from friction at the surface of the earth, and from the reaction of the denser medium constantly becoming greater, the centrifugal force is weakened, and when it is reduced so much that the inward pressure gains the ascendancy, as it must do in the end, the spiral revolution is established. The greater height of the barometer at the borders of the storm than in the centre, verifies the existence of an increased density. Mr. Redfield has found a warm opponent in Dr. Hare, of Philadelphia, whose strictures also extend to Professor Dove. As he has chosen to misunderstand and misapply the Professor's expression of a "rotary cylinder," and to insist accordingly, that the outer portion of the storm should be found to move with the greater rapidity, as if the expression were intended to represent the storm as a solid of revolution, he has spared us the trouble of an argument. Hare's objections are chiefly of a speculative kind. The best answer to them is to be found in the judicious language of Mr. Redfield. Dr. "If the supporters of a rotative or whirlwind action in tornadoes and hurricanes had chosen to maintain their cause in a speculative manner, the case might have been different. But when their facts and results were offered on the basis of direct observations, which had been set forth in many cases, with particularity and precision, it seems like a waste of words to assail these observed phenomena and results with strictures and objections of this character; volumes of which can never equal in value the direct observations which may be made of the phenomena of a single storm." Before we conclude this division of our subject, we must advert to the rules drawn up by Mr. Redfield for the government of the seaman when he encounters a revolving storm.§ They are prefaced by observations explaining its principal phenomena, and such general features of it as appear to be established. They are the honorable and valuable results of Phil. Mag. Vol. XIX.; also Silliman's Journal. t Silliman's Journal, Vol. XLIII. Ibid., Vol. XLIV.; Notice of Dr. Hare's "Strictures," &c. Mr. Redfield, in his "Meteorological Sketches," (Silliman's Journal, Vol. XXXIII., No. 1,) has presented some theoretical views concerning the prevailing cur rents of the atmosphere, which will not, we apprehend, receive the entire sanction of the scientific community. § Silliman's Journal, Vol. XXV.; Blunt's Coast Pilot. arduous and successful investigation. A certain degree of intelligence is requisite for their proper application, but they are simplified as far as the topic will admit. That they will prove beneficial to navigation, we have no doubt. The mariner will learn from them to avoid a storm, or, being involved in one, to escape from the destructive violence of the central portion. Surely, that person is to be ranked among the benefactors of mankind, who has lessened the perils of the sea, and added to the security of commerce. It remains for us to discuss Mr. Espy's theory. Before doing so, it will be just to make a hasty review of some of those principles established by the skilful experiments of Dalton, Daniell, and others, upon which it is founded. The experiments of the former upon the rarefaction and condensation of air in the receiver of an air-pump, similar to those made by Mr. Espy in his nephelescope, led him to discover the effect of the latent heat of vapor in raising the temperature of the atmosphere. The conditions of the experiments were varied, by using air in the different stages, from extreme dryness to a state of saturation; the thermometer suspended in the middle of the receiver experienced a fall, which was at its maximum with the dry air, and decreased according to the increase of moisture. Dalton has stated this result as follows: "Suppose that a part of the atmosphere contained one fiftieth of its weight of aqueous vapor, and that three fifths of this vapor, that is, one hundredth of the whole elastic mass, was converted into water, then the heat given out would be sufficient to raise the temperature of the remaining mass of air and vapor six or eight degrees.' He also estimated the change in the bulk of the atmosphere due to a change of temperature. Mr. Daniell enlarged our comprehension of the subject, by showing the elevation necessary to the condensation of vapor at certain temperatures, and the manner in which the admixture affected the gradation of temperature resulting from the decreasing density of the upper parts of the atmosphere. " * "The steam, [the slow and silent influence of which Daniell considers as one of the mainsprings of all the wonderful motions of the air and changes of the weather,t] as it reaches its * Manchester Memoirs, Vol. V., Old Series, pp. 523–525. Meteorological Essays and Observations, p. 263. point of condensation, must give out its latent heat, and during its precipitation, combining with a fresh proportion, it again ascends, and again evolves in the middle regions. It may thus be considered as carrying caloric from the surface of the sphere to the higher strata; and it is obvious how a considerable section of any one column may thus have its temperature equalized and fully saturated with aqueous particles. The currents thus become affected, both by the expansive powers of the vapor, and of the extricated heat."* Thence it follows, that the condensation of elastic vapor into clouds raises the temperature of the air; and this theoretical and practical conclusion is confirmed by the observation of M. de Luc, who found the thermometer to rise in a cloud. He noticed, also, that as the cloud became larger and more dense, it increased in elevation.