An analysis of the action of these trays is given, and it is concluded that they act very much the same as if the air was quite still. Under these conditions vapor will rise from the ground so long as the vapor-tension on the surface of the ground is higher than that at the top of the grass, and much of this rising vapor is, under ordinary conditions, carried away by the passing air, and mixed with a large amount of drier air, whereas the vapor 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 vapor during a dewy night, it must lose weight. A small turf about 6 inches (152 mm.) square was 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 vapor was rising from the ground while dew was forming, and therefore the dew found on the grass was formed of part of the rising vapor, trapped or held back by coming into contact with the cold blades of grass.
The difference between these experiments, in which the exposed 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 increase 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 conditions found in nature were favorable 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, among 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 considerable. From 10° to 18° 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. As a result of this difference of temperature, it is evident that vapor 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 vapor 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 vapor is being given off, while if they get dewed, the soil will also be condensing vapor. On all nights observed, these test-surfaces kept clear, and showed the soil to be always giving off vapor.
All these different methods of testing point to the conclusion that during dewy nights, in this climate, vapor is constantly being given off from grass land, and almost always from bare soil; that the tide of vapor almost always sets outward 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 the experiments are given, showing, from weighings, the amount of vapor 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 vapor condensed upon it. On some occasions the soil certainly got wetter on the surface, but the question still remains, Whence the vapor? Came it from the air, or from the soil underneath? The latter seems the more probable source; the vapor rising from the hot soil underneath will 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 vapor 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 vapor 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 surface 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 vapor-tension being higher underneath than above the stones, the result is, the rising vapor gets condensed 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 communication 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.