536 SCIENCE PROGRESS 



This apparently happens as the cyclone passes mto the region 

 of the prevailing westerlies, and is continued as the journey 

 proceeds. If, in the course of its travels, the cyclone should 

 find its base supplied with polar air, its energy is necessarily 

 paralysed. Though the death is a lingering one, it must die 

 when the velocity of the vast mass is sufficiently reduced and 

 its area correspondingly extended. Its existence might be 

 prolonged somewhat if it could make use of the convective 

 properties of the equatorial air in juxtaposition with the 

 polar front, but it would certainly be starved out of existence 

 if it had to feed exclusively upon polar air." 



If the theory outlined above is correct, then we must regard 

 convection as the operative force in the creation of both 

 tropical and extra-tropical cyclones, but not in the simple 

 manner generally assumed hitherto. Whether correct or not, 

 the theory is well worth studying in the original memoir, as 

 many of the arguments there brought forward have of neces- 

 sity been abridged, or even omitted, in this short summary. 



Dr. Harold Jeffreys adds (pp. 234-5) some further remarks 

 of a general nature to the theoretical discussion of the subject. 

 The two principal theories that are held as to the cause of 

 tropical cyclones are, he points out, the Convectional and the 

 Counter Current theories. The convectional theory has just 

 been dealt with at considerable length. The counter current 

 theory supposes that the whirls are eddies formed by the 

 interaction of extensive opposed wind currents, in a manner 

 analogous to that in which eddies are formed in a mill pond. 

 It is difficult on the convectional theory to explain why one 

 place rather than another in an extensive area of hot moist 

 air should be selected as the place of origin of a storm. It 

 may be, however, that the truth is contained in a combination 

 of the two theories, and that this difficulty may be overcome 

 by supposing that the conflict between opposed currents is 

 the earliest stage in the initiation of a whirl and that convection 

 continues and intensifies the motion in the manner described 

 by Sir Napier Shaw. In support of this theory Dr. Jeffreys 

 suggests that the South Atlantic, which is the only tropical 

 ocean in which cyclones are not experienced, is also the only 

 one which is not flanked by opposite wind currents. 



PHYSICS. By J. Rice, University, Liverpool. 



Recent Work on the Molecular Diffraction of Light. — It is many 

 years now since the late Lord Rayleigh first brought the subject 

 of molecular diffraction into importance by his well-known 

 papers on the blue light of the sky which results from the 

 scattering of sunlight by the gases of the atmosphere. Since 



