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The expense of the bridges was as follows:
lodges, stations, &c. , 3800 “
ART. W.-On Evaporation. By. J. FREDERic DANIELL, Esq. F. R. S. M. R. I. &c. [Jour. Roy. Institut.]
The subject of evaporation has occupied, at various times, much of the attention of natural philosophers, and many accurate and interesting observations have been recorded of the formation and diffusion of elastic fluids, from various kinds of liquids. The circumstances especially attending the rise and precipitation of aqueous steam in the atmosphere, are acknowledged to be important in the highest degree, as upon their silent influence depends the o of those important meteorological phenomena, with which is connected the welfare of all the organized creation. The labours of De Luc, De Saussure, and particularly of Mr Dalton, have thrown considerable light upon this never-ceasing process; but something appears to be still wanting to complete the investigation, and to follow up the results to their ultimate consequences. The following observations, however inadequate to fulfil this desirable purpose, may possibly attract some attention to the subject, and may be * of indicating the points which most require eluciatlon.
It is a well-known fact that water, under all circumstances, is endued with the power of emitting vapour, of an elastic force proportioned to its temperature. It is also well understood, that the gaseous atmosphere of the earth, in some degree, opposes the diffusion, and retards the formation of this vapour; not, as Mr Dalton has shewn, by its weight or pressure, but by its vis inertia. What is the
amount of this opposition, and by what progression it is connected with the varying circumstances of density and elasticity, have never yet been experimentally explained. It may facilitate the comprehension of the subject, to distinguish three cases with regard to the evaporating fluid: the first, when its temperature is such as to give rise to vapour equivalent in elasticity to the gaseous medium, and when it is said to boil; the second, when the temperature is above that of the surrounding air, but below the boiling point; and the third, when the temperature is below that of the atmosphere. With regard to the first, all the phenomena have been accurately appreciated. The quantity evaporated from any surface, under any given pressure, is governed, in some measure, by the intensity of the source of heat, and is in no way affected by the motions of the aerial fluid. The elasticit of the vapour is exactly equivalent to that of the air, whic yields en masse to its lightest impulse. When disengaged, it is immediately precipitated in the form of cloud, giving out its latent caloric to the ambient medium; and under that form is again exposed to the process of evaporation, according to the laws of the third division of the process. All the phenomena attending the process of boiling have been ably investigated by Gay-Lussac, Dalton, Ure, and Arch-deacon Wollaston: but, as they have but little connexion with the atmospheric relations, which are the particular object of the present paper, I shall proceed to the second case of evaporation. When the evaporating fluid is of a higher temperature than the surrounding air, but not so high as to emit vapour of equal elasticity to it, the exhalation is proportionate to the difference of temperature. The gaseous fluid, in contact with the surface, becomes lighter by the abstraction of portions of the excess of heat, ai rising up, carries with it, in its ascent, the entangled steam. This, as in the former case, is precipitated, and, in the form of cloud, exposed to the ū species of evaporation. This process is not only proportioned to the difference of temperature, and the elasticity of the vapour, but is also governed by the motion of the air. A current or wind tends to keep up that inequality of heat upon which it depends, and prevents that equalization which would gradually take place in a stagnant air. Such is the evaporation which often takes place in this climate, in autumn, from rivers, lakes, and sea, and which is indicated by the fogs and mists which hang over their surfaces.
