114 tITERATURE ON TORRENTS. 



even acquired that duU tint which prolonged exposure to the air gives to 

 them, it is indicative that the instability is very great. 



" The existence, then, of a torrential perturbation, attributable to matters 

 borne along, is demonstrable. But more than this, this perturbation is 

 subject to laws as constant as those which regulate the flow of water. The 

 a priori proof of this is the form taken by deposits which are the products 

 of this action. 



" Nothing is more irregular, to all appearance, than the floods of the great 

 torrents of the Alps. Those who have read the impressive descriptions of 

 them given by M. Surell, know that they look like chaos : blocks of stones 

 roUing along with powerful crashes, kaocking one upon another, and a 

 current black as ink, bounding over all obstacles, and spreading itself with 

 extreme mobility over a widely extended surface without being able to fix 

 itself any where. One is ready to believe that this enormous body of stones, 

 borne off by the waters, is about to be scattered abroad at hap-hazard, and 

 to form a confused mass, setting at defiance all rule ; on the contrary, it 

 is a curious fact of immense compass, that though the torrent, in pursuing 

 for ages its work of clearing away in the mountain, and of embanking in 

 the plain, may have multiplied indefinitely its floods and its transports of 

 material, the constant result of this continuous action — the one completed 

 result of all these elementary embankments — that which is designated the 

 lit de dejection — has assumed a geometrical form of the most perfect regul- 

 arity ! The determining of the geometrical law, by which the contour of 

 these deposits with its numerous distinct characteristics has been regulated, 

 presents considerable difficulties. I shall afterwards state what is my opinion 

 on this point ; but, whatever that law may prove to be, there evidently is 

 some such law ; and it is enough, at this stage of the discussion, that this 

 has been established." 



Alleging, then, that we may conclude with certainty that that work of 

 the torrent, in appearance so irregular, has been governed by laws, and 

 these the laws of torrentiality, he goes on to say, that it is desirable to 

 determine what these laws are, as they are likely to throw some light upon 

 the problem of inundations, and to indicate a rational solution of this, while 

 the solution of the problem is of no small interest to the science of terres- 

 trial physics, and even to that of universal cosmogony. 



In accordance with what is stated by M. Costa in regard to the con- 

 vexity of the flood of the torrent, when charged with earthy matter, are 

 the observations of M. Surell in regard to the convexity of the lit de dejec- 

 tion, the last form taken by the suspended earthy material as the water 

 subsided, though this convexity may be otherwise accounted for. Some- 

 thing similar may be observed in a flow of treacle, or of tar, or of quick- 

 silver, or other molten metal ; but something similar may be seen also in 

 a very rapid flow of water comparatively pure. 



By Marsh it is stated, in a foot-note appended to a passage in his treatise 

 on The Earth as Modified hy Human Action, — " Many physicists who ha\e 

 investigated the laws of natural hydraulics maintain that, in consequence 

 of direct obstruction and frictional resistance to the flow of the water of 

 rivers along their banks, there is both an increased rapidity of current and 

 an elevation of the water in the middle of the channel, so that a river pre- 

 sents always a convex surface. Others have thought that the acknowledged 

 greater swiftness of the central current must produce a depression in that 



