TRANSACTIONS OF THE SECTIONS. 43 



increase of latitude, it is evident that in our hemisphere the air must be drawn from 

 the greatest distance and with the greatest velocity on the south side of a storm ; and 

 this, taken in connexion with the constant eastward deflection of the moving mass 

 in its passage to the north, will account for the superior force of south-west winds in 

 the north temperate zone. 



The air pressing from the north and south to the place at which the greatest rain 

 occurs, must be deflected in opposite directions ; and on this principle it has been 

 proposed to account for the rotation of storms. But the eastward and westward 

 winds must cooperate in producing the same result, the former being deflected to the 

 south and the latter to the north, from an excess and a deficiency of centrifugal force. 

 The spiral motion, generated in this manner, prevents the atmospheric pressure, at 

 the centre of a storm, from being increased by the influx of the surrounding air, 

 and contributes to make the violent movement extend to the bottom of our aerial 

 ocean. 



As the air on the east side of the great vortex cools by retiring from the equator, 

 it becomes less capable of retaining its aqueous vapour, while an opposite effect takes 

 place on the west side, where the temperature of the air increases with the change of 

 latitude. From the greater abundance of rains which accordingly fall on its east side, 

 the focus of a rotating storm must be constantly shifted in an eastward direction ; 

 but between the tropics the movement depends on less effective causes, and the course 

 is mainly determined by the direction of the trade-winds. In temperate climates a 

 tendency of the storm to recede from the equator must proceed from the superior 

 violence of south-west winds, to which allusion has been already made. Accordingly 

 the present theory, without involving any new hypothesis, appears to furnish a very 

 satisfactory explanation of the leading facts which meteorologists have discovered, 

 respecting the rotary and orbital movements of tempests in different regions of the 

 earth. The constant change in the position of the focus, to which the whirling mass 

 of fluid is directed, appears to be the cause not only of the east or north-east course 

 which storms take in our climates, but also of the centrifugal motion of the air which 

 observers have occasionally noticed, and which Espy ascribes to the impulse of de- 

 scending drops of rain. 



On a System of Moving Bodies. By A. S. S. Wilson. 



Meteorology. 



On the Semidiurnal and Annual Variations of the Barometer. By John 

 Allan Broun, F.R.S., Director of the Observatories of His Highness the 

 Rajah of Travancore. 



In the twenty-second volume of Poggendorff's 'Annales' (pp. 219 and 493*), 

 M. Dove showed, in discussing observations made at Apenrade, that when the 

 tension of vapour in the atmosphere is subducted from the whole atmospheric 

 pressure (for each hour), the remaining diurnal variation of dry air pressure has a 

 period of twenty-four hours like that of the elasticity of vapour itself, only that the 

 maximum of the one occurs at the same time as the minimum of the other, these 

 epochs coinciding nearly also with those of highest and lowest temperaturef. 



M. Dove has shown that this result may vary under different circumstances ; 

 thus in a place far from the sea, to which no sea breeze can make up by day what 

 the ascending current carries away of vapour from the lower strata, the curves of 

 the elasticity of vapour and of dry air will march together ; since both fall at the 

 warmest time of the day, the dry air as well as vapour will be carried up by the 

 rising current, and flow off sideways. For a decidedly continental situation, then, 

 we may expect that the maximum of the morning will disappear in the combined 

 pressure measured by the barometer, which will happen for places in the neighbour- 



* Cited iu M. Dove's paper " Bericht," &c. der Wiss. zu Berlin, Marz 1846, p. 54, 

 t Ibid. p. 54. 