It is, however, the third modification of circumstances, which is the most interesting in the point of view which I have suggested, and from which I have merely distinguished the preceding, to free the subject from ambiguity. When the temperature of water is below that of the atmosphere, it still exhales steam from its surface; but in this case, the vapour, neither having the force necessary to displace the gaseous fluid, nor heat enough to cause a circulation, which would raise it in its course, is obliged to filter its way slowly through its interstices; and the nature of the resistance it meets with in this course is the first object of investigationThe force of vapour, at different temperatures, has been determined with great accuracy, and the amount of evaporation has been shewn to be cateris paribus, always in direct proportion to this force. The quantity is also known to depend upon the atmospheric pressure, but I know of no experiments which establish the exact relation between the two powers. I attempted to elucidate the point as follows: By enclosing in a glass receiver, upon the plate of an airpump, a vessel with sulphuric acid, and another with water, and by properly adjusting the surfaces of the two, it is easy to maintain, in the included atmosphere of permanently-elastic fluid, an atmosphere of vapour of any required force; or, in the usual mode of expressing the same fact, the air may be kept at any required degree of dryness. The density of the air, in such an arrangement, may, of course, be varied and measured at pleasure. Now there are three methods of estimating the progress of evaporation in such an atmosphere: the first and most direct, is to weigh the loss sustained by the water in a given time; the second, to measure, by a thermometer, the depression of temperature of an evaporating surface; and the third, to ascertain the dew point, by means of the hygrometer. ExPERIMENT 1.-The receiver which I made use of, was of large capacity, and fitted with a hygrometer. I placed under it a flat glass dish, of 73 inches diameter, the bottom of which I covered with strong sulphuric acid. The glass bell but just passed over it, so that the base of the included column of air rested every where upon the acid. In the centre of the dish, was a stand with glass feet, which supported a light glass vessel of 2.7 inches diameter, and 1.3 inches depth. Water to the height of an inch was poured into the latter, the surface of which stood iust three inches above that of the acid. A very delicate thermometer rested in the water, upon the bottom of the glass, and WOL. II. NO. I. 6
another was suspended in the air. It may be necessary to observe, that the sides of the vessel were perpendicular to its bottom, which was perfectly flat. The height of the barometer was 29.6, and the temperature of the water 56°. In twenty minutes from the beginning of the experiment, the hygrometer was examined, and no deposition of moisture was obtained at 26°. This being the greatest degree of cold which could be conveniently produced by the affusion of ether, the experiment was repeated, with a contrivance which admitted of the application of a mixture of pounded ice, and muriate of lime, to the exterior ball of the hygrometer. In this manner the interior ball was cooled to 0°, without the appearance of any dew. The temperature of the water and air were, in this instance, 58°, and the pressure of the atmosphere 30.5. From this experiment it appears, that in the arrangement above described, the surface of water was not adequate to maintain an atmosphere of the small elasticity of .068 inch; in other words, the degree of moisture in the interior of the receiver could not have exceeded 129, the point of saturation being reckoned 1000. How much less it was than this, or whether steam of any less degree of elasticity existed, the experiment of course did not determine. We may reckon, however, without any danger of error in our reasoning, that the sulphuric acid, under these circumstances, maintained the air in a state of almost perfect dryness. ExPERIMENT 2.—The same trial was made with atmospheres variously rarefied, by means of the pump. No deposition of moisture was, in any case perceived with the utmost depression of temperature, which it was possible to Fo and the state of dryness was as great, in the most ighly attenuated air as it was in the most dense. In the higher degrees of rarefaction, the water, however, became frozen. ExPERIMENT 3.−The water, which had been previously exposed to the vacuum of the pump to free it from any air in solution, was weighed in a very sensible balance, before it was exposed to the action of the sulphuric acid under the receiver. Its temperature was 45°, and the height of the barometer 30.4. In half an hour's time, it was again weighed, and the loss by evaporation was found to be 1.24 grain. It was replaced, and the air was rarefied till the gauge of the pump stood at 15.2 ; in the same interval of time it was reweighed, and the loss was 2.72; but its temperature was reduced to 43°. The loss from evaporation, in equal intervals, with a pressure constantly diminishing one-half, was found to be as follows:
Temperature. Loss. Pressure. Beginning. End. Grains. 30.4 45 45 1.24 15.2 45 43 2.87 7.6 45 43 5.49 3.8 45 43 8.80 1.9 45 41 14.80 .95 44 37 24. 16 .47 45 31 39.40
ExPERIMENT 4.—The o described in the last experiment, having been found adequate to maintain in the receiver a state approaching to that of complete dryness, I had no opportunity of judging whether the elasticity of the vapour, as it rose from the surface of the water, varied in any degree with the pressure of the air, or whether any part of the increase of evaporation were dependent upon such variation. To determine this point, I placed the sulphuric acid in a glass, of the diameter of 2.8 inches, so that its surface was very little more than equal to that of the water. The vessels were placed side by side, upon the plate of the air-pump, and covered with the receiver. The temperature of the water and air was 52°, and the height of the barometer 29.8. The following table shews the dew point, which was obtained at intervals of half an hour, at different degrees of atmospheric pressure:
The differences of these results are so extremely small, and are moreover so little connected with the variations of density, that there can be no difficulty in regarding them as orrors of observation, and we may conclude, that the elasticity of vapour, given off by water of the same temperature, is not influenced by differences of atmospheric pressure. The “ual surfaces of sulphuric acid and water here made use of, maintained, at the temperature of 52°, a degree of saturation