August 4, 1916] 



SCIENCE 



161 



in a perfectly quiet room, and there is no 

 more reason to expect the stick to fall one 

 way than another, therefore the stick will 

 not fall at all! Every one appreciates the 

 fallacy of this argument as applied to the 

 stick, and the moving ball does in fact 

 jump sidewise. 



To understand the behavior of the ball 

 let us think of the ball as standing still and 

 of the air as blowing past in a steady 

 stream. The air streams past the ball and 

 slides over a body of still air behind the 

 ball; the surface which separates the mov- 

 ing air and still air is called a vortex sheet, 

 and a vortex sheet is unstable. Any cause, 

 however minute, is sufficient to start an 

 eddy or whirl, and once started such an 

 eddy or whirl develops more and more. 

 Such an eddy or whirl means that the air 

 streaming past one side of the ball is 

 thrown inwards or outwards, and the reac- 

 tion on the ball pushes the ball sidewise. 

 This effect can be shown by dropping a 

 marble in a deep jar of water. Instead of 

 moving straight downwards the marble 

 follows an erratic zigzag path. This effect 

 is familiar to every one in the sidewise 

 quivering of a stick in a stream of water; 

 and the hissing of a jet of steam is due to 

 the rapid fluttering of the boundary be- 

 tween steam jet and air because of the for- 

 mation of innumerable eddies. 



METEOROLOGY 6 



There are three fairly distinct objects to 

 be attained in the analysis of weather ob- 

 servations, namely, (a) the determination 

 of systematic variations in time and place ; 

 (6) the elaborate classification of individ- 

 ual storm movements with respect to a 

 great number of measurable characteris- 

 tics, and the establishment of coefficients 



6 The proposal here set forth was mentioned in a 

 semi-humorous way in a very short article by W. S. 

 Franklin in Science, Vol. 14, pages 496-497, Sep- 

 tember 27, 1901. 



of correlation (statistical) between the 

 measurable characteristics of a given type 

 or class of storm on successive days so that 

 weather predictions can be made rationally, 

 that is, definite predictions qualified by 

 probable departures; and (c) the recog- 

 nition of critical states in an individual 

 storm movement (conditions of static or 

 dynamic instability) with the hope of de- 

 vising means for controlling the storm by 

 the suitable expenditure of a very small 

 amount of energy at the critical time and 

 place. If we are ever to control the 

 weather we must, as it seems, do it in this 

 way, and this would be singing Dan Tucker 

 to a hurricane not in accordance with Uncle 

 Remus 's idea. 



The above-mentioned objects are now 

 kept in view by meteorologists, but the 

 study of classifications and departures 

 should be increased a thousand-fold. The 

 point of view of the meteorologist has in 

 the past been the point of view of the 

 classicist in physics with his preconception 

 of a universe of one-to-one correspond- 

 ences; but statistical studies are the thing. 



STATISTICAL PHYSICS AND THE POSTULATE OP 

 INDETEBMINATION 



Whenever the postulate of erratic ac- 

 tion is set forth, and the probable depar- 

 ture of a natural phenomenon from the 

 most carefully considered prediction is 

 urged as in the nature of things inevitable, 

 we meet objections from two classes of men, 

 namely, the average man who thinks 

 frankly in terms of human values and the 

 classicist in science who idealizes nature in 

 one-to-one correspondences. Surely, the 

 classicist says, "if we knew all" the data 

 we could make an unqualified prediction 

 in any case. But, ignoring the hopelessly 

 unscientific attitude of mind of one who 

 can postulate infinite knowledge, let it be 

 understood that to speak of data in phys- 

 ics is to speak of a very narrow and limited 



