497 



DEW. 



DEW-POINT. 



498 



been the only attempts made in England to confirm and extend the 

 results at which Dr. Wells had arrived, until after the establishment 

 of the magnetical and meteorological department of the Royal Observa- 

 tory at Greenwich, when Mr. James Glaisher, F.R.S., the superin- 

 tendent (as an assistant of the astronomer royal) of that department, 

 instituted, carried on for some years, and afterwards communicated to 

 the Royal Society, an elaborate and most valuable series of observa- 

 tions and experiments " On the amount of the Radiation of Heat, at 

 night, from the earth, and from various bodies placed on or near the 

 surface of the earth." These were published in the ' Philosophical 

 Transactions ' for 1847, of which they occupy 113 pages, including 108 

 tables of observations. The subject of the production of dew was, of 

 course, only a part of Mr. Glaisher's researches ; but the most general 

 result he obtained on that subject, was entirely conformable with that 

 enunciated by Dr. Wells, and is stated in the following terms : 



" The formation of dew was found to depend solely on the tempera- 

 ture of the bodies upon which it was deposited, and it never appeared 

 upon them till their temperatures had descended below that of the 

 dew-point in their locality, as found by observations of a dry and wet 

 bulb thermometer placed in their vicinity. The amount of water 

 thus deposited was the greatest upon the substances whose tempera- 

 tures were the lowest." The dtic-point, as will be explained in the 

 next article, is the critical temperature below which the vapour of the 

 atmosphere begins to be condensed into moisture, the " point of reple- 

 tion " of Dr. Wells, as above. 



At the commencement of his experiments Mr. Glaisher made some 

 precise and important observations, on the difference between the 

 temperature of a thermometer and that of a body in contact with it, 

 which must here be noticed, on account of the bearing which they 

 have on the conduct of experimental inquiries of this nature. Among 

 the bodies found to radiate heat freely (in agreement with previous 

 knowledge), was glass, which thus very readily became wet with dew. 

 In consequence of this property, the tube of a naked thermometer, 

 which was lying on a substance entirely free from moisture, was fre- 

 quently found covered with dew, and therefore it seemed probable that 

 the temperature exhibited by the instrument was not that of the body 

 in question. On such instances occurring an attempt was made by 

 Mr. Glaisher to correct the error, by enclosing the thermometer stem 

 in a tube made of gilt paper ; the bulb alone, resting on the substance, 

 remained exposed to the sky. The differences between the readings of 

 a thermometer thus enclosed, and when naked, were found to be 

 sensible, but small in amount ; it was also observed, that when the 

 thermometer was wholly naked the stem was at times wet when the 

 bulb itself was dry ; and at all times much less moisture appeared on 

 the bulb than on the stem, unless the disposition of the substance in 

 question to become cold was the same with, or greater than, that of 

 the glass. The error arising from this cause was chiefly confined to ) 

 the consequent contraction of the mercury in the stem, and not in the j 

 bulb, and which was considered to be avoided by the use of gilt paper. 

 The error in all cases must have been small. Mr. Glaisher found, in 

 addition, that one of the many sources from which the reading of a 

 thermometer placed on the ground is affected, is the heat evolved 

 during the change of the watery vapour in the atmosphere into dew, 

 the amount of which appeared to be about 4 Fahr. from all the ex- 

 periments he made. The difficulty this fact occasioned to some of the 

 earlier theorists on dew has been noticed above. 



It was found, in the course of these experiments, that the differences 

 between the temperature of the air and of bodies on the earth, at 

 night, in equally calm and clear weather, were the same at every 

 period of the year, but it was also found that the amount of dew 

 deposited during such times was much greater in summer than in 

 winter. This is easily accounted for, from the well-known relation 

 existing between temperature and moisture. At all seasons of the 

 year, at night, the depression of the temperature of the dew-point 

 below that of the air is small, or the air is in a state of saturation 

 nearly, and therefore in summer a certain diminution of temperature 

 would <-ause much more vapour to be changed into water, than an 

 equal diminution in winter would do ; in agreement with the previous 

 reasoning of Dr. Wells. 



With respect to the occasional formation of dew in the day-time, 

 Mr. Glaisher remarks, " Radiation of heat from the earth to the 

 heavens must exist at all times, both day and night, and in all states of 

 the sky. Generally, when the sun is above the horizon, the heat 

 emitted by it to the earth will overbalance that which the earth 

 radiates upwards ; at times, however, in places shaded from its direct 

 beams, the amount of heat radiated exceeds that received from the 

 un and all other sources, and dew will be continually deposited 

 throughout the day." Some instances of this are exhibited in his 

 Tables. Dr. Joseph D. Hooker, we may add (in his 'Himalayan 

 Journals,' vol. i., p. 196), has recorded, ->in conformity with this prin- 

 ciple, that in East Nepaul, the sun, hi many places, does not reach the 

 bottom of the valleys until 10 A.M., and is off again by 3 P.M. ; and that 

 the radiation to a clear sky is so powerful that dew frequently forms 

 in the shade throughout the day. But in these regions the clearness 

 of the* sky is such, that at night Dr. Hooker found the upper blanket 

 of his bed to become coated with moisture, from the rapid abstraction 

 of heat by the tarpaulin of his tent, itself frozen by its own radiation. 

