810 



THE CIVIL ENGINEER AND ARCHITECrs JOURNAL, 



[OCTOBEB, 



From tlie adopted Mean Temp<?ratur«9 of Air end Kvnporation, tlip ft)]Iowin3 hygro- 

 metrical results are to be calculated by Glai»lier's IIj gromelrical Tables. 



ThP Temperature of the Dew-point .. .. .. ■• 



Tlio elastic force of V«pour 



Tlie weijibt of Vapour in a cubic foot of Air 



The 3'lJitioual weight of Va^/our requireil to saturate a 

 cubic foot of Air ., .. .. ,. ,. ,. .. 



The fJepiee of Humility (complete saturation = 1) ., 

 The areraije weight of a cubic foot of Air 



The original register, or a verified copv, Ib to be transmitted, at the end of every 

 rocnih, to the Secretary of the Erttish Meleorologioal Society, 13, Dartmouth 

 Terrace, Blacklieath, Kent. 



The atmosphere which surrounds our globe is suhject to agita- 

 tions far more extensive than t!ie swell of the ocean; waves as 

 broad as the Atlantic itself pass over us from time to time. Inde- 

 pendently of these atmospheric waves, hourly variations in the 

 pressure of the air have been determined, vvhidi seem to follow a 

 definite law. One of the causes of these fluctuations is variation 

 of temperature; another is the union with the aerial of an aqueous 

 vapour. Sucli is the tendency of the air to unite with the vapour 

 of water that it m.iy be said, in no case, to be found in a state of 

 absolute dryness: the supply is obtained from rivers, the ocean, 

 the surface of the soil. The very laws of nature tend to its 

 distribution — w inds and atmospheric currents lead to its equable 

 diffusion. ^V'e may, indeed, consider the globe to be surrounded 

 with two co-existing atmospheres — the one of air, and the other of 

 aqueous vapour — not chemically combined, but commingled, or 

 mechanically united; the one being, as it were, diffused throughout 

 the pores of the other, as water through the substance of a filtering 

 vessel or the pores of sponge; and they may be, for the sake of 

 experiment, as readily separated as the sponge may be relieved of 

 its aqueous burden by compression. As the sponge, moreover, by 

 its elasticity will recover its size, as before, so if we deprive a 

 cubic foot of air of its moisture it will occupy the same space, 

 though its density will be less. The e.xperiments of D.alton and 

 otliers have proved this remarkable fact, that under similar circum- 

 stances as to temperature and pressure, the quantity of watery 

 vapour existing in air will be exactly equal to what it would be in 

 a vacuum of equal capacity; and that, if we have the means of 

 computing the pressure or vveight of vapour in vacuo, we shall be 

 able to determine with equal accuracy the actual weight of moisture 

 in a given volume of air by the same means; in fact that, iu either 

 case, the pressure will be the same. 



The capacity of air for moisture increases with the temperature; 

 but there is a limit to its power of h(dtling acjueous vapour in 

 solution; when this limit is attained, the air is in a state of complete 

 saturation. If from any cause a volume of air in this condition 

 should be suddenly cotded, a deposition of moisture succeeds; the 

 air parts with aqueous vapour in minute particles; and if it be free 

 from agitation these appear in the form of dew, which is witnessed 

 in perfection after the removal of the heat of the sun on a still 

 autumnal night. The effect of a temperature below the freezing 

 point will be to convert tlie dew into hoar frost, as is visible when 

 winter approaches. 



Should the air part from its moisture at a distance from the 

 earth's surface, the aqueous particles will separately descend but 

 slowly, or not at all. By the law of aggregation, they will unite 

 in globular forms, and their united weight being now sufficient to 

 overcome atmospheric resistance, they will be attracted towards 

 the earth, and fall in the form of drops of rain. That electricity is 

 concerned in the production of rain is more than probable; and 

 this opinion is confirmed by the fact that copious and heavy 

 showers fall during a thunder-storm. 



Dry air is denser than the vapour of water, and a mixture of the 

 two w ill be lighter than the same vidume of dry air alone. Thus 

 we find the mercury of the barometer will indicate a decrease of 

 pressure on a damp foggy day; whereas it will be found to rise in fair 

 dry weather. Most ot the variations indicated by the barometer 

 arise from the greater or less degree of moisture existing in the 

 air; and the interchange of aerial strata of various densities, and 

 in various conditions as regards heat, produces all tlie fluctuations 

 of pressure observed. \V'hether these are only the lower strata of 

 the atmosphere — how high they extend — whether the heiglit of the 

 atmosphere above the earth's surface is invariable — are questions 

 for science to determine. 



