Yet, within the last few months, Mr Aitken, in a communication to the Royal Society of Edinburgh, has brought forward many observations, and the results of numerous experiments, which appear to prove that Dr Wells’ theory of dew is not, after all, correct. The essential difference between the old and the new theories is as to the source of the moisture which forms the dew. Instead of being condensed from the air above by the cooled vegetation, Mr Aitken maintains that it comes from the ground. The author of the original theory admitted that some of the dew might come from below, but affirmed that it must be an exceedingly small proportion. Mr Aitken’s experiments, on the contrary, seem to prove that most if not the whole comes from the ground. It is quite clear that the grass and soil do get rapidly cooled on dewy nights; but if they are below the temperature of the air above, the ground just under the surface is much warmer. Thermometers placed on the surface of grass were often found ten to eighteen degrees lower than those placed under the surface among the stems. In such circumstances, vapour must be rising from the soil, and part of it will condense on the grass, which has been cooled by radiation. By carefully weighing small squares of turf cut from a lawn before and after the appearance of dew on them, it was always found that they _lost_ weight. If the dew had condensed out of the surrounding air, the turf would have _gained_ in weight by the amount of dew deposited. It was thus clear that vapour was rising from the ground, only part of which was condensed on the grass, the remainder passing into the air. Another experiment, pointing to the same conclusion, was made by inverting thin trays over the grass. On dewy nights these trays were always found wet on the under surface; and the grass below them was always much wetter than that freely exposed outside. The moisture rising from the ground was evidently trapped and condensed, instead of being allowed to pass freely into the atmosphere. The explanation of the absence of dew on the surface of stones, roads, and other hard surfaces, on the old theory was, that these, being poor radiators, did not get much cooled. But closer observation shows that dew does form on stones and clods and gravel, only it is chiefly on the under surfaces. Thus, slates laid over both hard and gravelly roads are always found dripping wet on their under surfaces on dewy nights; while their upper surfaces and the surrounding roads are dry. During frost, too, clods and stones on the surface of the soil are almost always found to be covered with hoar-frost, showing that the moisture is trapped as it rises from the soil.