† An accumulation of aqueous vapor, and a consequent unequal expansion of the atmospheric columns, are accompanied by a decrease of density, and are made known by a fall of the barometer. A constant stream then rushes in with increasing force, augumenting by its condensation the cause of its velocity. ‡ Mr. Dalton, and afterwards Professor Leslie, demonstrated, that the mean temperature of the atmosphere diminishes from below upwards, in a regular gradation. This diminution has since been determined to be about one degree for every one hundred yards of ascent. Mr. Daniell discovered, that the elasticity of aqueous vapor does not decrease with the gradual decrease of the temperature and density of the air, as we ascend, but remains stationary at great heights.§ He also accounted for the apparent permanency and stationary aspect of a cloud. Clouds amongst mountains, and upon hills by the sea-side, appear to be perfectly immovable, although a strong wind is blowing upon them. The fact is, that the vapor, wafted by the wind, is precipitated by contact with the cold mountain; and is urged forward in its course till, borne beyond the influence which caused its condensation, it is again exhaled, and disappears; and this process must also take place on either side of the planes of precipitation in the atmosphere, and give an apparently fixed position to clouds in the free air. Gay-Lussac has also ex Meteorological Essays and Observations, pp. 77, 67, 129. t Ibid., pp. 101, 105. Ibid, pp. 111, 112. § Ibid., p. 128. Ibid., pp. 123, 124, and Man. Mem. Vol. V., New Ser. plained the same phenomenon by the constant ascent of warm currents during the day. The theory of the formation of heavy masses of dense clouds, the cumuli, for instance, seen in fine weather, which sometimes accumulate in piles of stupendous magnitude and beauty, deeply shaded at the base, and terminating in lofty summits of silver lustre, is given by Daniell.* The sun dissolves the morning vapors, and, during the heat of the day, they rise from the surface of the land and waters, and reach their point of condensation in greater or less quantities at different altitudes. "And now the mists from earth are clouds in heaven; Sublimer than a storm." We have no room to devolope this hypothesis, but it is adopted by Mr. Espy, as one of the most striking illustrations of his theory. Dalton and Daniell had also provided invaluable helps to the meteorologist, in their very complete tables, exhibiting the specific gravity, elasticity, temperature, dew point, &c., of the atmosphere, at different altitudes, for every ten degrees of latitude from the pole to the equator. Mr. Espy's combination of the foregoing facts and principles is equally novel and ingenious. The first formation of a cloud is occasioned, he conceives, by the uprising of a column or stratum of air from the surface, where it has become more heated, or more highly charged with aqueous vapor, and thus specifically lighter than the air in its vicinity. As this column ascends, it is cooled, by the expansion due to diminished pressure, at the rate of about one degree for every one hundred yards of ascent, allowance being made for the corresponding decrease in the elastic force of the vapor. If it rise high enough, the cold thus produced will condense some of its vapor into cloud. The highest temperature, at which this can take place, is the dew-point; and the height, therefore, which the uprising column must attain, before this condensation can occur, or the cloud be formed, depends upon the difference between the temperature of the air and the temperature of the dew-point, one degree of this difference answering to one hundred yards of altitude. As soon as the cloud begins to form, the latent caloric of the condensed vapor, being liberated, raises the temperature of the air in the column, and prevents it from cooling so fast in its further progress upward as it did to that point, keeping it constantly at a higher temperature, and consequently less heavy than the air of the surrrounding region. An upmoving power is thus created, and rapidly increased by the latent heat, or caloric of elasticity, of the vaporous portion of the atmosphere in the column. This may be regarded as the first step of the process, and Mr. Espy thinks, that it is going on simultaneously in other parts of the atmosphere for many miles round, as appears from the number of detached clouds existing at the same time. All these columns have a tendency to approach and unite into one large cloud. A mechanical disturbance of the atmospheric equilibrium has now taken place, which nature will make efforts to restore. First, there must be a new accession of air towards the rarefied column, increasing its upward steam power; secondly, the air, carried into the upper parts, is to be disposed of. The necessary supply is afforded by the rush of air on all sides towards the centre of the ascending column; and here we have Mr. Espy's converging winds. We speak of this as the second step. The third is the disposition of the air in the upper part of the column. The great expansion of the air in the newly generated cloud will cause it to spread outward in all directions above. The outspreading of the air will form an annulus all round the cloud, in which the air will sink from its greater weight, and increase the velocity of the wind near the surface of the earth towards the centre of the column; whilst, on the outside of the annulus, the pressure will cause a gentle wind outwards. The upmoving currents are now supplied by the air within the annulus, and that which descends in the annulus itself. Any general currents, that may exist at the time, will modify these motions by their transverse action, and may prevent the formation of the cloud, by breaking off and carrying away the highest portion of the column before it has reached the plane of condensation. These are the fundamental propositions of Mr. Espy's theory, and in connexion with them, one important fact is to be specially noted. The ascending column originates in the smaller specific gravity of a stratum of air. As the column rises, the specific gravity continues to diminish according to the elevating power; or rather, it is more proper to say, that |
