Page:Popular Science Monthly Volume 39.djvu/675

Rh sheet is cut into two veins, which, losing trace of their common origin, remain definitely separated (Fig. 14).

When the sheet is very shallow and very wide, like that which spreads over a river dam, insignificant inequalities at the dam are enough to provoke a division into many distinct sheets. Consider two of these near one another (Fig. 12, xxiii). It may happen that cord a of the left arc and cord c of the right arc, coming very near to each other, join; thus are united molecules of water which an instant before had come out of the sluiceway at a considerable distance apart. Inversely, two molecules, neighbors till then arrived together at the same point b of the dam, are separated, one passing by b1, the other by b2 to reach the bottom widely separated. So also with the molecules a1 and a2. In the most regular fall, not a single molecule reaches the bottom by the most direct way. The same molecules which were traveling parallel in the bed of the canal, quietly keeping alongside of one another, seem, as soon as they reach the dam, to be suddenly taken with a frantic thirst for liberty, throwing themselves to the right and the left, joining, separating, and joining again; it is a go and come that never ceases till they find themselves newly imprisoned in the bed of a brook. Hence the serpentine course of the threads of water, the sparkling, the tremulousness which are common to cascades with broad sheets, and constitute their charm. The photographic reproductions (Figs. 18 and 19) represent, as well as can be done graphically, most of the characteristics which we have just described.

This is what occurs normally, under the influence of weight, cohesion, and inertia. When water runs out from a regular orifice, or over a horizontal dam, how complicated everything becomes when these conditions of regularity are not fulfilled, or when other forces come into play! One force that plays an important part in this matter is adherence between the molecules of the water and the walls of the vessel. This it is which often causes a liquid to follow a different way from that which is mechanically pointed out to it by inertia, weight, and cohesion. It then springs out into prohibited roads; instead of going to the cup held out to receive it, it chooses to follow the rim of a bottle and to spatter itself over the white cloth. Instead of precipitating itself into the pool with its companions, some capricious vein allows itself to be tempted by a nothing, to glide along a wall of rocks, and to trace those silvery threads which are often more graceful than the cascade itself. Yet adherence is a physical force, the effects of which may be foreseen and calculated up to a certain point. There are other factors, varying infinitely, but also calculable, which constitute, we might say, a part of the cascade, and contribute to impress a special, individual character on