Evaporation and condensation of aqueous vapour tend, in various 

 ways, to diffuse heat more equally throughout the globe. As the 

 power of the air to imbibe moisture increases with the temperature, 

 evaporation goes on with most rapidity in warm climates, and tlie 



heat absorbed in the process has a cooling tendency. Strata of 

 warm air, elevated by the in-rush of cooler draughts, are driven to 

 regions where the temperature is lower; here the vapour is con- 

 densed, and its latent heat is given forth to mitigate the severity 

 of the colder climate. 



If two saturated volumes of air of unequal temperature, and 

 therefore of varying capacities for moisture, meet each other, their 

 tendency will be to unite and equalise both the temperature and 

 the moisture. The moisture, however, will always be in excess, 

 for the two processes will not proceed at the same rate. Thus, 

 supposing two volumes of air, one of the temperature of 40°, and 

 the other of 60^, to unite, the mean temperature of the mass will 

 be 50'. The elastic force of vapour at W of temperature (as will 

 be explained more fully hereafter) is '261 measured as pressure in 

 inches of mercury; at 60- it is -o^S; but at 50°, the mean of the 

 two temperatures, the elastic force is '373, which is less than SSi 

 (the mean of the others) by -O'il : this represents the tension of that 

 portion of moisture which would be set -free. If this vapour, in its 

 liberated state, meet with a stratum of air not saturated with 

 moisture, it will be re-absorbed either partially or entirelv; if only 

 partiallv, the remaining portion, consisting of aqueous vapour in 

 an extremely minute state of subdivision, may be arrested in its 

 descent, and float in the atmosphere in the form of clouds. 



From the preceding observations it will appear that the pressure 

 shown by tlie barometer is compounded of that of the air itself, 

 and the co-existing vapour of water, from which it is never entirely 

 disunited. It will now be shown in what manner these two forces 

 may be separated, and the proper value assigned to each. We shall 

 then be led to appreciate the simplicity of the process of deducing 

 the hygrometric state of the atmosphere from simply observing the 

 difference betw een dry and wet bulb thermometers; in other words, 

 by remarking the difference between the temperature of the air 

 and the temperature of evaporation. 



Tension of Aqueous Vapour. 



Tlie experiments of Dalton, and more recently of Dr. Vre, 

 Regnault, and others, have been the foundation of tables, showing 

 the elastic force of vapour as measured in inches of mercury for 

 every degree of temperature. Since many of the deduced results 

 depend upon these, a description of the method employed by Dr. 

 Ure in determining the tension of vapour may tend to give 

 confidence in the results. 



D L £ is a syphon barometer, the leg E being closed, and the 

 other open at D. On the admission of the mer- 

 cury there will, of course, be an equilibrium when 

 the column in the closed leg balances the atmo- 

 spheric pressure on the surface of the mercury 

 in the other, and the space above G will be a 

 vacuum; a glass vessel is adapted to the outside 

 of this portion of the tube, and rings of platinum 

 wire on the exterior of the tube serve to mark the 

 height of the mercury in each leg. A drop of 

 water is now intioduced into the vacant space 

 above G, which is forthwith changed to vapour, and 

 the vessel A is filled with w aler, the temper.iture 

 of which is shown by the thermometer inserted 

 therein. The tension of the vapour will of course 

 cause the mercury to descend in the tube E, and 

 rise in the tube D; a )iortion of the metal is now 

 poured into the open tube until itsweiglit counter- 

 balances the tension of the aqueous vapour, and 

 brings the mercury to its original level at G. Let 

 O L be the space occupied by the additional quan- 

 ., J, tity of mercury; this space, accurately ascertained, 



\^Z^ will be the measure, in inches of mercury, of the 



elasticity of aqueous vapour at the temperature 

 shown by the immersed thermometer. By varying the heat of the 

 water, bv using treezing mixtures for the low temperatures, and 

 boiling oil fir the higher. Dr. L're obtained results ranging between 

 21^ and 312' of heat; the elastic force for the former being 0- 17 

 inch, for the latter, Itil inches. 



The Greenwich Meteorological Observations contain "J Table, 

 shotrhig the Elastic Force of Vapour in inches of Jlcrcuryfor every 

 tenth of a degree, from 0° to 90" ;" an extract from it is here gi\'en, 

 to illustrate its use in the subsequent deductions. 



