4.^0 PHYSICAL GEOGRAniY OF THE SEA, AND ITS IMETEOROLOGY. 



mastered l)y opposinf^ forces, always require the wind, when not 

 blowino- round in spirals and a whirl, to haul in the southern 

 bemisphore. Kow, paradoxical as it may at first seem, it is also 

 the forces of diurnal rotation that give that same wind, when it 

 is blowing round in spirals, its first impulse to march round in 

 the contrary direction, or (§ 78G) ivitli the hands of a watch ; but 

 this is as it should be — it hauls one way, and marches the other. 

 After passing a, and each of the other stations, a' «', the rush of 

 wind is sufficient, let us suppose, to create a whirl. The Avind 

 at a' a' a', continuing on with a circular motion, is represented 

 thenceforward in its course by the curved arrows a c, a' e'. The 

 wind coming from the east and the west has no direct impulse 

 from diurnal rotation, but the wind on either side 'of it has, and 

 hence the p-me vertical wiufl is carried around with the rest. If, 

 now, we imagine the disc C to be put in motion, and the storm 

 to become a travelling one, we shall have to consider the com- 

 position and resolution of other forces also, such as those of 

 traction, aberration, and the like, before we can resolve the 

 ^whirlwind. 



789. Bernouillis formula. — But the cyclonologists do not locate 



their storms in such high latitudes as the parallels of Cape Horn. 



Hence we might safely infer, one M^ould suppose, that in high 



southern latitudes a north wind has a tendency to incline to the 



^westward and a south wind to the eastward ; and the cause of 



this tendency is in operation, whether the place of low barometer 



be a disc or an oblong, for it is in obedience to the trade-wind 



law, as expounded by Halley, that it so operates; and it will 



also be the case whether the wind be caused by an influx into 



the place of low, or the efflux from the place of high barometer ; 



or, as is generally the case, by both together. If the distance 



between the jDlace of high and low barometer were always the 



same, then a given difi'erence of barometric pressure would 



always be followed by a wind of the same force of velocity. By 



<3xpanding Bernouilli's formula for the velocity of gas jets under 



given pressures, Sir John F. W. Herschel has computed* the 



-volocitj^ and the force with which currents of air or winds would 



issue under certain differences of barometric pressure. Under 



the most favourable conditions, i. e., when the places of high and 



of low barometer are in immediate juxtaposition, as on the inside 



/md outside of an air-pump, an etlectivc difference of O.OOG inch 



* See article Meteorology, Encyclojviodia Britannica, 1857. 



