282 THE ATMOSPHEEE AIS'D METEOEOLOGY. 



surface of the ocean. According to Bridet,* the speed of translation 

 in the hurricanes of the Indian Ocean is comprised between tlie 

 extremes of one mile and 20 miles an hour. 



The movement of the cyclone has the efifect of hollowing into a 

 funnel all the central part of the whirlwind^ and hurling the masses 

 of air towards the circumference of this enormous wheel which turns 

 in the atmosphere. It is thus that in the rivers, and even the smallest 

 tributaries, the eddies are always depressed in the centre, because of 

 the centrifugal force which carries the waters along in a circle. The 

 diminution of the aerial column makes itself instantly felt, by a cor- 

 responding diminution of weight, and the mercury sinks in conse- 

 quence, as soon as the hurricane commences to be formed in the high 

 regions of the atmosphere. The storm which is approaching thus 

 announces its proximity, and those whom it threatens can take their 

 precautions so as to escape entirely from the disaster, or so as to diminish 

 its effects. The sailors whose vessel is anchored in a sure port, double 

 their moorings ; those who are lying in an open road, exposed to 

 the fury of the w^inds, as at Reunion, hasten to obey the signal gun, 

 and fly to the open sea so as to withdraw from the centre of the hurri- 

 cane. The barometer has been seen to fall by IJ, 2, and even 2|- 

 inches, t that is to say, nearly a tenth of the total height of the mer- 

 cuTj, and each of these perturbations has not failed to be the signal 

 of a storm all the more terrible the higher the barometer had pre- 

 viously risen. At times the rarefaction of the atmosphere is accom- 

 plished in such a sudden manner, that the air contained in the houses 

 suddenly expands, explodes, so to say, and hurls windows and doors 

 far away. For this reason, says Fitzroy, the habitations are left open 

 in certain places to avoid such accidents. 



In the sea, the waters rise to a greater or less height in consequence 

 of the lessening of the atmospheric pressure, and move with the 

 centre of the cyclone ; thus a " tempest-wave " is raised, whose force 

 is added to that of the formidable surf which the wind has excited. 

 This is the principal cause of those terrible tidal '' races," no less 

 dangerous than earthquakes which roll over the neighbouring coasts. 

 During the hurricane of Barbadoes, in 1831, the waves which 

 broke against the northern promontory of the island were 72 feet 

 higher than the mean level of the water. At the great cyclone of 

 Calcutta, in October, 1864, the Hooghly rose 22 feet all along the 

 lower part of its course, and inundated several islands. More recently 



* Etude sur les Oiiragans de V Hemisphere Austral. 

 t On board the Duke of York, in 1833, at the mouth of the Hooghly. 



