VIEWS OF RUNNING WATER. 657 



mary 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 dis- 

 tance apart. Inversely, two molecules, neighbors till then arrived 

 together at the same point b of the dam, are separated, one passing 

 by bi, the other by b 2} to reach the bottom widely separated. So 

 also with the molecules a 1 and a 2 . 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 re- 

 productions (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 im- 

 portant 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 me- 

 chanically 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 al- 

 lows 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 cas- 

 cade, and contribute to impress a special, individual character on 



VOL. XXXIX. 48 



