precipitated, appears as a cloud, the particles of which collect into rain-drops and fall to the earth. Thus the air suddenly loses much of its weight, and instead of restoring equilibrium to the troubled atmosphere, it introduces a new source of disturbance. Though the weight of the air is diminished by the fall of rain, yet the bulk is increased by the expansive force of the latent heat which the condensed vapors set free. Thus the rainy air expands upwards and flows outwards, and no longer able to balance the pressure of the surrounding air, it is carried still higher by inblowing winds, which rise in turn and continue the process, often extending the storm over vast areas. The force of these movements is measured partly by the force of latent heat set free, and partly by the mechanical power of the rain-fall, a very small fraction of which constitutes the water-power of all our rivers. Such a fruitful source of disturbance, generated by so slight an accident as the upward movement of the saturated air, expanded by its own agency to so great an extent, so sudden and discontinuous in its action, so obscure in its origin, and so distinct in its effects,—such a phenomenon defies the powers of mathematical prediction, and rouses all the winds to sedition.
A storm not only disturbs the lower winds, but its influences reach even to the upper movements. The sudden expansion and rising of the rainy air delay these movements, which afterwards react as violent winds.
The forces stored away by the gradual rise of vapor and its absorption of heat, and then suddenly exhibited in a mechanical form by the effects of rain, afford an illustration of that principle of conservation and economy of power, of which there are so many examples in modern science. No power is ever destroyed. Whether exhibited as heat or mechanical force, in the products and forces of chemical or of vital action, in movement or in altered conditions of motion,—whether changed by the growth of plants into fuel or into food, and converted again to heat by combustion or by vital processes, and brought out as mechanical power in the steam-engine or in the horse,—it is still the same power, and is measured in each of its forms by an invariable standard. It first appears as the heat of the sun, and a portion escapes at once back into space, while the rest passes first through a series of transformations. A part is changed into moving winds or into suspended vapor, and a part into fuel or food. From conditions of motion it is changed into motion; from motion it is changed by friction or resistance into heat, electric force, molecular vibrations, or into new conditions of motion, and passing through its course of changes, it remains embroiled in its permanent effects or escapes into space as heat.
Though mechanical science will probably never be able to predict the beginning or duration of storms, it will yet, doubtless, be able to account for all their general features, and for such distinct local peculiarities as observation may determine. Great advancement has already been made in the determination of prevailing winds and in the study of storms. Two theories have been brought forward upon the general movements of storms; both have been proved, to the entire satisfaction of their advocates, by the storms themselves; and probably both are, with some limitations, true. The first of these theories we have already described. According to it, the winds move inward toward the centre of the storm; according to the other theory, they blow in a circumference around the centre.
Observations upon storms of small extent, such as thunder-storms or tornadoes, show very clearly that the winds blow toward the stormy district. But when observations are made upon the winds within the district of such extensive storms as sometimes visit the United States, the directions of the wind are found to be so various, that the advocates of either theory, making due allowance for local disturbances, can triumphantly refute their adversaries. In such storms
