144 SATURDAY LECTURES. 



iiiade, and a l)rief review of the history of such explorations 

 and the part taken in them by our own country. 



The determination of the depth of the sea, at least to a 

 certain distance from the surface, is a necessary preliminary 

 to navigation, and hence has grown with commerce from 

 the time when the primitive savage tirst launched his rude 

 canoe in the Nile delta until the project of an Atlantic 

 cable rendered it necessary to plumb the depths of ocean. 

 The use of a weighted line for this purpose probably long 

 antedates the historic period. The ordinary mariner's lead- 

 line is familiar to nearly everybody, and consists simply of 

 an elongated piece of lead with a slight indentation in its 

 bottom, and a hole in its smaller end by which is attached 

 a stout cord, large enough not to cut the hands when being 

 hauled in, marked at regular intervals with tufts of red, 

 white, and blue flannel, or small leather tags, to indi- 

 cate the length of line run out. In the cavity in the bot- 

 tom of the lead is usually placed a little tallow or hard 

 soap, Avhicli will bring up a few particles serving to indicate 

 the kind of mud, sand, or gravel, of which the bottom is 

 composed. In ordinary depths the line runs out rapidly 

 until the bottom is reached, and the thump of the lead on 

 hard sand is distinguishable in still water at a depth of 

 nearly a quarter of a mile. 



But in greater depths and in waters, or from a vessel, more 

 or less in motion, the accurac}^ of the soundings possible by 

 the common method becomes rapidly less, and at depths of 

 1,000 fathoms (about a mile) the determination becomes 

 quite untrustworthy. 



This was not at first realized by investigators, and more 

 or less confidence was placed in depths, such as those re- 

 ported by Walsh, Denham, and Parker, who ran out from 

 six to ten miles of line, in the Atlantic without recognizing 

 that the bottom had been reached, in regions where we now 

 know the depth does not vary much from two miles. The 

 mystery and uncertainty which thus became associated with 

 the conception of the depths of ocean had, as will be shown 

 hereafter, an important effect in retarding attempts at ex-' 

 ploration of the deep sea. 



