336 Dove on the Law of Storms. 



of the Alps enormous masses of rain fell, and Venice, Genoa and 

 Nice were overflowed. In Appenzell the tempest was such as 

 the oldest inhabitants had never witnessed ; it raged with pecu- 

 liar force in the valleys; the mountains presented such an obsta- 



cle to the pressure of the stream of air, that the barometer stood 

 much higher on their southern than on their northern declivity. 

 We see that those barometric minima of the temperate zone, 

 which are caused by the entrance of tropical whirlwinds, are dis- 

 tinguished from the same phenomena in the torrid zone by their 

 greater extension as well as by the different direction of their 

 course. In the case of the minimum of the 2nd of August, 1837, 

 the difference of barometric pressure at St. Thomas and Porto 

 Rico, places scarcely twenty miles apart, was 15 lines. On the 

 24th December, 1821, the difference of pressure at Brest and Ber- 

 gen was only 12 lines, the amount of the absolute minimum be- 

 ing the same in both instances. On the 2 1st of May, 1823, on 

 the Hidgelee coast, the barometer fell on board the Duke of York, 

 between 8 a. m. and 11a. m., from 326"' to below 298'", or 27 

 lines in three hours, as shown both by the barometer and sym- 

 piesometer, (the fluid in both instruments having sunk for the 

 space of half an hour below the visible part of the tubes, which 

 began at 298 /// ,) the simultaneous fall at Calcutta having been 

 only 8 lines. We see thus, that the fall of the barometer previ- 

 ous to the minimum, and its subsequent rising, take place much 

 more rapidly within the tropics than in the temperate zone; but 

 if we consider the total diminution of pressure, we shall find that 

 it is much greater in temperate than in tropical regions. In the 

 former it may be compared to an extensive valley with gentle 

 declivities, in the latter to a deep ravine with precipitous sides. 

 Besides the causes of diminished atmospheric pressure in the 

 tropical regions, an additional cause comes into play in the tem- 

 perate zone, viz. the high temperature brought from lower lati- 

 tudes by the rapid movement of the air from that direction. On 

 the 24th of December this was very considerable. In Tol mezzo 

 the thermometer rose to 25° Reaumur in the shade ; at Geneva 

 it rose suddenly 5° in the night of the 24th and 25th ; at 1 &■ *• 

 on the 25th it reached 12°.5, which was its highest point. In 

 Boulogne, Paris, and Hamburg the temperature was unusually 

 high. It seems evident that when so warm a current of air was 

 flowing towards the pole over Europe, the cold air displaced by 



