Page images
PDF
EPUB

charged with aqueous vapor* than others, are thus made specifically lighter, and consequently rise, and when the dew point is high, these upmoving currents do not find their equilibrium until they are sufficiently expanded by the diminished pressure to which they are subjected to re. duce their temperature to the point of forming dew, when a cloud will be. gin to appear.

The reduction of temperature which would thus be produced by the expansion of ascending air, Mr. Espy finds by experiment to be about one degree for every one hundred yards of ascent; and hence, if an upmoving current of air is ever produced in the operations of nature, it is easy to calculate how high it must rise before it begins to condense its vapor into visible cloud. For example: if, in a summer's day, the ther. mometer stands at 80°, and the dew point is 70°, then air must be cooled 10° before it will begin to condense its vapor into cloud. Consequently, if it cools one degree for every one hundred yards that it rises, then when it attains an elevation of ten hundred yards, it will be cooled down to the point of forming dew, when its vapor will begin to condense, and the base of a forming cloud become immediately visible. The bases of all forming clouds in the same neighborhood should therefore be nearly on the same level.

Again : it is known to every chemist that vapor cannot be converted into water, without releasing a large quantity of caloric, known in technical language as the caloric of elasticity, and thus producing a considerable amount of sensible heat. If ice is exposed to heat, caloric combines with it and forms water; if water is exposed to heat, caloric combines with it and forms steam or vapor; and when vapor is converted back to water, this caloric (heat) must necessarily be released; and, according to Mr. Espy, its agency in producing wind, rain, hail, barometric fluctuations, and all the sublime and astonishing phenomena which attend our most violent storms, has hitherto been altogether overlooked. He finds, by calculating according to well known chemical laws, that the caloric of

elasticity released during the condensation of vapor while a cloud is forming, will expand the air in the cloud about eight thousand cubic feet for every cubic foot of water formed by the process of condensation.

The expansion of the air in a cloud during the formation of water, is also proved by an instrument which Mr. Espy uses, called a Nephelescope, or cloud examiner. It consists of a glass vessel [b.] communicating with a bent tube [c.) containing mercury, and having a forcing pump [a.] attached to it, by means of which any desirable

quantity of air may be pressed into the receiver 6 or glass vessel [b.] When the instrument is

charged, the pressure on the inner leg of the mercury forces it up in the outer, and by care. fully measuring the difference between the two, a given amount of pressure can be produced. When the air within (which is heated by the pressure) acquires the temperature of the air

[graphic]

* Vapor is five eighthe the specific gravity of air.

without, the stop-cock is turned and the air permitted to escape until the mercury in both legs of the bent tube is on a level, when the stop is again closed.

Now as the stop is closed at the moment the greatest cold is produced by expansion, the mercury in the outer leg will begin to asccnd, and that in the inner leg to descend, and the difference of level at which they settle will indicate the reduction of temperature produced by a given expansion. But what the general reader is chiefly concerned to know in this experiment, is the fact that when moist air is used, and a cloud is formed in the receiver, the mercury in the outer leg of the bent tube is forced up higher than when dry air is used and no moisture is condensed, showing that the caloric of elasticity causes the air to occupy much more space when it is set free than when it is united to water in the form of vapor. *

If this is true, and it seems to be placed beyond a doubt, then the air within a cloud is both lighter and warmer than that by which it is surrounded. That it is warmer is proved by actual observation as well as by Mr. Espy's experiments. Sausseur tells us that when he was enveloped in a cloud on the side of a mountain, his thermometer rose higher than in the sun ; and both Durant and Gay-Lussac note the same fact while passing through clouds in a balloon. The uniform depression of the barometer under large clouds and during all our great storms, would seem also 10 confirm Mr. Espy's other position, and place beyond a doubt the fact that the air in the cloud is warmer, and therefore lighter than the sur. rounding atmosphere.

If, then, a cloud can be formed by a current of air moving upwards, and the cloud thus formed is lighter than the circumambient air, it necessarily follows that the equilibrium of the atmosphere must be more or less disturbed by every formation of this character. For if a lofty cloud by the evolution of its latent caloric, makes the air within it warmer and lighter, then will the air around it rush from all sides towards its base, and upwards into its centre; and as the wind in its upward course comes under less pressure, it will become gradually colder until it reaches the tempera. ture of the dew point, when it will begin to condense its vapor, thus feeding the cloud with fresh materials for its expansion and perpetuity, and communicating to it, as it were, a self-sustaining power by which it moves on perhaps for days together, as we often behold in the operations of nature, enlarging as it advances, causing high winds wherever it passes, and fertilizing the earth with its refreshing showers.

