&5 



closely, but is pressed back towards the south by the influence of the first; 

 traverses Germany on the 17th, arrives at the Mediterranean on the 18th, 

 and prevails there till the 21st. The way is opened anew towards the south, 

 and from that date till the 10th foul weather is scarcely ever intermitted 

 OJi the coasts of Portugal. 



In proportion as instances are multiplied, we become more and more«per- 

 Buaded that there can be no great tempest in our regions without rotary 

 movement, whatever be the cause which produces it. The velocity of the 

 air, except under some local circumstances, scarcely ever surpasses that of 

 a wind rather strong (assezfort,) which mariners call a stiff gale. But where, 

 in the midst of this mass of air in motion there happens to be produced a 

 whirling movement around a centre more or less vertical, there occur, on 

 the periphery of the space occupied by the phenomenon, points where the 

 velocity of rotation is opposed to the velocity of general tranfslation, and 

 at these points the velocity of the wind may seem to be arrested; but there 

 occur also, on the other hand, regions where these two velocities have the 

 same direction, and by re-enforcing one another produce winds of an extreme 

 violence. The rotary movement has then as a result the diminution of the 

 velocity of the wind at certain points, and its translation to the opposite 

 points whei-e it will consequently be in excess. 



Under ordinary circumstances cyclonic storms participate in the general 

 movement of the mass of air in which they originate. They cannot 

 deviate from it, excepting when they find a point of resistance in the pro- 

 jections of the surface, or in some neighboring cyclone ( tourbillon ) . Now, 

 we are approaching the time when our northern hemisphere will begin to 

 grow warm, while the southern hemisphere will be becoming cold. Tlie ten- 

 dency of the air from the equatorial regions towards our own, under the 

 form of the upper trade-winds between the equator and the north tropic, and 

 under the form of winds from the southwest beyond that region, will go on, 

 therefore, diminishing, while the translation of the air, under the form of 

 northeast trade-winds, will assume, on the contrary, more activity, in order 

 to furnish to the southern hemisphere the excess of which it will have need. 

 The orbit described by the general currents has commonly less prolation to- 

 wards the north in spring than in autumn; it ascends not so high towards 

 the pole, penetrates less deeply into the east of the continent, and is more 

 readil}^ modified by the prominences of central Europe. We are entering on 

 the season of gusts of wind. Under these conditions of the atmosphere 

 accidental deviations, more or less considerable, may be produced as the 

 effect of changes of temperature, and, above all, great condensations of va- 

 p ')X which accompany those gusts. Such are the accidents which diversify, 

 without end, the meteorological conditions of each day. 



To what cause should we attribute these great cyclonic movements of the 

 air ? Many theories have been advanced and sustained by high scientific 

 authority. The contact and struggle of opposed currents, the eartli's move- 

 ment of rotation, the convergence of the meridians, atmospheric electricity, 

 great condensations of vapor, and the rapid refrigeration which accompanies 

 them, have been, by turns, invoked. The very diversity of opinions main- 

 tained by eminent mcteoi-ologists shows how much obscurity still rests 

 upon the question, and how much new researches are required The first 

 point to be solved, is to learn whence come the cyclones, for it is by being 

 present, so to say, at their origination that we shall be able to discover the 

 mechanism of their formation. This is a question of time. In a practical 

 point of view, there is another of more pressing interest : to seek, by all 

 possible means, every indication which may enable us to foresee the arrival 

 of a storm, its intensity, and the path which it will pursue. The discussion 



