The physics of waterfalls have received but little attention, though it might have been anticipated that they must involve considerations of an interesting nature, and of some philosophical importance ; for in them we have the principal cases in nature of the mechanical agitation and division of seater, resulting chiefly from its own inertia, gravity, and momentum. In the greatest waterfall in Europe, for example, the Wiring Fos, near the Hardanger Fiord, in Western Norway. we have very large body of water, and a clear fall of NO feet. It deecen,.s into a narrow bare chasm, the sides of which are very nearly perpen dicular. " The spray dashes up from below with such force, that another waterfall, which descends on the opposite side of the chasm, is actually stopped by it and dispersed before it can reach the bottom." (11. F. Tozer,' Vacation Tourists in 1860.) The late Captain Basil Hall, in his ' Travels in North America,' thus describes the involution of air in the Horse-shoe Fall of Niagara, and its subsequent disengagement. " This enormous cataract, in its descent, like evcry other cascade, carries along with it a quantity of air, which it forces far below the surface of the water, an experiment which any one may try on a small scale by pouring seater into a tumbler from a height. The quantity of air thus carried down by so vast a river as Niagara must be great, and the depth to which it is driven in all probability considerable. It may also be much condensed by the pressure; and it will rise with proportionate violence both on the outside of the cascade, and within the sheet or curtain which forms the cataract" He mentions also the blast of wind which rises accord ingly from the pool on the outside of the sheet. To the explosion, as it way be termed, of the bubbles, thus constituting this enormous and perpetually renewed accumulation of bursting froth, we must ascribe the greater part of the sounds which combine to produce the roar of the Niagara falls. In doing this coo apply to the subject the observa tions of Professor Tyndall, on the sound of breakers and the roar of the ocean. Vacation Tourists in 1860,' p. 307, note.) The impact of water against water, he has shown, is a comparatively subordinate source of sound ; though in this case, from the enormous masses of seater concerned, and the momentum which those of the river acquire from the height from which they descend, the sounds proper to that impact must be very considerable. The sonorousness of the roar, also, agrees with this ascription of it, principally, to the violent impact of air against air, the air impinged upon beiug that of the free atmo sphere, by which the sound is communicated to the ear. The ex plosions of the bubbles must take place, in many instances, with enormous force, owing to the resistance presented by the immense cohesive power of films of water, and occasion corresponding loudness of the sounds, which, bosvever, in coalescence and rapid succession are heard as an interminable roar.
To these, no doubt enormous bubbles of condensed air, rushing upwards to the surface of the pool, and oxpandiug as they rise, we must also probably attribute the production of the sharp-pointed cones of water which are continually projected upwards from the pool, on the outside of the fall, sometimes to the height of a hundred and ten or twenty feet. They are further described by Captain Hall (who, however, expresses no opinion as to their origin), as resembling some comets in form, their point, or apex, which is always turned upward, being quite sharp, and not larger, he estimates, " than a man's fingers and thumb, brought as nearly to a point as possible. The conical tails which stream froni these watery meteors may vary from one or two yardsto ten or twelve, and are spread out on all sides in a very curious manner." The actual production of these cones, and their emergence from the surface of the pool, which must be their beginning above seater, if the origin hero ascribed to them be the true one, are concealed by the clouds of spray in which the bottom of the falling sheet is constantly hidden from tho view. Out of this they are at all times seen darting up.
This cloud or mist of spray is itself an interesting phenomenon, characteristic more especially of great falls, like those of Niagara, uniting depth of fall and quantity of water. It arises from the bottom, where the impact of the descending upon the still or merely flowing water takes place ; and it involves several physical considerations which seem to have escaped notice. Ac produced by the Niagara falls,
its extent and visibility from a diet:Ince have often been described. The lower part of the great fall, according to Captain B. !tall, is always concealed by this rolling cloud of spray, which waves backwards and forwards, and rises at times to the height of many hundred feet above the falls, Isaac Weld describes it cos coneistiug of thick volumes of whitish mist, having much the appearance of smoke rising from heaps of burning weeds ; such smoke, it may bo added, itself consist• ing priueipally of steam condensed into globules of water, or steam• cloud. The same traveller relates that this "cloud formed from the spray" of the Niagara falls was seen by him and his party on Lake Ontario, at the distance of fifty-four miles, as "a small white cloud in the horizon," which remained steadily fixed in the same spot, its shape, as observed by a telescope, "varied every instant, owing to the continued rising of the mist from the cataract beneath." But it is only seen at such a distance, he adds, when the "air is very clear and there is a fine blue sky." Weld also describes the now well-known appear ance of coloured bows in the spray, similar to rainbows, and for which it will be convenient in this article to use that appellation.
Notwithstanding the production of these rainbows, proving the mist to consist of separate drops of water, in the condition, in fact, of small rain, it has frequently been described as vapour, and identified in nature with fog and the clouds of the sky,* although rainbows do not occur in them, and although we have no reason to believe that the conversion of water into vapour can ever take place by mechanical action, however violent and continued. Were this possible, indeed, it could not but take place at these falls, where the water is perpetually exposed to the consequences of greater mechanical force than under any other circum stances whatever, either in nature or art ; where indeed all the circum stances are most favourable to such conversion, were it physically possible.
We have seen in the example of the Voring Fos, how enormous is the force with which the spray into which the descending is divided by the momentum with which it falls upon the still water, and partly, no doubt by the resilient elasticity of the latter, is driven upwards. Such spray, but greatly varying in magnitude, number, and approximation of particles, and therefore in their aggregate effect and appearance, may be produced under conditions in which water impinges with great force upon water, or upon solid bodies, or solid bodies upon it ; also as in storm-spray, by the action of the wind on the surface of the sea, by which, as by the perpetual action of waterfalls, persistent mist is fre quently occasioned. Further, it is produced to a sufficient extent for the appearance of rainbows, when the waves of the sea, at their sum mits, curl and break into foam under the force of a breeze. But the spray-mist of great waterfalls is in all probability occasioned, in the greatest degree, and as consisting of the most minute particles, by the violent bursting of the multitudinous air-bubbles already described, forbaing the froth or foam of the falls, by which the films of water inclosing the air are rent into minute dust, as it were, of water, incalculable in the number of particles in a given space, and capable of suspension in the air, like the_globules of which the clouds of the sky themselves consist. [CLouns.J These particles, however, differ greatly from those of the clouds, and the connection of their history with the subjects of the smallness of the particles into which water is thus mechanically divisible, and the true nature of the clouds, and of the globules or particles composing them, is in several respects instructive. The spray of waterfalls and of the crests of waves consists of globules or particles of water, result ing, exclusively, from its mechanical division ; but the production of perfect rainbows in it evinces that it is identical, within certain limits, in respect of the magnitude of the particles, their distance from each other, and their distribution in a given space, not with true cloud, but with tL3 minute rain in which the rainbow of the sky occurs under ordinary circumstances. We have, therefore, in the production of that meteor, a kind of measure of the magnitude, constitution, and dis tribution of the particles, which, in all these cases, it is demonstrable must be geometrically solid.