culation of Mr. Thompson's theory. Both these regular movements are, however, greatly disturbed, and especially the latter, by winds which are occasioned by local and irregular rains.
In these movements and their causes we have the general outline of our subject, within which we must now sketch the weather. The causes of atmospheric movement, which we have thus far considered, are the unequal distribution of the sun's heat, the absorption and precipitation of moisture, the direct and the inductive action of the earth's rotation and friction. If to these we should add the tidal action of the sun's and moon's attractions, we should perhaps complete the list of veræ causæ which are certainly known to exert a more or less general influence upon the atmosphere. But this short list is long enough, as we shall soon see.
If the earth were wholly covered with water of a uniform depth, its climates would be distributed with greater regularity, and the perturbations of climate would be comparatively small and regular; though even under such circumstances there would still exist a tendency to discontinuity and complexity of movements from that influence of rain, the peculiar character of which we shall soon consider.
The irregular distribution of land and water, and the peculiar action of each in imparting the heat of the sun to the incumbent air,—the irregular distribution of plains and mountains, and their various effects in different positions and at different altitudes,—the distribution of heat effected by ocean currents,—all these tend to produce permanent derangements of climate and great irregularities in the weather. To these we must add what the astronomer calls disturbing actions of the second order,—effects of the disturbances themselves upon the action of the disturbing agencies,—effects of the irregular winds upon the distribution of heat and rain, and upon the action of lands and seas, mountains and plains. Though such disturbances are comparatively insignificant in the motions of the planets, yet in the weather they are often more important than the primary causes.
The aggregate and permanent effect of all these disturbing causes, primary and secondary, is seen in that irregular distribution of climates, which the tortuous isothermal lines and the mottled raincharts illustrate. The isothermal lines may be regarded as the topographical delineations of that bed of temperatures down which the upper atmosphere flows from the equator toward the poles, till its downward tendency is balanced by the centrifugal force of its eastward motion. This irregular bed shifts from month to month, from day to day, and even from hour to hour; and the lines that are drawn on the maps are only averages for the year or the season.
In the midst of these irregular, but continuous agencies, the rain introduces a peculiar discontinuity, and turns irregularity into discord. We have shown that the rain is an immediate cause of wind; but how is the rain itself produced? For so marked an effect we naturally seek a special cause; but no adequate single cause has ever been discovered. The combination of many conditions, probably, is necessary, such as a peculiar distribution of heat and moisture and atmospheric movements; though the immediate cause of the fall of rain is doubtless the rising, and consequent expansion and cooling, of the saturated air.
The winds that blow hither and thither, vainly striving to restore equilibrium to the atmosphere, burden themselves with the moisture they absorb from the seas; and this moisture absorbs their heat, retards their motion, and slowly modifies the forces which impel them. Now when the saturated air, extending far above the surface of the earth, and carried in its movements still higher, is relieved of an incumbent weight of air, it becomes rarefied, and its temperature and capacity for moisture are simultaneously diminished; its moisture, suddenly pre-
