« ΠροηγούμενηΣυνέχεια »
the elevating power is wholly due to the loss of specific gravity caused by expansion, and the heat emitted by the vapor as it condenses. Therefore, the barometer must fall to its lowest level in the column, or under the middle of the cloud. And such is the observed fact. Sir John Herschel, and afterwards Sir David Brewster,* objected to Mr. Espy's theory, that the inward rush of air ought to make the barometer rise in the centre of a storm. These distinguished philosophers had not given to the subject the attention it deserves. The converging winds endeavour to restore an already disturbed equilibrium ; but no sooner do they arrive at the point where
; they deflect upwards, than they also lose their gravity by expansion and calorification. A hạsty perception of Mr. Espy's misapplication of the principle of spouting fluids may have occasioned their mistake. This principle applies to the converging tendency of the surrounding air. Mr. Espy commits the error of applying it, also, to the ascent of the air in the column, which is owing to its buoyancy alone.
We have yet to show how Mr. Espy's theory explains the common operations of nature. As the force of the ascending current increases, together with the quantity of condensed vapor, the cloud will change from the fleecy cirrus, or massy cumulus, to the black and threatening nimbus, when the moisture will be precipitated in the form of rain. The more elevated drops of rain may be forced up by the powerful draught, and be discharged from the annulus into the clear, cold air, whence, being frozen, they will descend to the earth as hail. As the centre of the spout passes over the ground, its lifting power may take up light substances, which will afterwards reappear in those showers of dust, grain, &c., which have excited the curiosity of the philosopher, and the terror of the superstitious. Į If the air is very hot below, while no cross currents disturb the perpendicular elevation of the spout, and the whole atmosphere is highly saturated with vapor, then the inward and upward motions will
* Edinburgh Review, No. 138.
# A shower of grain took place in 1840, at Rajket, in Kattywar. The nalives flocked to Captain Aston, to ask hís opinion of this phenomenon ; for not only did the heaven's raining grain upon them excite terror, but the omen was aggravated by the fact, that the seed was not one of the cultivated grains of the country, but entirely unknown to them.
be very violent. The rapid diminution of pressure will lower the point of condensation until the base of the cloud touches the earth. In this case, we have the phenomenon known as a tornado on land, or a water-spout, at sea. this, the rarefaction of the air is extreme ; passing over a building, it may cause a sudden expansion of the air within, which will blow out the sides and roof. If a tornado should be arrested in its progress by meeting a mountain, it would disgorge at once its accumulated weight of water and ice in one immense flood, that would cut its way down the side of the mountain, carrying with it the loose earth and stones, and leaving a deep gully to mark the track of its ruin. This is Mr. Espy's mode of accounting for the meteoric waterfalls, one of which overwhelmed the unfortunate Willey family in the pass of the White Mountains. As the air spreads out more rapidly above than it runs in below, the causes whieh occasion the first formation of a cloud, may enlarge the circuit of their influence, so as finally to embrace a region of considerable extent, the violence of their action increasing with its time and extent ; the meteor then becomes a storm, or hurricane, whose motion, en masse, is governed by the pres dominating upper currents.
We must content ourselves with these general outlines of Mr. Espy's beautiful and original theory, regretting that want of space alone, and by no means a want of respect for its merits, compels us to withhold the further development of its details. “ This beautiful mechanism, whether it has been adopted by nature or not, is most worthy of her, and Mr. Espy may honorably be proud of its conception. The reception of this theory, by the French Academy, amounted to applause. A report upon it was made by a committee consisting of MM. Arago, Pouillet, and Babinet, names eminent in physical science, who, regarding Mr. Espy's facts as fully confirmed by his documents, and confiding in his observations, accepted his hypothesis in terms of the highest approbation. One qualification, and one prolific of doubts, was admitted ; it was, that Mr. Espy had not taken
; into account the effect of electricity in these phenomena. Mr. Espy's “ Paper
Paper on Storms was read before the British Association, in September, 1840. It excited uni
* “ Espy's Theory of Storms,” by Professor Peirce. Camb. Miscellany, No. 3.
versal interest, and his lecture room was so thronged, that numbers were obliged to turn from the door, after a vain attempt to procure admission. Mr. Espy has subjected his theory to the rigid test of mathematical analysis, by computing, from the data of Dalton and Daniell, the amount of heat given out by the condensed vapor, the decrease of density in the ascending column, the effect of the diminished pressure upon the barometer, and the force and velocity of the upward motion. Among other objections made at the meeting of the British Association, it was stated by Professor Forbes, that these calculations were erroneous. We should have been content, without further examination, to reply, that the committee of the French Academy pronounced them “tres suffisamment exactes," had not Professor Peirce of Harvard University repeated them in one instance, and proved, that "there are cases in which rain does fall, and in which Mr. Espy's uprising columns cannot be sustained ;" from which he concludes, that “ his views do not embrace the whole subject, and some other mode of accounting for the continuance of the rain remains to be ascertained.»*
Professor Forbes further objected, that the central funnel or spont would be insufficient to vent the air, rushing into it. An upward motion, however, of ten miles an hour, in a space of moderate dimensions, such as is indicated by the calm in the centre of a storm, would give vent to air rushing towards it from every direction with the velocity of one hundred miles an hour. But this reply to Professor Forbes injures Mr. Espy's theory of the formation of hail. Mr. Arch Smith's remark, that a rotary motion must inevitably result from a centripetal one, unless we imagine the infinite improbability, that all the radial forces precisely counterbalance each other, is so plainly justified by an elementary principle of dynamics, that we wonder it escaped Mr. Espy's penetration. But to objections of a theoretical character, like these last, Mr. Espy answers by an appeal to facts, and the recorded phenomena of nature. Taking facts, then, as our argument, we intend to add a few comments.
