Jan. 14, 1886] 



NA TV RE 



subject, tliouglu It (..ime neither from above no.- from below, but 

 was condensed out of the air near the surface of the earth. He 

 combated Gersten's idea that it rose from tlie earth, and showed 

 that all the phenomena observed by Gersten and others which 

 were advanced to support this tlieory could be equally well ex- 

 plained according to the theory that it was simply formed from 

 the vapour present at the time in the air, and which had risen 

 from the ground during the day, and concluded that if any did rise 

 from the ground during night, the quantity must be small, but, 

 with great caution, he adds that "he was not acquainted with any 

 means of determining the proportion of this part to the whole." 

 A few observations of the temperature of the ground near the 

 surface and of the air over it, first raised doubts as to the correct- 

 ness of the now generally-received opinion that dew is formed of 

 vapour existing at the time in the air. These observations, made 

 at night, showed the ground at a short distance below the sur- 

 face to be always hotter than the air over it ; and it was thought 

 that so long as this excess is sufficient to keep the temperature 

 of the surface of the ground above the dew-point of the air, it 

 will, if moist, give off vapour ; and it will be this rising vapour 

 that will condense on the grass and form dew, and not the 

 vapour that was previously present in the air. 



The first question to be determined was whether vapour 

 does, or does not, rise from the ground on dewy nights. One 

 method tried of testing this point was by phacing over the grass, 

 in an inverted position, shallow trays made of thin metal and 

 painted. These trays were pul over the ground to be tested 

 after sunset and examined at night, and also next morning. It 

 was expected that, if vap )ur was rising from the ground during 

 dewy nights, it would be trapped inside the trays. The result 

 in all the experiments was that the inside was dewed every 

 night, and the grass inside was wetter than that outside. On 

 some nights there was no dew outside the trays, and on all 

 nights the inside deposit was heavier than the outside one. 



An analysis of the action of these trays is given, and it is con- 

 cluded that they act very much the same as if the air was quite 

 still. Under these conditions vapour will rise from the ground 

 so long as the vapour-tension on the surface of the ground is 

 higher than that at the top of the gr.ass, and much of this rising 

 vapour is, under ordinary conditions, carried away by the 

 passing air, and mixed with a large amount of dryer air, whereas 

 the vapour rising under the trays is not so diluted ; and hence, 

 though only cooled to the same amount as the air outside, it 

 yields a heavier deposit of dew. 



Another method of testing this point was employed, which 

 consisted in weighing a small area of the exposed surface of the 

 ground, as it was evident that if the soil gave off vapour during 

 a dewy night, it must lose weight. A small turf about 6 inches 

 (152 mm.) square, \\as cut out of the lawn and placed in a small 

 shallow pan of about the same size. The pan with its turf, 

 after being carefully weighed, was put out on the lawn in the 

 place where the turf had been cut. It was exposed for 

 some hours while dew was forming, and on these occasions it 

 was always found to lose weight. It was thus evident that 

 vapour was rising from the ground while dew was forming, and 

 therefore the dew found on the grass was formed of part of the 

 rising vapour, trapped or held back by coming into contact with 

 the cold blades of grass. 



The difference between these experiments in which the ex- 

 posed bodies lose weight, and the well-known ones in which 

 bodies are exposed to radiation, and the amount of dew formed 

 is estimated by the incresc in their weight, is pointed out. In 

 the former case the bodies are in good heat-communication with 

 the ground, whereas in the latter, little or no heat is received by 

 conduction from the earth. 



Another method employed for determining whether the condi- 

 tions found in nature were favourable for dew rising from the 

 ground on dewy nights, was by observations of the temperatures 

 indicated by two thermometers, one placed on the surface of the 

 grass, and the other under the surface, amongst the stems, but 

 on the top of the soil. The difference in the readings of these 

 two thermometers on dewy nights was found to be very con- 

 siderable. From 10° to lS° F. was frequently observed. A 

 minimum thermometer placed on, and another under, the grass, 

 showed that during the whole night a considerable difference was 

 always maintained. .\s a result of this difference of tempera- 

 ture, it is evident that vapour will rise from the hotter soil 

 underneath, into the colder air above, and some of it will be 

 trapped by coming into contact with the cold grass. 



