191G] 



on Illusions of the Upper Air 



607 



away radially on all sides, becomes at length imbued with a lateral 

 motion due to the above-mentioned cause, though acting in a dif- 

 ferent manner and in opposite directions." * 



Out of that there gradually grew the conception, on the one hand, 

 of the central area of a cyclone on the map as a centre of centripetal 



ISOTHERMES DE L ATiVl OSPH ER E LIBRE 

 AU DESSUS DE LA REGION DE PARIS 



PAR M L TElSSERENC OE BORT 



DU 3 AU 18 MARS 1903 



DU 5 AU 16 MAI 1904 



Fig. 1. — IsoPLETHS of Height (in kilometres) and Tempera- 

 tures (in degrees centigrade) for sixteen days in March 1903 and 

 twelve days in May 1904. The sequence of temperatures in the 

 upper regions is not quite the same as at the surface, but there is no 

 suggestion of changes becoming less pronounced aloft. 



' motion, a focus of attraction for the surrounding air, and of the 



general area of the cyclone as a region of ascending warm air pro- 



I ducing rain or snow ; round the central region the air moves inward 



Proceedings of the Royal Society, 1862-1863, vol. xii, p. 385. 



