532 SCIENCE PROGRESS 



of the vortex theory is furnished by the velocity of translation 

 of the extra-tropical cyclones, which velocity is on the average 

 in close agreement with that of the circumpolar circulation 

 of westerly winds at a height of 4,000 metres, as computed 

 by Tessereinc de Bort, and shows a seasonal variation similar 

 to that of these winds. Tropical hurricanes, as well as " secon- 

 dary depressions " and tornadoes, all admitted to be in most 

 instances examples of vortex motion, show likewise a tendency 

 to move with the prevailing winds in the regions of their occur- 

 rence. We may assume, therefore, that the phenomenon to be 

 explained is the initiation of a violent circular vortex in the 

 Doldrums, and the maintenance of such a vortex practically 

 unchanged throughout its sojourn in the tropics, also on 

 certain occasions its expansion and diminution of intensity 

 on entering higher latitudes, where it may continue for days 

 or even weeks when circumstances are favourable. 



Sir Napier Shaw, after pointing out the fact that tropical 

 hurricanes originate in regions where the equatorial belt of 

 easterly winds blow over conspicuously warm sea-water, 

 passes to a discussion of the detailed manner in which a whirl 

 may be initiated. His remarks on this part of the subject 

 are of particular interest, and may be quoted here : 



" The selection by tropical cyclones of the localities of 

 hottest sea-water for their place of birth or nurture is certainly 

 suggestive of convection from the surface as their cause ; and 

 the recent investigation of the upper air enables us to say 

 that conditions have been ascertained which, if brought into 

 juxtaposition, could produce certain results of the proper 

 order of magnitude for tropical cyclones. For example, 

 from the soundings of air at Batavia on the island of Java, 

 we obtain an average or normal representation of the lapse of 

 temperature with height in the equatorial region, and we know 

 also from Neuhoff's diagram ^ and equation the effect upon 

 temperature of adiabatic changes of pressure in the case of air 

 saturated with water vapour at, for example, 300° C. (Abso- 

 lute). We can set out these side by side, and we see at once 

 that air saturated with water vapour at 300° a. would be in 

 unstable equilibrium at the surface at Batavia. If it began 

 to rise it would not find itself at the same temperature with 

 its surroundings, and therefore not permanently in equilibrium, 

 until the level of 15 kilometres had been reached, and only 

 then if we suppose it to be loaded with its condensed water 

 as drops. After they had fallen out, further height would be 

 required to bring the density of the air to that of its environ- 

 ment. There is nothing to excite surprise in this result, 



^ Meteorological Glossary, Meteorological Office, London, p. 16. 