In order that a theory may possess the requisite degree of plausibility, it ought not only to be correct in its fundamental
principles, but sound and accurate in its deductions ; not only symmetrical in its general structure, but perfectly harmonious in its agreement with authentic observations. The primary principle of this theory leads its author to maintain, that artificial fires, by creating an ascending stream of heated air, under favorable circumstances, may produce rain. In the spirit of an adventurer rather than a philosopher, he offers to display this feat, if a promise of sufficient reward be given in case of success.*
But the statements with which Mr. Espy enforces his opinions point also to the generation of electricity, by chemical decomposition arising from combustion, as a probable cause of the shower. If his theory were strictly correct, we might expect, that the heat communicated to the atmosphere by the combustion of fuel, and by the animal respiration in great cities, would prove a frequent source of rain.t Yet the table of Mr. Dalton shows, that less rain falls at London, Liverpool, and Manchester, than in the country. The amount at London is eight inches below the mean of Great Britain, as determined by Dalton. The meteorological observations throughout the Philosophical Magazine show the same result. One table exhibits the quantity of rain, for a series of years, in more than one hundred places in both hemispheres. The mean annual amount of the whole table is about 42.5 inches. The annual amount at London, according to Howard, is 25.2 inches ; according to Daniell, 22.2 inches ; and according to Dalton, from forty years' observation, 20.7 inches. The annual amount at Paris is 19.9 inches. We have already shown, that the centripetal tendency of the winds in a great storm, advocated by this theory, is contradicted by the facts. The whirling motion in whirlwinds, waterspouts, and tornados, also, is proved by the testimony of accurate witnesses. Mr. Espy thought so in 1833, before he had contrived his theory; then he said, “When a waterspout occurs, it is acknowledged on all hands, and by every observer, that it is attended by a whirlwind.”! We may refer particularly to the descriptions of that able seaman and hydrographer, Horsburg, who, having prepared his ship, carried her through the centre of several of
Espy, p. 497. † Mr. Hutchinson's Objections, Espy, p. 463. † Franklin Journal for Nov. 1833. VOL. LVIII. No. 123.
them.* The accounts of Captain Beechey, the report of the tornado that sunk the steamboat Tigris in the river Euphrates, on the 21st of May, 1836, and the observation of Colonel Reid, are distinct and explicit upon this fact. M. Pellis, professor of mathematics in the college of Sainte Foy, describes a tornado that happened at Flanjaques. It turned a mill round, and twisted the trees, until their tops were forced round almost the entire circle. Several were noted in the last century, at the lake of Geneva, and Limay in France, of a similar character.§ And Mr. Espy, in his extracts from M. Peltier, has included a very striking instance of one, which devastated the communes of Fountenay and Châtenay, (near Paris,) during which "there arose a frightful whirlwind of dust, and light bodies." || The same remark applies to the hurricane of Stow, described by Professor Loomis, to the New Haven tornado, by Professor Olmsted, and the Providence tornado, by Mr. Redfield. The only exception of weight is the New Brunswick tornado, the traces of which were carefully examined by Professors Bache and Henry. They did not witness the tornado itself. Their conclusion was, "that the effects indicated a moving column of rarefied air, without any whirling motion, at or near the surface of the earth." We shall not endeavour to set aside these high authorities, or take any thing from their valuable opinions in this particular case. Still, the whirling motion in the others remains to be accounted for, upon other principles than those of Mr. Espy's theory. But for authorities, we may cite the distinguished names of Pouillet¶ and Despretz,** both of whom speak of tornados as characterized by a whirling movement.
The most minute and satisfactory relation of a whirlwind is to be found in a letter from Dr. Franklin to Peter Collinson. Franklin accompanied it three quarters of a mile; he found the circular motion amazingly rapid, bending some tall trees round in a circle swiftly, and very surprisingly." This whirlwind, the tornado of 1741 at Lake Geneva, and that quoted from M. Pellis, were unattended with rain or
Reid, p. 9.
Comptes Rendus, Vol. I. Histoire de l'Académie, 1741, p. 20; 1742, p. 26; 1764, p. 32. Espy, p. 357. Elémens de Physique et de Météorologie. **Traité Elémentaire de Physique, 4th ed. p. 865. tt Franklin's Works, Sparks's ed. Vol. VI. p. 201.
Idem, chap. 11.