TORRENTS, ETC., BY COSTA DE BA8TELICA. 117 



ascei'talned, the imagination set to work to seek out fanciful explanations 

 of what was seen. 



" Thus, for example, has it beeii with the generally alleged fact, that 

 at the moment of flood large atones set off of themselves, rolling in advance 

 before the current had touched them, under impulse from a current of air 

 preceding the advancing head of waters. 



"Eye-witnesses, and these grave men, have affirmed this fact to myself; 

 and M. Surell has collected numerous testimonies of this phenomenon, and 

 has sought to account for it theoretically. In reality, the fact as reported 

 is absolutely impossible. Kesistance increases as the square of the velocity. 

 Let a calculation be made, from the velocity necessary to a current of air to 

 displace a stone no larger than an egg ; what velocity would require to be 

 imparted to a current of air capable of displacing a stone such as some of 

 those of which this has been told, which must have been at least 50 centi- 

 mfetres, or 20 inches in diameter ! The thing alleged is physically impossible ; 

 and it must be remarked that the people who allege they have seen those 

 things occur under their eyes, at a few paces from them, do not dream that 

 if they had been caught in such a current of air they would, at least, have 

 felt it ! 



" When these witnesses are cross-questioned, they all declare that they 

 have seen the stones rolling dry before them ; but no one says he has seen 

 these stones begin to move. These witnesses are trust-worthy, in so far as it 

 is true that the stones were seen rolling before their eyes ; but the point in 

 which they deceive themselves is the explanation of the phenomenon. They 

 have attributed this to the force of the current of air ; there is the mistake. 



" The fact is a very simple one, and easily explained in accordance with, 

 what has been evolved by the study of the effect of a sudden retardation on 

 a current of matter. Through the velocity acquired, and the upward direction 

 given to their movement, the stones, detaching themselves, are projected 

 forward from the water by which they were borne along. 



" We find that it is towards the contraction of a water-course, occasioned 

 by a bridge, that the phenomenon manifests itself with most intensity. It 

 is, moreover, at such points that it ought to be most easily observed. At 

 the time of a great flood, there are few spectators in the deserted gorges of 

 the torrents. 



" From the moment that we are in possession of principles, nothing is 

 more easy than to account rationally for all the effects, and all the accidental 

 incidents which they may produce." 



And in a similar way does M. Costa account for other remarkable pheno- 

 mena which have been observed in connection with torrents. 



In a third part of the work, M. Costa discusses at great length the 

 extinction of torrents. In his statement of this question, he says, — "According 

 to the opinion of all the engineers who have had to contend with torrents 

 with a view to the protection of valleys against their ravages, works simply 

 defensive have been acknowledged to be in most cases, if not useless, at least 

 altogether insufficient, and often dangerous, intensifying at times the evil. 



" M. Surell more especially has put this fact in a clear light ; and he has 

 established beyond all controversy that it is necessary to carry into the basin 

 the works designed for the protection of the land, that the evil may be 

 attacked at its source. 



" He has demonstrated, not less triumphantly, that boisement is the most 

 potent means of extinguishing torrents, as by vegetation we can act, at one 



