254 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 19 31 



actually the two investigations were complementary; the one ex- 

 cluded all save radial star streaming, the other excluded all save 

 transverse star streaming. Between them they established that 

 neither radial nor transverse nor any other direction of star stream- 

 ing is compatible with strict dynamical equilibrium. It seems to be 

 clearly established by observation that radial star streaming exists; 

 but Jeans's investigation indicates the price that must be paid — the 

 galaxy can not be in a steady state. Nor would a transverse direction 

 of star streaming have saved it; the determination of the direction 

 merely decides which horn of the dilemma our galaxy shall impale 

 itself on. 



Here lies the chief difficulty in pursuing dynamical theories of 

 the galaxy. A perfectly steady model having been proved to be 

 impossible, we must seek some compromise among the incompatible 

 conditions for permanence, which will give a semipermanent model 

 satisfying our minimum requirements of duration. I do not think 

 that much serious progress has been made toward such an adaptation 

 of the steady state theory. Oort and Lindblad have obtained certain 

 theoretical relations between the intensity of star streaming (the 

 prolateness of the Schwarzschild velocity-ellipsoid) and the mag- 

 nitude of the differential rotation and found a rather impressive 

 agreement with observation; but it may be urged in criticism that 

 they have selected one out of a number of incompatible conditions for 

 a steady state, and it is not at all clear why this (rather than the 

 rejected conditions) should be retained in the appropriate compro- 

 mise. My own impression is that it is the least essential of the 

 conditions for longevity. 



ROTATION OF THE COSMIC CLOUD 



My survej'' of the dynamics of the galaxy must necessarily be 

 superficial, since it would be idle to enter into details without recourse 

 to mathematical formulae. I should like, however, to refer to an 

 aspect of the problem which has not as yet received much attention. 

 The recognition of a cosmic cloud of rarified gas extended through 

 interstellar space and sharing in the rotation of the galaxy opens up 

 a new field of theoretical investigation; and the djmamical equilib- 

 rium of the cloud is a problem equally important with, and consider- 

 abl}'' easier than, the dynamical equilibrium of the stellar configura- 

 tion. 



We find that in our own neighborhood the motion of the mean of 

 the stars agrees with the motion of the cosmic cloud ; the difference 

 can not be put higher than 2 or 3 km per sec. Moreover, it appears 

 that this agreement is not confined to our immediate neighborhood, 

 but extends at least 1,000 parsecs away from us, since Plaskett's inves- 

 tigation of the differential rotation of the cloud covers this range. 