“ When a cloud begins to form from an ascending column of air, it will be seen to swell out at the top, assuming successively the appearances of 1, 2, 3, generally called cumuli : or, if the upmoving current should be driven out of its perpendicular motion by an upper current of air, the clouds which might then form would be ragged and irregular, called broken cumuli, as 4. These will always be higher than the base of cumuli, but much lower than cirrus. While the cloud continues to form and swell up above, its base will remain on the same level, for the air below the base has to rise to the same height before it becomes cold enough, by diminished pressure, to begin to condense its vapor into water; this will cause the base to be flat, even after the cloud has acquired great perpendicular height, and assumed the form of a sugar loaf. Other clouds, also,

* When dry air is used in the experiment, the temperature, according to Mr. Espy, is reduced about twice as much as when moist air is used.

for many miles around, formed by other ascending columns, will assume similar appearances, and will moreover have their bases all on the same or nearly the same horizontal level ; and the height of these bases from the surface of the earth will be greatest about two o'clock, when the dew point and temperature of the air are the greatest distance apart.” 1 2

3

[graphic]

“ When upmoving currents are formed by superior heat, clouds will more frequently begin to form in the morning, increase in number as the heat increases, and cease altogether in the evening, when the surface of the earth becomes cold by radiation. The commencement of upmoving columns in the morning, will be attended with an increase of wind, and its force will increase with the increasing columns; both keeping pace with the increasing temperature. This increase of wind is produced partly by the rush of air on all sides at the surface of the earth towards the centre of the ascending columns, producing fitful breezes; and partly by the depression of air all round the ascending columns, bringing down with it the motion which it has above, which is known to be greater than that which the air has in contact with the asperities of the earth's surface. The rapid disturbance of equilibrium, which is produced by one ascending column, will tend to form others in its neighborhood; for, the air being retarded on the windward side, will form other ascending columns, and these will form other annuli, and so the process will be continued."

But, it may be asked, if the air in a cloud is lighter than that which surrounds it, and in consequence possesses a self-sustaining principle, why all forming clouds do not increase till they produce rain ? We shall answer this question by another quotation from Mr. Espy's book. In his introduction, on page 16, he says: “Neither can clouds form of any very great size, when there are cross currents of air sufficiently strong to break in two an ascending current, for the ascensional power of the upmoving current will thus be weakened and destroyed. Immediately after a great

rain, too, when the upper air has yet in it a large quantity of caloric, which it received from the condensation of the vapor, the upmoving columns which may then occur, on reaching this upper stratum, will not continue their motion in it far, from the want of buoyancy; therefore, they will not produce rain, nor clouds of any kind, but broken cumuli. Besides, as the

air at some distance above the surface of the earth, and below the base of the cloud, is sometimes very dry, and as much of this air goes in below the base of the cloud and up with the ascending column, large portions of the air in the cloud may thus not be saturated with vapor, and, of course, rain in this case will not be produced. These are some of the means contrived by nature to prevent upmoving columns from increasing until rain would follow. Without some such contrivances, it is probable that every upmoving column which should begin to form cloud when the dew point is favorable, would produce rain, for as soon as cloud forms, the upmoving power is rapidly increased by the evolution of the caloric of elasticity.”

The cloud which produces water-spouts, land-spouts, and tornadoes, differs somewhat from other clouds, and can be formed only when the dew point is very high, the atmosphere devoid of cross currents, and the air in the neighborhood comparatively quiet, or rather, moving in the direction of the main current above. When these circumstances concur, and a cloud begins to form by an ascending column, there is nothing to prevent its rapid generation, and it shoots upward to a vast height, while it occupies only a small space in a lateral direction. The effects which follow the generation of such a cloud, must necessarily be more or less violent, because the whole force of the cloud is spent on a very small space. Extending upwards to a great height, and being lighter than the surrounding atmosphere, it takes off from the air below much of its accustomed pres. sure, and the wind consequently presses in towards its base from all sides, and rushes up into the cloud itself with fearful velocity, carrying with it all light substances, uprooting trees, bursting off the roofs of houses, barns, and other buildings, and sometimes lifting into the air heavy timber, animals, and in one instance which we recollect, a cart loaded with potatoes.

