METEOROLOGY 531 



original method, it becomes possible to determine the parallaxes 

 by indirect means of stars of any type except exceptional stars 

 such as those of the very earliest type. 



METEOROLOGY. By E. V. Newnham, B.Sc, Meteorological Office, 

 London. 



Hurricanes and Tropical Revolving Storms, by Mrs. E. V. 

 Newnham, M.Sc. {continued). — In the January number of 

 Science Progress a brief summary was given of the portion 

 of this memoir which deals with the observed characteristics 

 and geographical distribution of cyclones throughout the 

 world. The theoretical side of the subject will now be con- 

 sidered. 



In his introductory note entitled " The Birth and Death of 

 Cyclones," Sir Napier Shaw points out that the tropical 

 hurricanes, together with the smaller " tornadoes," are gener- 

 ally admitted to be examples of travelling vortices, and to 

 these may be added, as a result of his own researches, the small 

 " secondary " depressions that occur so frequently in the 

 prevailing westerly winds of temperate latitudes, and which, 

 he has shown, travel with approximately the same speed as 

 that of the winds in their immediate neighbourhood. Any 

 theory, however, which purports to explain the mode of origin 

 and the structure of tropical hurricanes must take into account 

 the fact that these often pass into temperate latitudes, and on 

 entering the zone of prevailing westerly winds are indistinguish- 

 able from the large primary " depressions " of these regions, 

 to which the " secondaries " just referred to are related in 

 much the same way that a satellite is related to its primary. 

 Considerations of continuity would lead one to expect that 

 if the tropical hurricane is a vortex, then these primary de- 

 pressions will also have the same structure, seeing that the 

 transition from a tropical hurricane to a temperate depression 

 is a gradual one, and the resemblance between the two types 

 of storm is considerable. On this point, however, some 

 divergence of opinion exists among meteorologists, the structure 

 of a temperate depression suggested by Prof. V. Bjerknes 

 showing lines of discontinuity of a non-circular shape by no 

 means suggestive at first sight of vortex motion. Sir Napier 

 Shaw maintains, however, that at suitable heights, roughly 

 between 4,000 metres and 8,000 metres, the temperate cyclone 

 is actually a symmetrical whirl, for at these levels the correlation 

 between pressure and temperature is so high as to exclude 

 the likelihood, perhaps even the possibility, of appreciable 

 asymmetry. That asymmetry exists, however, at lower levels 

 he is fully prepared to admit. Further evidence in support 



