28 



DEPTHS OF THE OCEAN 



and of the steel line (piano wire) afterwards adopted for 

 sounding. It will be obvious at once what a saving of space 

 is obtained by the use of a steel line. This 

 will be clear, too, if we look at Sir William 

 Thomson's sounding machine, the principle 

 of which is clearly illustrated by the follow- 

 ing instructive figure from Sigsbee's book 



(P'g- 5)- , . , , 



The wire is wound m by a large wheel 



consisting of a drum 2 feet 6 inches in cir- 

 cumference between two thin galvanised iron plates 6 feet in 

 circumference, the object of making this wheel of such a size 

 being to enable the line to be paid out and hauled in quickly. 



In taking soundings the art consists in getting the wheel 

 and line to stop the moment the plummet touches the bottom. 



00 



Fig. 4. — Sounding-Line 

 AND Wire. 

 a and l>, Circumference of 

 the hemp sounding-line of 

 the "Challenger" ; c, piano 

 wire. (From Sigsbee.) 



The line drifts when free of the lead, as it is, of course, relieved 

 of the weight as soon as the bottom is reached, but there still 

 remains the weight of the line itself, while the momentum of 

 the wheel will cause it to continue revolving for a little while. 

 The wheel must consequently be made as light as possible, and 

 a resistance of some sort must be provided, rather stronger at 



