DEW. 



The dew, therefore, is a deposit from that por- 

 tion of vapour which enters into the composi- 

 tion of common air, and which is swept in 

 contact with objects at or near the surface of 

 the earth, like breath thrown upon the blade 

 of a knife or other polished surface. When 

 the sky is clear, as in starry and moonlight 

 nights, then do grass, leaves, and other objects, 

 throw off their heat most rapidly and become 

 cooler than the air immediately above them, and 

 the colder they get the more dew is condensed 

 upon them. Different substances part with 

 their heat more or less rapidly, and this ex- 

 plains the cause why different proportions of 

 dew are observed on objects similarly exposed 

 to the atmosphere. A gravel walk will have 

 little or no dew upon it, whilst the grass on 

 each side will be reeking wet: because the 

 grass not only radiates its heat more rapidly 

 than the walk, but does not derive warmth 

 from below to compensate for the loss. Be- 

 sides, the moisture falling upon the gravel 

 walk is absorbed more rapidly than the dew de- 

 posited upon plants. 



The temperature at which dew begins to 

 form i> railed the //nr-//(;i/if, and may be ascer- 

 tain - 1 vi TV accurately. Ttfhs, by lay in u' H 

 thermometer on the grass in the evcnini:. as UK- 

 o parts with the warmth collected 

 : the day, and gets colder, the moment 

 obules or particles of dew are observed 

 l, the degree at which the mercury 

 in th" thermometer stands shows the dew-point 

 or temperature at which the watery vapour 

 condenses. It has been observed already, 

 iss possesses a faculty of radiating or 

 parti ni? with its heat very rapidly, on which 

 account it quickly becomes considerably colder 

 than the air immediately above. If, when a 

 thermometer is placed upon the herbage, an- 

 other H suspended in the air two or three feet 

 above, this last will not fall so low by many de- 

 grees, the difference being sometimes as great 

 as 10 or 15 degrees of Fahrenheit 



In making this experiment, the instrument 

 suspended in the air must of course have its bulb 

 covered from the sky by means of a piece of 

 tin-foil, or other non-radiating substance, to pre- 

 vent its heat from passing off, in which case 

 the instrument would itself radiate, and thus 

 represent its own temperature and not that of 

 the surrounding air. It may be often observed 

 that in the morning, whilst the grass is reeking 

 wet with dew, a polished substance, lying upon 

 it, such as the blade of a knife, will have little 

 or no moisture on its surface. This shows that 

 polished surfaces part with heat and become 

 cooled down to the dew-point very slowly. The 

 most dew will of course always be found on 

 substances which have the power of cooling 

 most rapidly, and few objects do this so readily 

 as grass and the leaves of plants and trees. 



The degree of cold necessary to be acquired 

 by grass and other objects, before they can 

 have dew deposited on them, can always be 

 ascertained beforehand. Thus, take a thin 

 tumbler of cut-glass, having polished sides ; 

 fill this about half full of pump or ice-water. 

 Plunge into it 1he bulb of a thermometer, and 

 the moment a film of dew or misty cloud is 

 seen to form on the polished 01 > ; de surface, 



DEW. 



note the degree at which the therm >metei 

 stands, and this will be the dew-point. ShouM 

 pump-water not be sufficiently cool to produce 

 a cloudiness on the polished surface of the 

 glass, some ice may be added; or common salt 

 and nitre, sal ammoniac, or some other sub- 

 stance employed in the production of artificial 

 cold. The temperature at which atmospheric 

 vapour condenses to form dew is generally 

 several degrees below the temperature of the 

 atmosphere. But this is only the case during 

 clear weather, since, when there is a fog, or a 

 rain, the deiv-point will be found to correspond 

 with the temperature of the air; showing that 

 any cause which contributes to brim: down the 

 atmospheric temperature to the dew-point, will 

 directly promote the condensation of its vapour 

 or moisture into mist, cloud, rain, snow, or hail. 

 The many relations which the dew-point, or de- 

 gree at which vapour condenses, holds with 

 atmospheric phenomena, may be understood 

 from this. And let it be borne in mind that 

 the dew-point is almost continually rising or 

 falling, like the temperature of the atmosphere, 

 being usually, in clear weather, some four, six, 

 eight, or ten degrees lower than common air, as 

 indicated by the thermometer. 



The very simple means just described, by 

 which the dew-point can be ascertained, ap- 

 proximately, with the aid of a tuml>i-T and 

 :neter, is by no means the only modi; 

 practised for the purpose. On the contrary, it 

 is the most primitive plan, and one requiring 

 u't'-at >kill and judgment to ensure tolerable 

 accuracy. Mr. Daniells, a learned meteorolo- 

 gist, of London, several years since invented 

 a contrivance called a H 'T, for deter- 



mining the dew-point, which is rather compli- 

 cated and too costly for general use. It will, 

 we think, be entirely superseded by an instru- 

 ment very recently invented by Prof. A. D. 

 Bache, of Philadelphia, of which the following 

 general description may furnish some idea. 



One of the forms of this instrument consists 

 of a square bar of highly polished steel, about 

 half an inch in depth and breadth, and ten inches 

 long, one end of which fits into a case attached 

 to a box of tinned iron or copper. This box is to 

 contain cold water, ice, or a freezing mixture, 

 according to the season or rather to the tempera- 

 ture of deposition, or dew-point. The end of 

 the bar which fits into the case has its tempera- 

 ture brought below the dew-point, while the 

 other end is at the temperature of the air; one 

 of the sections of the bar is, therefore, at the 

 dew-point Between this section and the box 

 vapour is deposited on the side of the bar, and 

 beyond it, in the highly burnished surface of the 

 steel, appears in strong contrast, the line of 

 junction being very well defined. To ascer- 

 tain the temperature of the line where the de- 

 posit of dew commences, cylindrical holes are 

 made perpendicularly downwards in the bar, 

 at intervals of about half an inch apart, 

 throughout the whole length, large enough to 

 admit the bulb of a very small thermometer, 

 and deep enough to carry the bulb entirely 

 i into the substance of the bar. If the line 

 of deposition is opposite the middle of on* 

 of these holes, the thermometer then gives 

 the dew-point; if between two of them, the 



405 



