I850."| 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



311 



Dew-Pobit. 



If a volume of air of the tenipevature of 35' be saturated with 

 moisture, it follows that the teu^iion of such moisture is eijuivalent 

 to -222 of an inch of mercury. Hence, if the barometer reading 

 be 29'8S6, the pressure due to air alone, supposing it to be deprived 

 of its vapour, is 29-886 — 0-222; or, 29-(i6+. 



Complete saturation, however, is of comparatively rare occur- 

 rence: only, indeed, when the readings of the wet and dry bulb 

 are alike. In every other case the air is capable of retaining a 

 greater quantity of acpieous vapour. The knowledge of the dew- 

 point here comes to our assistance. Tlie dew-point gives a tempe- 

 rature to which, if we suppose any volume of air reduced, it 

 would be saturated with the moisture contained within it. This 

 point may be ascertained immediately by means of Daniell's hy- 

 gi-onieter, as was shown in my last paper. Entering tlie table then 

 with the temperature of the dew point, or that temperature at 

 which the air would be ready to part with its moisture, we ascer- 

 tain, as before, the tension of aqueous vapour under any given 

 condition of the atmosphere. 



Thus, supposing that dew begins to be deposited at 31% tbe air 

 being of any temperature, we enter the table with 31°, and find 

 the tension of aqueous vapour 0-192 inches; %Thich, subtracted 

 from the reading of the barometer, will give the pressure of dry 

 air. 



If the dew-point is not directly ascertained, it must be inferred 

 from simultaneous observations of the dry and wet bulb thermo- 

 meters, by means of Glaisher's factors, to be spoken of i)resently. 

 In Mr. Glaisher's practice, he at times experienced difficulties in 

 the use of Daniell s hygrometer; and at times he found that the 

 simultaneous results of the dew-point, as found from Daniell's 

 hygrometer and the dry and wet bulb thermometers, were dis- 

 cordant: and on investigating tliese causes, he found that the 

 error rested alone with Daniell's hygrometer. The times at wliich 

 these discordances existed were in those particular states of the 

 air when great dryness was prevalent; and the depression of the 

 temperature of the dew-point below that of the air was great, and 

 a long time elapsed after the dropping of ether on the white ball 

 before dew was deposited on the black ball. Such would require 

 the long continuance of the observer near the Instrument, and this 

 necessarily would influence both the hygrometrical state, as well 

 as the temperature of the air around the instrument; and this 

 would be especially tlie case if the observer be short-sighted, and 

 obliged to approach the instrument very nearly. And he makes 

 the following objections to the use of this hygrometer: — 



"Supposing the inclosed thermometer to be one of extreme deli- 

 cacy, which it is not, it would then indicate the temperature of 

 the portion of ether only in which its bulb was in contact, and 

 which portion may be very different from that which is below it; 

 and may be very different indeed from that part of the outside of 

 the glass upon which the dew is deposited. And if the ether be 

 dropped very slowly upon the white bulb, so that evaporation 

 should proceed slowly, the evil of long-continued watching is 

 required; and if more quickly, then the different layers of the 

 inclosed ether is of different temperatures. It must also be recol- 

 lected that the situation of the black ball, upon which tlie deposit 

 of dew takes place, is not very far from the white ball; and in 

 cases where large quantities of ether are necessary, such must 

 influence materially the hygrometricd state of tlie air in the space 

 included by both bulbs." 



In consequence of these sources of error in the use of Daniell's 

 hygrometer, together with its expense in use and trouble in using, 

 Mr. Glaisher made many attempts, by different combinations of 

 the results derived from the observations of the dry and wet bulb 

 thermometers, to deduce the temperature of the dew-point from 

 them; and at last found that the difference between the tempera- 

 tures of air and dew-point, divided by the difference between the 

 temperatures of air and evaporation, was constant at the same 

 temperature; but that this value was different with every different 

 temperature. 



He then collected all the simultaneous observations which had 

 been made at Greenwich, everv two liours, from the year 1840 to 

 181-i, and from them deduced the following table: — 



Tabic of Fjclm-s, by which the Difference of Seadinffi of the Dry Bulb 

 and Ji'el Bulb is ia be mtdtiplied. in order to produce the Difference be- 

 tween the Ueiidiwjs of the Dry Bulb and DewPoinI Thermometers. 



On the Weight of Vnpotir in a Cubic Foot of Air. 



On the Wciglit of a Ciihic Foot of Air, 



On the Aynoiint of Vii]H}ur required for Complete Saturation. 



On tlie Degree of Humidity of the Atmosphere. 



It has been experimentally determined by M. Gay-Lussac, that 

 air expands tstj"^ P'"^*' ^'f ^^^ bulk for every addition of I'' of heat, 

 inasmuch as it expands equally with equal increments of heat from 

 the freezing to the boiling point, to the amount of f of its bulk. 

 Taking a cubic foot of dry air at a pressure of 30 inches, .and a 

 temperature of 32° as unity, a simple proportion will give the 

 space it will occupy when subject to any given degree of tempera- 

 ture — say 44°. 



Now, by the addition of 180°, we find the expansion | of the 

 volume (viz., from 32° to 212°); required the expansion for 44 — 32, 

 or 12°. 



180° : 12° : : I : Jj, so that the cubic foot of air becomes 1'025 ft. 



From determinations of the weight of a mass of dry air under a 

 pressure of 30 inches by Sir George Shuckburgh, Biot, and Thenard, 

 .it is inferred tliat a cubic foot of dry air, at 32°, under pressure of 

 30 inches, weighs 563 grains; whence we may determine its weight 

 after expansion by heat (say, at a temperature of 44°), by the fol- 

 lowing proportion: — 



1 -025 feet : 1 foot : : 5S3 grains : 549-27 grains. 



The next step is to ascertain the enlargement which a volume of 

 dry air receives when saturated with vapour at any degree of 

 temperature; but in the examples, 44° will be the degree assumed, 

 and a cubic foot the volume. 



If a cubic foot of dry air, of known elasticity, be mixed with a 

 cubic foot of vapour, also of known elasticity, and if the mixture 

 be compressed into the space of one cubic foot, the elasticity of 

 the mixture will be the sum of the two elasticities of the air and 

 vapour; or, if it be allowed to expand till its elasticity is equal to 

 that of the unmixed air (suppose 30 inches), it will occupy a larger 

 volume, in the proportion of the sum of the two elasticities to the 

 elasticity of the air .alone. Now, from the table, we know the 

 elastic force of vapour for every degree of temperature; let it be 

 requii-ed to find the space occupied by a mixture of a cubic foot of 

 dry air and moisture at the temperature of 44°. 



Tension of aqueous vapour at 44° = 0-304 inches 



Tension of dry air 30- 



30-304 inches. 



Then, TjTf.Vrrj- : ^ '■'■ 1 c. f. : 1-01012 c. f., which is the space 

 ccupied by the mixture of the two aerial fluids. 



The following formula will give the result in more general 

 terms: — 



Let ;) := the atmospheric pressure, as measured by the inches of 

 mercury in the barometer. 

 E = the elasticity of vapour at a given temperature (mea- 

 sured in the same way.) 

 n = the bulk of a certain quantity of air, when dry, at the 



given temperature, and under the pressure y>. 

 ri =: the bulk of the same quantity of air when saturated 

 with vapour at the same temperature, and under the 

 same pressure p. 

 Then, since the elasticity varies inversely as the volume, the 