As the cloud is small in circumference, and is moved forward with considerable velocity by the main current in the higher region of the atmosphere, its progress brings it suddenly over the place which is to be the scene of its devastation ; the accustomed pressure of the atmosphere is removed almost instantaneously; the barometer falls sometimes as low as two inches in the course of a few minutes, and the effect is analogous to that of an explosion. H. Tooley, who communicated to the secretary of the Albany Institute an account of the Natchez tornado, which took place on the 7th of May, 1840, has called particular attention to this last mentioned circumstance, and cited the following strong cases.

“ 1. The garret of a brick house occupied by Thomas Armat, Esq., as an office, was closely shut up, both ends bursted outward, and such was the force of the explosive power, that some of the bricks of the windward end were thrown upon a terrace nearly on a level with the end, and at a distance of not less than twenty feet in the face of the storm.

“ 2. A brick house on the north side of Main street, belonging to John Fletcher, had the leeward gable end thrown out, the windward end remaining uninjured.

“3. The windward gable end of a large house adjoining the Commercial Bank, bursted outward against the face of the storm ; the leeward end was uninjured.

“ 4. The gable ends of a large three story brick house on Franklin street, owned by Rowan and Cartwright, were thrown outward with great force.

“5. The front ends (leeward to the storm) of two brick stores ownea by Eli Montgomery, were thrown outward with great force, the windward ends being uninjured.

“6. Another large brick house, near the last just mentioned, owned by Watt, Burke & Co., had the leeward side nearly demolished.

7. Another brick house adjoining the last mentioned, had the wind. ward gable end thrown outward.

8. The Theatre, a large brick building, had the entire roof blown off and thrown some ten feet forward, and the walls demol. ished.

“9. The leeward walls of two front rooms of the Tremont House on Wall street, were thrown outward with great force, without destroying or moving the furniture therein, and where the storm could have no access.

“10. The roof of the fire-proof brick office of the Probate Court, exploded to windward, that side, it is presumed, being the weakest.

“11. The gable ends of a large brick store on Main and Pearl streets, were thrown outward with great force.

“ 12. The southern side, and the northern and western gable ends of the brick Insurance buildings on Pearl and Market streets, were thrown outward with such force as to nearly demolish the building.

“13. The roof of Dr. Merrill's house on State street was saved by the explosive power bursting open a large trap door in the roof, thereby making an outlet for the expanded air.

“14. The leeward wall of a new wooden house owned by Rhasa Parker, on Washington street, was thrown outward by the explosive power, the windward side end remaining unbroken excepting the glass of the windows.”

Professor Johnson in his description of the New Brunswick tornado, which occurred on the 19th of June, 1835, has called attention to the same curious fact. He says: “In a few cases, in which the ridge of a building lay in a northerly and southerly position, the eastern slope of roof was observed to be removed, or at least stripped of its shingles, while the western slope remained entire. Many buildings were likewise observed with holes in their roofs, whether shingled or tiled,

but otherwise not much damaged, unless by the demolition of windows. These appearances clearly demonstrated the strong upward tendency of the forces by which they were produced, while the half unroofed houses, already mentioned, prove that the resultant of all the forces in action at the moment was not in a perpendicu. lar to the horizon, but inclined to the east. Such a force would apply to the western slope of the roof some counteracting tendency, or relieve it from some portion of the upward pressure. Had there been no other facts to show the powerful rushing of currents upward, the above would, it is conceived, have been sufficient to settle the question, but taken in connection with the circumstance that roofs so removed, were carried to a great height, and their fragments distributed over a large extent along the subsequent path of the storm, that beds and other furniture were taken out of the upper stories of unroofed houses, that persons were lifted from their feet or dashed upward against walls; and that in one instance, a lad of eight or nine years old, was carried upward and onward with the wind, a distance of several hundred yards ; and particularly that he afterward descended in safety, being prevented from a violent fall by the upward forces, within the range of which he still continued. In connection with

« PreviousContinue »