninteresting it would be, for example, to 

 study the minutest details of a conflagration (assuming such study to 

 be possible), recording the height and breadth and the irregular and 

 evanescent distribution of temperature throughout each flicker of con- 

 suming flame, the story of each crackling sound and the extent and 

 character of every yield and sway of timber and wall ! The fact is that 

 we are immersed in an illimitable sea of phenomena, every single detail 



