STORMS, HURRICANES, AND TYPHOONS. 



405 



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 place of high and low barometer were always the same, then 

 a given difiference of barometric pressure would always be fol- 

 lowed by a wind of the same force of velocity. By expanding 

 Bernouilli's formula for the velocity of gas jets under given pres- 

 sures, Sir John F. W. Herschel has computed* the velocity 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 and 

 outside of an air-pump, an effective difference of 0.006 inch in 

 the barometric pressure would create a breeze with a velocity of 

 seven miles the hour. Such a wind is capable of exerting a 

 horizontal pressure of 0.2 lb. the square foot, thus : 



Changes, however, in the barometer, amounting to five or six 

 times these differences, are observed to take place at sea without 

 producing v/inds exceeding in velocity the rates above. This is 

 because the places of high and low barometer at sea are far apart, 

 and because, also, of the obstructions of the winds afforded by 

 the inequalities of the earth's surface. 



790. But, in this view of the subject, the importance of a daily 

 Predicting storms, systoui of wcather roports by telegraph on shore, 

 and across the water between Europe and America when the sub- 

 Atlantic cable is well laid, looms up and assumes all the propor- 

 tions of one of the great practical questions of the age. We may 

 * See article Meteorologj', Encyclopaedia Britannica, 1857. 