 From some of Mr. Glaisher's tables, containing the results of obser- 

 ABTS AJID sci. Drv. VOL. in. 



rations when the excess of air-temperature above that of long grass 

 amounted to 5, 6, 7, or 8, it appears that at such times the sky was 

 frequently cloudy, and when wholly so, that the clouds were high ; 

 that the air was in frequent motion, and at times the wind was blowing 

 with a pressure of 4 Ibs. on the square foot ; that the atmosphere was 

 occasionally thick by haze or vapour, and that under these circum- 

 stances, dew was seldom deposited. Other tables contain the results 

 of the observations when the excess was 9*, 11, or 12, and at those 

 times the sky was generally clear, the air calm, and the atmosphere 

 frequently hazy or misty, or vapour or fog was prevalent, and dew was 

 also frequently deposited; in two cases, one in February and the other 

 iu November, the sky was wholly covered by high and thin clouds, the 

 radiation of heat from which was thus inadequate to counteract that 

 of the earth, as had been before observed by Dr. Wells. 



It was also observed by Mr. Glaisher, that the various amounts of dew 

 deposited, at the same time, on different bodies at night, were, as near 

 as could be determined, proportional to the amount of the depression 

 of their temperature below that of the dew-point. " Hence, it is 

 evident," he concludes, " that all hygrometers formed of any of these 

 substances [a variety of bodies in common use, chiefly belonging to the 

 mineral kingdom, as chalk and graphite, but including the common 

 metals also], or of any animal or vegetable substance, when exposed to 

 the clear sky at night, will be cooled by the radiation of their heat, 

 and will cool the air iu contact with them ; and thus indicate a 

 greater degree of humidity than actually exists (as had been pointed 

 out by Dr. Wells, with respect to the organic substances) ; and par- 

 ticularly so, should their temperature descend below that of the 

 dew-point, and dew be actually deposited upon them." 



Contemporary, as it would appear, with the researches of Mr. 

 Glaisher, and equally elaborate and satisfactory, were those of the 

 lamented Italian philosopher Melloni, who had made almost the entire 

 subject of radiant heat emphatically his own. They were read to the 

 Royal Academy of Sciences of Naples, very shortly after Mr. Glaisher's 

 paper was read before the Royal Society of London ; seem to have 

 been undertaken, in part, on account of the obstinate and perhaps 

 traditionary retention in certain Italian schools of science of the 

 ancient notions on the formation of dew ; aud are throughout, in the 

 most remarkable manner, confirmatory of the discovery of Dr. Wells, 

 while they correct some of the tacit admissions of his theory and 

 elucidate others. They are also, in a great degree, comparable in detail 

 with those of Mr. Glaisher, with which they present an accordance, 

 that when characterising the labours of two physicists accurately 

 versed in their subject, is itself strong evidence of the truth. But 

 M. Melloni adds to the causes of complication in the actual phenomena 

 of dew, as investigated by the English meteorologist and his pre- 

 decessors, that of the great emissive or radiant power of water, which 

 has important bearings on every part of the series of phenomena, 

 after the first actual deposition of dew, and in all probability is the 

 reason why the thermometer wetted with dew, described by Mr. Glaisher 

 as above, did not introduce any serious error into his observations. 



A great part of Melloni's three memoirs is devoted to the experi- 

 mental verification of the proposition that almost all the facts which 

 precede and accompany the formation of dew result from the presence, 

 of shorter or longer duration, of the air arouud the radiating surfaces. 

 He investigates with great success the circulation of temperature, and 

 of local amount of vapour in the different atmospheric strata adjacent 

 to the earth's surface, under the influence of nocturnal radiation. For 

 the results, however, as well as for a discussion of the geographical 

 distribution of the varying phenomena of dew, we must refer to the 

 author's original memoirs ' On the nocturnal cooling of bodies exposed 

 to a free atmosphere in calm and serene weather, and on the resulting 

 phenomena near the Earth's surface,' ' Ann. de Chirn. et de Phys.,' 

 Feb. and April, 1848; or (Taylor's) 'Scientific Memoirs,' vol. v., 

 pp. 453-476, 530-552. [CLODD, HOAK-FROST, HYGKOMETKY, METEOR- 

 OLOGY, RADIATION OF HEAT, RAIN, VAPOUB.] 



DEW-POINT. The critical temperature, usually expressed in ther- 

 mometrical degrees, of the double atmosphere of air and aqueous 

 vapour (or of any portion of the earth's atmosphere in its natural state), 

 below which the latter becomes condensed into liquid water, and above 

 which that atmosphere will convert into vapour and absorb into itself 

 (augmenting the quantity of the vapour) water with which it may be 

 in contact. In the former case the water becomes condensed into 

 minute globules like those of the clouds, which, if the condensation 

 take place against solid surfaces of lower temperature, become attached 

 to those surfaces in the form of dew ; hence the origin of the term. 



Viewing the same subject in a slightly different aspect, Sir John 

 Herschel defines the dew-point as "the temperature which the air 

 ought to have, so as to be exactly saturated with the quantity of vapour 

 it actually contains." 



The phrase depression nf the deio-point denotes the quantity of heat 

 to be abstracted, or the number of degrees of the thermometer below 

 the actual temperature of the atmosphere which it requires to be 

 cooled, in order that the vapour it contains may become so condensed. 



The following examples will illustrate the use of these expressions. 

 In the Greenwich Meteorological Observations for 1857 it is recorded 

 that, on July 15, the mean daily reading of the thermometer, or the 

 air-temperature, was 72'7 Fahr., and that of the dew-point 58 0t 2, the 

 difference of which, 14'2, is the depression of the dew-point ; and that 



K K 



