24 A TEDIOUS BRIEF EXPLANATION 



principle that underlies it if he can imagine the sinkers 

 to be threaded on to the sounding-tube in a row, 

 exactly like a row of beads on a needle. If the needle 

 be held perpendicularly, point downwards, the beads 

 will, of course, slip off But if, when the needle is in 

 this position, the lowermost bead be kept from falling 

 by a loop of wire passing down beneath it, then so 

 long as this loop is kept taut from above, the beads 

 cannot slip off, since the lowermost bead is kept in 

 place by the pull on the loop. But the moment the 

 tension on the loop is relaxed, the lowermost bead 

 will fall off and the others will slip off after it. ''Well, 

 then, that's the humour of it," as Corporal Nym says ; 

 but if any reader wishes to have an exact acquaint- 

 ance with the apparatus, I would refer him to pages 

 60 and 61 of the first volume of the Narrative of the 

 Voyage of H.M.S. Challenger. 



It must be mentioned with regard to the hollow 

 part of the sounding-tube — the part that sinks into 

 and brings away a sample of the ocean-bottom — that 

 its mouth is valved, so as to let the mud in but to 

 prevent its falling out again. As for the sinkers that 

 carry the tube down, they slide off on the bottom 

 and are no more seen. 



For deep-sea temperature observations, maximum- 

 minimum thermometers, specially protected against the 

 enormous hydrostatic pressure to which they are ex- 



