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THREE CRUISES OF THE "BLAKE. 



EQUIPMENT OF THE "BLAKE." 



The principal object of the hydrographer is to ascertain the 

 depth of the sea at any given point, and this seems at first 

 glance a very simple process. Sounding in a few fathoms with 

 an ordinary lead-line and a heavy sinker presents no difficulties, 

 and even in one hundred fathoms the hand lead-Hne can be used 

 with a moderate degree of accuracy. Beyond this, the problem 

 is very different. I well remember my own first experience of 

 sounding, in Lake Titicaca, in not more than one hundred and 

 fifty-four fathoms, — when more than five hundred fathoms of 

 line were payed out and ''no bottom" reported. This unsatis- 

 factory result was due simply to the insufficiency of weight of 

 the sinker. The experience of those who with the ordinary ap- 

 paratus have attempted to sound at still greater depths in the 

 oceanic basins has been the same. As the sinker descends, the 

 weight of the rope to which it is attached becomes, of course, 

 greater and greater ; the friction also increases to an alarming 

 extent ; and the action of the currents, where there are any, on 

 the immense surface presented to them by say one thousand or 

 fifteen hundred fathoms of two-inch rope, is sufficient not only 

 to counteract the very moderate weights used as sinkers, but to 

 exert a lateral force so strong in many cases that the depth 

 seems to increase in proportion to the amount of line payed out, 

 as if it were indeed fathomless. 



It is not astonishing, therefore, to find recorded such extraor- 

 dinary depths in some parts of the Atlantic as eight thousand 