While the experiments were being conducted on grass land, 

 parallel observations were made on bare soil. Over soil the 



inverted traps collected more dew inside them than those over 

 grass. A small area of soil was spread over a shallow pan, and 

 after being weighed was exposed at the place where the soil had 

 been taken out, to see if bare soil as well as grass lost weight 

 during dewy nights. The result was that on all nights on which 

 the tests were made the soil lost weight, and lost very nearly 

 the same amount as the grass land. 



Another method employed of testing whether vapour is rising 

 from bare soil, or is being condensed upon it, consisted in placing 

 on the soil, and in good contact with it, small pieces of black 

 mirror, or any substance having a surface that shows dewing 

 easily. In this way a small area of the surface of the earth is 

 converted into a hygroscope, and these test-surfaces tell us 

 whether the ground is cooled to the dew-point or not. So long 

 as they remain clear and undewed, the surface of the soil is 

 hotter than the dew-point, and vapour is being given off, while 

 if they get dewed, the soil will also be' condensing vapour. On 

 all nights observed, these test-surfaces kept clear, and showed the 

 soil to be always giving off vapour. 



All these different methods of testing point to the conclusion 

 that during dewy nights, in this climate, vapour is constantly 

 being given off from grass-land, and almost always from bare 

 soil ; that the tide of vapour almost always sets outwards from 

 the earth, and but rarely ebbs, save after being condensed to 

 cloud and rain, or on those rarer occasions on which, after the 

 earth has got greatly cooled, a warm moist air blows over it. 

 The results of some of the experiments are given, showing, 

 from weighings, the amount of vapour lost by the soil at night, 

 and also the heat lost by the surface soil. 



It seems probable that when the radiation is strong, that soil, 

 especially if it is loose and not in good heat-communication with 

 the ground, will get cooled below the dew-point, and have vapour 

 condensed upon it. On some occasions the soil certainly got 

 wetter on the surface, but the question still remains. Whence 

 the vapour? Came it from the air, or from the soil underneath? 

 The latter seems the more probable source : the vapour rising 

 from the hot soil underneath w ill be trapped by the cold surface- 

 soil, in the same way as it is trapped by grass over grass-land. 

 During frost, opportunities are afforded of studying this point in 

 a satisfactory manner, as the trapped vapour keeps its place 

 where it is condensed. On these occasions the under sides of 

 the clods, at the surface of the soil, are found to be thickly covered 

 with hoar-frost, while there is little on their upper or exposed 

 surfaces, showing that the vapour condensed on the surface-soil 

 has come from below. 



The next division of the subject is on dew on roads. It is 

 generally said that dew forms copiously on grass, while none is 

 deposited on roads, because grass is a good radiator and cools 

 quicker, and cools more, than the .-.urface of a road. It is shown 

 that the above statement is wrong, and that dew really does 

 form abundantly on roads, and that the reason it has not been 

 observed is that it has not been sought for at the correct place. 

 We are not entitled to expect to find dew on the surface of roads 

 as on the surface of grass, because stones are good conductors 

 of heat, and, the vapour-tension being higher underneath than 

 above the stones, the result is, the rising vapour gets condenfed 

 on the under sides of the stones. If a road is examined on a 

 dewy night, and the gravel turned up, the under sides of the 

 stones are found to be dripping wet. 



Another reason why no dew forms on the surface of roads is 

 that the stones, being fair conductors, and in heat-communica- 

 tion with the ground, the temperature of the surface of the road 

 is, from observations taken on several occasions, higher than 

 that of the surface of the grass alongside. The air in contact 

 with the stones is, therefore, not cooled so much as that in 

 contact with the grass. 



For studying the formation of dew on roads, slates were found 

 to be useful. One slate was placed over a gravelly part of the 

 road, and another over a hard dry part. Examined on dewy 

 nights the under sides of these slates were always found to be 

 dripping wet, while their upper surfaces, and the ground all 

 round, were quite dry. 



The importance of the heat communicated from the ground 

 is illustrated by a simple experiment with two slates or two iron 

 weights, one of them being placed on the ground, either on 

 grass or on bare soil, and the other elevated a few inches above 

 the surface. The one resting on the ground, and in heat-com- 

 munication with it, is found always to keep dry on dewy nights, 

 whereas the elevated one gets dewed all over. 



The effect of wind in preventing the formation of dew is 

 referred to. It is shown thai, in addition to the other ways . 



