But although this is the correct view, and though physicists had made a noteworthy advance in getting rid of erroneous notions, yet a theory of dew still remained to be formed; for it was not yet shown how the cold, which causes the deposition of dew, is itself occasioned. The remarkable effects of a clear sky and serene weather in encouraging the formation of dew, were also still unaccounted for. On the explanation of these and similar points, the chief interest of the subject depends. Science owes the elucidation of these difficulties to Dr. Wells, a London physician, who studied the subject of dew in the commencement of the present century. His observations were made in a garden three miles from Blackfriars Bridge. Wells exposed little bundles of wool, weighing, when dry, ten grains each, and determined by their increase in weight the amount of moisture which had been deposited upon them. At first, he confined himself to comparing the amount of moisture collected on different nights. He found that although it was an invariable rule that cloudy nights were unfavourable to the deposition of dew, yet that on some of the very clearest and most serene nights, less dew was collected than on other occasions. Hence it became evident that mere clearness was not the only circumstance which favoured the deposition of dew. In making these experiments, he was struck by results which appeared to be anomalous. He soon found that these anomalies were caused by any obstructions which hid the heavens from his wool-packs: such obstructions hindered the deposition of dew. He tried a crucial experiment. Having placed a board on four props, he laid a piece of wool _on_ the board, and another _under_ it. During a clear night, he found that the difference in the amount of dew deposited on the two pieces of wool was remarkable: the upper one gained fourteen grains in weight, the lower one gained only four grains. He made a little roof over one piece of wool, with a sheet of pasteboard; and the increase of weight was reduced to two grains, while a piece of wool outside the roof gained no less than sixteen grains in weight. Leaving these singular results unexplained for a while, Dr. Wells next proceeded to test the temperature near his wool-packs. He found that where dew is most copiously produced, there the temperature is lowest. Now, since it is quite clear that the deposition of dew was not the cause of the increased cold—for the condensation of vapour is a process _producing heat_—it became quite clear that the formation of dew is dependent on and proportional to the loss of heat.