536 



NATURE 



\Oct, 2 1, 1875 



actions to which they have been subjected. Hence, it is 

 argued, they must have been suspended in the air by 

 some force to allow time for these alternate actions to 

 take effect. Is the force in question not that of elec- 

 tricity? Let us suppose two clouds, superimposed the 

 one above the other, to be charged with opposite electri- 

 cities ; if the crystals of ice which are often to be met with 

 in the upper regions of the atmosphere happen to be in 

 the interval between the two clouds, they will be attracted 

 by the nearest, and thereafter repelled as soon as they 

 have received by contact the electricity with which it is 

 charged. Instantly being attracted by the other cloud, 

 they rush towards it and. are immediately charged with 

 the opposite electricity ; and this alternate play, during 

 Avhich the hailstones receive successive accretions from 

 the vapour abstracted Irom the clouds and congealed by 

 the cold of the original hailstone or of the space inter- 

 vening, will go on till the hailstones acquire a weight too 

 great for them to be any longer suspended, or till an elec- 

 tric discharge has destroyed the opposite electricities 

 which have accumulated on the surfaces of the clouds. 

 At this instant the hailstones fall to the ground by the 

 simple effect of their own weight. 



To the same cause the formation of waterspouts has 

 been attributed. Let us suppose a low cloud highly 

 charged with electricity and producing by induction on 

 the water of the sea a powerful accumulation of statical 

 electricity of the opposite sign on its surface. The mu- 

 tual attraction of these two electricities, the cloud on the 

 one hand, the sea on the other, while powerless to pro- 

 duce contact, will nevertheless give rise to two opposing 

 protuberances in the oppositely electrified bodies. At 

 that point the electricities will acquire a tension the 

 greater as the protuberances continue to assume forms 

 more elongated. As the attractive action goes on increasing, 

 these two protuberances will gradually approach each 

 other between the sky and the earth, and will ultimately 

 unite, the protuberance descending from the sky passing 

 over a greater space than the other. Then, by the con- 

 ductor thus quickly formed of water and an elongated 

 fragment of cloud, the electricity of the upper regions 

 will escape into the ground, exerting a destructive action 

 over all obstacles in its way. It is also to be noted that 

 the instant when the waterspout is thus completed, 

 thunder ceases to roll in the clouds, the reason being 

 that the electricity has found a silent mode of escape. 

 M. Peltier, the accomplished physicist to whom science 

 is indebted for many ingenious researches on atmospheric 

 electricity, endeavoured to reproduce in miniature the 

 phenomenon thus described ; but in bringing a highly 

 electrified conductor close to the surface of a sheet of 

 water, he was unable to show any other sensible mechani- 

 cal effect than a more rapid evaporation. 



We shall not, however, insist on the electrical theory of 

 waterspouts. The theory is now rejected, equally with 

 the electrical theory of hail, because if a few waterspouts 

 have exhibited traces of an electrical action, the greater 

 part of the observed facts show nothing of it. Water- 

 spouts and typhoons are mechanical phenomena, in which 

 electricity plays not the principal part, but a part alto- 

 gether subordinate. There was a time when the tendency 

 was to explain everything in meteorology by electricity. 

 Whenever any question became obscure, electricity was 

 resorted to as a convenient explanation, and any difficult 

 point was considered as cleared up by an adroit appeal to 

 some laboratory experiment — such as the explanation of 

 the theory of hail from the dance executed by pith-balls 

 between two brass plates. It came, however, to lie recog- 

 nised that, in seeking to identify meteorological phenomena 

 w-ith laboratory experiments, the risk was run of losing 

 sight of the real circumstances of nature and putting in 

 their stead those of the laboratory. The clouds of Volta 

 are real plates of brass, and the spark of the induced 

 conductors, as they are brought near each other, always 



forgets to manifest itself when the two fragments of water- 

 spouts unite together. 



2. Barometer. — The question of the barometer is more 

 difficult. The diminution of pressure which precedes 

 and accompanies cyclones has always been considered as 

 a proof of aspiration. It is certain that the continual 

 lowering of the barometric column — a lowering the maxi- 

 mum of which occurs in the very centre of the hurricane — 

 is a phenomenon so constant as to serve as an infallible 

 warning to sailors. In certain seas and at certain times 

 of the year, the sailor ought to keep his eye on the baro- 

 meter as much as the compass. But what is the signifi- 

 cance of this diminution of pressure ? Does it prove 

 that the air over our heads is rarefied.? If a vertical 

 column of air was rarefied, the equilibrium would be re- 

 established not at its lower part only, that is to say at 

 the expense of the lower stratum ; to effect this, a solid 

 envelope would be necessary to isolate the column through 

 its entire length, leaving only a free opening at its base. 

 But the column, on the contrary, being everywhere in 

 communication with the atmosphere, the equilibrium 

 would be quickly restored by a simultaneous afflux of the 

 strata at all heights, and not merely by the afflux of the 

 lower stratum alone. This, however, is not how things 

 take place. The diminution of the barometer does not 

 indicate a vacuum in the upper regions, but is the result 

 of a movement. Involuntarily, when we speak of the 

 barometer, we always regard pressure in the statical con- 

 dition, as if the atmosphere was constantly in equilibrium, 

 whilst in reality it is in continual motion. If there was 

 reason to believe that the different layers of air do not 

 mix in crossing each other, it could not be denied that the 

 aqueous vapour in its continual ascent from the ground 

 and the sea does not traverse the successive strata on its 

 way to the more elevated regions of the atmosphere, to 

 be there condensed into minute crystals of ice. And when 

 under the action of other causes, the whole strata of the 

 atmosphere flow almost horizontally, like vast rivers, 

 between strata absolutely immoveable, producing every- 

 where condensation of vapour and heavy rains, it is 

 doubtful if even the statical principle of the equality of 

 pressure could in every sense be employed ; the baro- 

 metric pressure and its variations ought no longer to be 

 interpreted from the statical point of view only, especially 

 if it arises from gyratory movements on a vast scale. 

 There is here a question belonging to the dynamics of 

 fluids which mathematicians have not yet explained ; but 

 in the meantime we ought not to forget the essential dis- 

 tinction between dynamical and statical pressure so as to 

 suppose that every rapid fall of the barometer indicates a 

 sudden rarefaction of air overhead, and consequently 

 aspiration from above downwards. 



3. Trade Winds. — The question of the trade winds is 

 connected with the preceding subject. If the air be con- 

 sidered only by itself, it will arrange itself in a state of 

 equilibrium, in homogeneous layers of varying densities, 

 which decrease with the height. These layers will be 

 bounded by ideal level surfaces enveloping the globe, and 

 which may here be regarded as spheres. If the action of 

 the sun, whose heat-rays are specially absorbed by the 

 lowest strata and by the aqueous vapour, is felt more 

 energetically over the torrid zone than in higher latitudes, 

 the inferior strata will expand, and pressing upv/ards will 

 raise the upper strata to a higher level. The equilibrium 

 being thus disturbed, it will tend to re-establish itself by 

 a general flow towards the coldest regions after the 

 manner of ocean currents, or like immense rivers which 

 have for their beds level surfaces, of which we are about 

 to speak. 



On the other hand, the temperate and cold zones re- 

 ceiving this overplus of air, their lower strata taking 

 a movement inverse to the above, set in towards the 

 large space of the equatorial regions where the density of 

 the air is less ; and leaving out of view the effects of the 



