Fogs have a considerable influence on the winter. In the summer of 1783, an uncommon fog prevailed all over Europe, and great part of North America. It was dry, of a permanent nature, and the rays of the sun had but little effect in dissipating it, which they easily do in moist fugs arising from water. The effect of the rays in heating the earth was exceedingly diminished : hence its surface was frozen early, the first snows remained on it undissolved, and received continual additions; the air was more chilled, and intensely cold, and the winter of 1783 and 1784 was exceedingly severe.
The spring fogs are most detrimental to such young fruit, and other trees, as are planted in low situations; tuations ; because they moisten the young shoots, and thus render them more liable to the injuries of the frosty nights succeeding them, but which they escape when placed in more elevated situations. - These fogs are converted into rime during the night, which thus falls on the trees, and is in some circumstances believed to shelter the vegetables by the heat it emits at the moment of its freezing : hence black frosts, which are not accompanied with rime, are said to be more prejudicial. But Dr. Darwin remarks, that where dew or mist descends on vegetables, before the act of freezing commences, and is partly absorbed by them, they become more succulent, and are thus destroyed by their fluids being converted into ice. To obviate this inconvenience, he proposes to make temporary sheds in the walls of gardens, projecting eight inches from the walls, and to be held by hooks that may be easily removed, when no more frosts are to be apprehended. Dr. Darwin successfully tried this expedient with an apricot-tree, which was preserved uninjured, either by the fog, or the frosts that followed it, during the vernal nights.