The habitual dryness of the American climate increases, as we advance west and north-west from the Missouri, where there frequently is not a drop of rain for six months. This is owing to the great distance from any sea, the superior elevation, and the comparative want of timber, combined with the greater intensity and longer duration of the north-west wind, which sweeps with unobstructed force over the naked plains. It appears, then, that more rain falls in fewer days, in America, than in Europe; and that there are fewer cloudy days, more fair days, and quicker evaporation. It is to this last circumstance we must ascribe those immense dews, unknown in European climates, which occur in America, and which are so copious in summer, as to resemble heavy showers of rain. But it must also be observed, that dews are comparatively unknown in the tract watered by the Upper Missouri; and which, in all probability, is owing to the want of timber, wood being limited to the banks of the rivers, which are commonly bordered with trees. GENERAL REMARKS ON CLIMATE. It is the opinion of Professor Leslie, that all the varieties of climate are reducible to two causes; distance from the equator, and height above the level of the sea. ‘Latitude and local elevation form, indeed,’ says he, ‘the great basis of the law of climate, and any other modifications have only a partial and very limited influence.’ Climate is generally treated of under four divisions: the cold and humid; cold and dry; warm and humid; hot and dry. But these climates do not always exist according to the full import of the terms by which they are designated. They are subject to modifications, principally of two kinds; the one arising from the alternation of two different climates in the same region, the other from the greater or less prevalence of either of the four elements. Thus when heat, dryness, and humidity are duly combined, they render the climate comparatively temperate. In Egypt, for instance, the combinations of heat and humidity, during the inundation of the Nile, and of heat and dryness during the rest of the year, temper a climate, without which these alternations would be insupportable. In Holland the cold humidity of the autumn is succeeded by frost, which increases the salubrity of the climate, that would not otherwise be so healthy.