NATURE 



yoct. 7, 1875 



dice, which is constantly receiving new life and vigour by 

 the persistent testimony of observers already prepossessed 

 in its favour. There is another reason equally good which 

 accounts for this mode of explaining phenomena. Of all 

 physical inquiries, the most difficult are those which 

 belong to the order of mechanics, which as little admit 

 of sentiment in dealing with them as pure mathematics. 

 In those fields of inquiry where pure mechanics can no 

 longer guide us, the crudest hypotheses take root and 

 grow : witness the wild dreams of the astronomers of the 

 seventeenth century. Now the department of mechanics 

 to which falls the exposition of the gyratory movements 

 of liquids and gases, and on which depend exactly the 

 atmospheric phenomena we speak of, does not yet exist, 

 except as a first and most imperfect draft. 



Taken thus at unawares, as it were, and compelled to rely on 

 evidence altogether illusory and suggesting unhesitatingly 

 the idea of aspiration on avast scale, modem meteorology 

 strives at least to free itself from conflicting impossibilities. 

 Thus, instead of making the waterspout suck up water in 

 its ordinary form, it is assumed that this water is first 

 blown into spray by the conflict of the winds at the base 

 of the waterspout, and then whirled aloft in this form. A 



curious experiment was even made in. 1852 at Washing- 

 ton, for the purpose of showing that this is the case. The 

 following account of it is taken from the " Fourth Me- 

 teorological Report to the Senate of the United States," 

 by Prof. Espy : — 



* The effect produced by the ascent of a column of air 

 in a narrow space may be thus shown : — If we produce a 

 simple rarefaction of two or three inches of mercury in 

 the upper part of a vertical tube a few feet in length and 

 five inches in diameter, by putting it in connection with 

 the central opening of a machine in full blast, the air will 

 rush into the tube by the lower orifice with a speed pro- 

 portional to the square root of the diminution of pressure, 

 or about 240 feet per second for an inch of mercury. 

 Then, if a basin filled with water is placed under the 

 opening of the tube and the surface of the water be 

 brought to about 7.\ inches from the end of the tube, the 

 water in the basin will be quickly sucked up and ascend 

 the tube, and produce in miniature what takes place in a 

 waterspout. If the tube is glass and of the same dimen- 

 sions, the water will be seen rising in spray in the form of 

 an inverted cone. This experiment was made in a foundry 

 at Washington in the spring of 1852, in the presence of 



Prof. Henry and several distinguished members of Con- 

 gress." 



It is singular that none of those present at this experi- 

 ment remarked the difference there is between a tube of 

 metal or of glass and an almost ideal tube whose bound- 

 ing surface is only thin insubstantial vapour. The expe- 

 riment is, however, a proof of the resoluteness with which, 

 in this age even, a belief in the powerful upward suction of 

 waterspouts is entertained. 



In order that an ascending current may take place in 

 the atmosphere for some seconds, it is essential that a 

 mass of a lower stratum of air be heated a little more 

 than the air surrounding it. It thus becomes lighter than 

 the layers above it, and consequently rises. In ascend- 

 ing, however, it expands and cools, and soon all further 

 ascent is arrested at a height where the pressure and tem- 

 perature equal the pressure and temperature of the 

 ascending mass. It is, moreover, replaced from below by 

 air of a lower temperature from all sides. Up to this 

 point there is little, if any, resemblance to a waterspout ; 

 there is, however, already the beginning of a movement 

 of ascension, and by means of some new additional hypo- 

 theses the phenomenon is completed by giving to it the 

 essential characteristics of a real waterspout. 



^^' -' S... 



Moist air ascends, it is affirmed, more quickly and to a 

 greater height than dry air. Prof. Espy maintains even 

 that it will rise till the limits of the atmosphere be reached 

 in this way : — Moist air in ascending expands and 

 becomes colder ; a portion of its aqueous vapour is con- 

 densed into mist, and the heat set free in the act of 

 condensation maintains the mass of ascending air con- 

 stantly at a higher temperature than the stratum of air 

 through which it is ascending. Some physicists consider 

 that these views, thus pushed to exaggeration, are erro- 

 neous, but the belief is pretty general, that " the heat due 

 to the condensation " of aqueous vapour is sufficient to 

 raise an ascending column of moist air to a much greater 

 height than an equal column of dry air. Be that as it 

 may, the result would be that when the layers of air rest- 

 ing on the ground are heated by the noonday sun and by 

 radiation, and above all by contact with the ground itself, 

 the equilibrium of the air is disturbed ; we should see con- 

 stantly appearing everywhere a stratum of mist obscuring 

 the rays of the sun. It is useless to point out that this 

 does not represent what takes place. We accept it, how- 

 ever, and proceed. 



If we advert to the phenomena of mirage, we find there 

 combined, according to the writers whose theory we are 

 expounding, all the conditions which favour the produc- 

 tion of a permanent local indraught of air, and conse- 

 quently the essential conditions of the waterspout. When 

 the air is perfectly calm and the soil highly heated, the 

 lowest strata of the air are highly heated and thus become 

 specifically lighter than the strata resting over them. But 



