so B^SUM^ OP SUBBLIi's STUDY OF 



velocity of a strong wind. Applying this to a torrent through A canal 8 

 mfetres, or 27 feet in breadth, and 2 mfetres, nearly 7 feet in depth, he 

 shews that it gives a flow of 228'48 cubic metres per second, while the 

 Garonne gives only a flow, in the ordinary state of the river, of 150 cubic 

 metres, and the Seine of 1 30 cubic mtoes, per second ; and thus is the 

 brief duration of the flow of a torrent accounted for. The calculation is 

 founded on a formula given in D'Aahuisaon S'^drmil — (p. 133), in which, 

 representing the fall per m^tre by p, the section of the body of water 

 by s, the perimefre mouille, or circumference of the wheel, by c, the 

 velocity = 51 square p s 4- o. 



It is founded on the observation that in such rapid currents the resistance 

 to the flow is proportional to the square of the velocity ; and extending the 

 computations to determine the size of blocks of stones which may be carried 

 down by such torrents, he shews that such a torrent as is supposed is capable 

 of moving a stone of the heaviest kind equivalent to a cube of 5 '15 mtoes. 

 But referring to the circumstance that a torrent 2 metres, or 7 feet in 

 depth, could not act on such a block over the whole of its side, he shews 

 that this will give only an equivalent of 2'74 cubic mtoes ; and then he states 

 that, in accordance with this, it is not rare to find blocks of 20 cubic metres 

 near slopes of 6 centimfetres per mfetre ; and that in the last preceding irrup- 

 tion of the torrent of Chorges the waters left on the bed de dejection 

 a hundred blocks of 30 cubic mtoes, and some even which measured 

 upwards of 60 cubic mfetres. 



Section II. — Natural History of Torrents in the High Alps. 



The most striking and characteristic featui'e of torrents — understanding 

 by that term what in English would be called the bed of the toiTent — is, 

 according to M. Surell, the deposit known technically as the lit de dijection, 

 though this can only be considered a product of the flow of water by which 

 that bed of the torrents is produced, for, if the waters had not carried off 

 the material deposited, then there could have been no deposit; and by this 

 are supplied indications of the comparative age or antiquity of many 

 torrents now extinct. 



Often, says M. Surell, are we struck, in passing through the department, 

 with the appearance of a flattened mound, situated at the opening of a 

 gorge, presenting a fan-shaped surface with very reg-ular slopes, — it is the 

 bed de dijection of an ancient torrent. 



" Sometimes careful continued observation is requisite to the discernment 

 of the original form, concealed as this is by massive trees, by cultivated 

 fields, and often even by houses and towns. But when it is examined with 

 care, and looked at under different aspects, the outline so characteristic of 

 beds de dejection comes out at last most clearly, and it becomes impossible 

 to mistake it. Along this mound flows a little streamlet which proceeds 

 from the gorge, and peacefully traverses the fields. It is this which has 

 ormed the ancient torrent, and in the depth of the mountain may be dis- 

 covered the old basin de reception, recognisable also by its form. 



" These extinct torrents, if such a phrase may be used, are more numerous 

 than one at first thought would expect. When once the key to be employed 

 in the search has been obtained, and attention is directed to them oreat 

 numbers are discovered. ' ^ 



