METEOROLOGY. 



571 



radiators of heat are the best vaporizers of 

 moisture. The evaporation from a given sur- 

 face of still water appeared to be | that of an 

 equal surface of " calico " (i. e ., cotton cloth) ; 

 while the latter, which vaporizes better than 

 flannel, has very nearly the same power with 

 woollen cloth, unsized paper, bran, or fine 

 sand. 3. The evaporation from equal surfaces, 

 of the same or nearly the same material, in a 

 quiescent atmosphere, is very nearly the same 

 in amount, whatever inclination the surfaces 

 may have; as, whether upward, or downward, 

 &c. This tact would appear to show that vapor 

 does not. as has been supposed, pass from the 

 evaporating surface by force of an ascensional 

 current, except so far as it may at the same 

 time be aided by a greater heat in the source, 

 but that it is in reality carried into the air in 

 the way of diffusion. 4. The rate of evapora- 

 tion is greatly affected by the elevation at which 

 the damp surface is placed above the level of 

 the ground ; and damp fabrics close to the earth 

 may, when the sun's rays are feeble or wanting, 

 even gain moisture by absorption, while similar 

 ones a few feet above them are losing moisture 

 in vapor. 5. The rate of evaporation is affected 

 by the radiation of surrounding bodies ; so that 

 even a screen at a little distance above a damp 

 surface caused the evaporation to go on from it 

 more rapidly than from similar surfaces at the 

 same time not exposed to such source of radi- 

 ation. 



Dalton s Theory of an Independent Vapor 

 Atmosphere questioned. Prof. Lament, of Mu- 

 nich, in a recent letter to the distinguished 

 meteorologist, Prof. Kamtz, calls in question 

 the long-received theory of Dr. Dalton respect- 

 ing the relations of the watery vapor of the 

 atmosphere to the air proper, or dry air. Any 

 dry gas flows into and diffuses through a space 

 occupied by another gas, in the same manner 

 as if such space were a vacuum. In accordance 

 with this fact. Dr. Dalton was led to consider 

 the actual atmosphere of the earth as a combi- 

 nation of as many distinct atmospheres as it has 

 gaseous and vaporous constituents, each of these 

 pressing only on itself, so that the actions of 

 each might be treated irrespectively of the 

 others. Meteorologists have since generally 

 adopted this view ; and in practice they have 

 endeavored to separate the pressure of the aque- 

 ous vapor from the whole barometric pressure 

 of the atmosphere, and thence to infer the sep- 

 arate " pressure of the dry air." 



To this question, so important not only 

 to a right investigation of the barometer, but 

 also in connection with general physics, as to 

 whether the aqueous vapor does, or does not, 

 form an atmosphere independent of the air, 

 Prof. Lamont addresses himself. Dalton's ex- 

 periments showed that the same amount of 

 water evaporates into a space filled with air as 

 into an equal one destitute of air ; and that, 

 while evaporation takes place rapidly into a 

 space void of air, it goes on very slowly in a 

 space filled with air not in motion. Those ex- 



periments in reality afford no information as to 

 the mutual relations subsisting between the 

 vapor and air : this deficiency Dalton supplied 

 by interposing the hypothesis already stated. 

 It has since been too much overlooked that the 

 idea of an independent vapor atmosphere stands 

 only as a possible, not as a necessary result of 

 the experiments on which it was based. Ob- 

 jections to the theory were, however, from 

 time to time presented, and especially by Bes- 

 sel, Brown, Jelinek, and Espy. 



Prof. Lamont believes that he himself first 

 brought forward a decisive proof of the incor- 

 rectness of Dalton's theory (Denlcschriften der 

 Munchner Akad., &c., 1857). He showed by 

 observations extending over many years that 

 the mean reading of the barometer stands quite 

 as high in cases of a small, as in those of a 

 great, vapor pressure. In the same place, he 

 described an experiment in which a mass of 

 vapor and a mass of air, placed in communica- 

 tion with each other, mutually preserve a state 

 of equilibrium, without the vapor penetrating 

 the air, or the reverse ; and he laid down the 

 proposition that the vapor of the atmosphere 

 exerts a pressure on the air proper, and the 

 Air on the vapor. He has more recently en- 

 deavored to furnish the experimental proof that 

 Dalton's laws themselves contain an essential 

 error. The reasoning which led to the new 

 experiments may be briefly stated as follows : 

 If a little water be introduced into the bottom 

 of an upright closed tube, and there gradually 

 evaporated into the air above it, then, on 

 Lament's theory, that the vapor and air exert 

 a mutual pressure, the expansive force of the 

 vapor and air at any moment will act on any 

 part of the interior of the tube by an amount 

 equal to the sum of the expansive forces of the 

 two ; and while the vapor is yet undiffused 

 through the air, its pressure, say upward 

 against the top of the tube, will be just as great 

 as after it has reached that surface. On the 

 contrary, if Dalton's theory be true, the vapor 

 must diffuse itself in the interstices of the air, 

 and with no mechanical effect, until it becomes 

 a confined body, that is, until it has reached 

 the top of the tube. The conditions here sup- 

 posed can be rendered sufficiently permanent 

 to admit of testing the principle, by an arrange- 

 ment in which the space furnishing the vapor 

 is kept at a higher, and that representing the 

 top of the tube at a lower temperature. To 

 effect this, a glass tube having at one end a 

 bulb, and a little above this bent at right 

 angles, is so placed as to extend horizontally, 

 and being farther on bent into a depending 

 U shape, it runs horizontally again from this, 

 terminating in an open end : into this last 

 horizontal part a globule of mercury is intro- 

 duced, to show, by the extent to which it is 

 pushed along the tube, the pressures at differ- 

 ent times exerted upon it by the confined at- 

 mosphere within the bulb, and included portion 

 of the tube. The diameter of the tube used, 

 was 1.1 Paris lines ; and a point extending from 



