May 22, 1896.] 



SCUENGE. 



761 



The dykes or embankments which con- 

 fine the river on either side are about 25 

 yards wide and rise in two, or sometimes 

 three, terraces as approached either from 

 the plain or from the river, as if a wide 

 dyke of moderate height had j ust been i 

 made, along the summit of which a nar- 

 rower dyke had subsequently been raised. 

 The height of the dykes was estimated to 

 be about 26 feet, and being well grassed 

 over they do not present that strikingly 

 artificial character which might be ex- 

 pected. An excellent road runs along the 

 summit of the southern dyke. The dykes 

 thus, although not so high as the majority 

 of the houses in the villages on either side, 

 overtop the smaller houses and outbuild- 

 ings, while, standing on the bridge at the 

 middle of the river, seven feet above the 

 level of the stream, only the roofs and 

 upper stories of the buildings on either side 

 of the river can be seen. 



With regard to the level of the waters of 

 the Po as compared with that of the ad- 

 jacent plains many contradictory state- 

 ments have been made. The statement of 

 Lyell that at Ferrara it was as high as the 

 roofs of the houses was derived fi-om 

 Cuvier's ' Discours sur les Revolutions de 

 la Surface du Globe,' although not quoted 

 quite correctly, where the statement is 

 made on the authority of M. de Prony, an 

 Inspector-General of Bridges and Eoads,who 

 had been directed by the goverment to in- 

 vestigate the means of preventing the dis- 

 astrous floods caused from time to time by 

 the Po overflowing its banks. 



These very old observations were subse- 

 quently shown by Lombardini in 1847 to 

 be erroneous. This observer proved by 

 accurate measurements that, at the time 

 these were carried out, the mean height of 

 the Po only here and there rose above the 

 level of the plains and was generally con- 

 siderably below it, and that even during 

 the great flood in 1830 the surface of the 



river was scarcely ten feet above the pave- 

 ment in front of the Palace at Ferrara 

 (Geikie, Text-book of Geology, p. 368). 



Since this time, however, these conditions 

 have altered in a marked manner, the 

 more recent investigations of Zollikofer 

 having shown that in the normal condition 

 of the river the surface of the water in the 

 neighborhood of Ferrara is somewhat over 

 8 feet above the surrounding plains, while 

 in flood time the water in some places rises 

 from 16 to nearly 20 feet above the plain on 

 either side (Kovatsch — ' Die Versandung 

 von Venedig' — Leipzig, 1882, p. 35). 



At the time of my visit the surface of the 

 water was certainly higher than the level 

 of the plains, and the deep furrows in the 

 dyke on the left bank of the river showed 

 that in flood time the river now rises at 

 least as high as the top of the first terrace 

 of the embankment, which would be equiva- 

 lent to the height given above by Zollikofer. 

 That the river at times threatens to rise 

 even higher is shown by the fact that where 

 the upper terrace of the dyke is cut through 

 to allow the passage of the road from 

 Ferrara a brick wall has been constructed, 

 so arranged that by the insertion of planks 

 the highest level of the dyke may be main- 

 tained. 



The city of Ferrara, therefore, although 

 it might be subjected to disastrous in- 

 undation should the dyke on the right bank 

 of the river break, is not so seriously 

 threatened as might be inferred from Lyell's 

 statement, and the Po, which in flood time 

 ' hangs suspended, so to speak, over the sur- 

 rounding plains,' is now much-less dreaded 

 than in times past, owing to the irrigating 

 channels which tap it, as well as to a 

 secondary series of lateral embankments 

 which, placed at a considerable distance 

 from the dykes on either side, border the 

 whole course of the river below Cremona. 

 Frank D. Adams. 



McGiLL Unia^eesity, April, 1896. 



