10 THE OCEAN. 



imperfect, and, except for inconsiderable depths, do not give results of 

 rigorous accuracy. In those latitudes where the water is many 

 hundreds, or even thousands of fathoms in depth, they cannot risk the 

 taking a sounding unless the atmosphere and the waves are in an ex- 

 ceptional state of tranquillity ; and even then the slenderness of the 

 cord, the weight of the apparatus, the enormous pressure it endures 

 as it descends, and which increases at the rate of one atmosphere 

 for every 11 yards of immersion, and finally, the long hours which 

 must be employed in this delicate operation, greatly endanger the 

 final success. Unless instruments furnished with electrical bells, like 

 those of Schneider or of Gareis and Becker,* and others more easily 

 employed, more rapid and sure, are used, " bathymetric " observ- 

 ations will be always at great distances from each other, and it will 

 not be possible to construct a submarine map in relief, such as is 

 being constructed of the surface of the continents. Besides, it is 

 very rarely that sailors take soundings in the deep seas simply 

 for the scientific pleasure of investigating the depth of the ocean. 

 It is solely for the requirements of navigation, of commerce, and of 

 industry, that they have observed the depth of the sea, either in 

 gulfs like the Adriatic, or in parts that are filled with sand-banks 

 like the North Sea, in the neighbourhood of coasts and rocks laid 

 down in ancient maps, or in those parfs of the ocean which are 

 destined to receive electric cables. In the open sea ships sail 

 almost entirely over unmeasured depths. 



Owing to its elongated form and to the amphitheatre of lofty 

 mountains, which all but wholly surround it, the Adriatic ofiers a 

 very remarkable example of the continuation of the continental slopes 

 below the level of the sea. The bed of the northern part of this gulf, 

 which is a continuation under water of the level plains of Yenetia, has 

 an exceedingly gentle slope, much less, in fact, than that of the plains 

 of Lombardy, which seem horizontal. f The sounding-lead shows 

 only a depth of 54 fathoms beyond the narrows formed by the islands 

 of Zara and the headland of Ancona ; thus more than a third of the 

 Adriatic is found not to exceed in mean depth rivers like the Mis- 

 sissippi and the Amazons. Farther south the submarine declivity, 

 which continues on one side that of the Apennines, on the other that 

 of the Alps of Dalmatia, becomes comparatively greater, and the 

 sounding-lead descends to about 110 and even 170 fathoms below the 

 surface. At this spot the sea forms a sort of hollow, bounded on 



* Physiographie des Meeres, 1 867. 

 + G. Collegno, Geologia delV Italia, p. 12. 



