14 THE OCEAN 



freezing point. Even in comparatively shal- 

 low watGT the temperature varies but slightly 

 and under the equator the water in the deep 

 sea is scarcely warmer than at the Arctic and 

 Antarctic Circles. 



All these facts we know from actual investi- 

 gation, for by means of weights attached to 

 strong piano-wire, self-registering thermom- 

 eters, pressure-recorders and other specially- 

 designed instruments we have learned a great 

 deal about the conditions at the bottom of the 

 sea. 



Even the simple matter of sounding and 

 securing samples of the bottom at great depths 

 was a most difficult problem to solve. If a 

 large rope or line was used it required enor- 

 mously heavy weights to carry it straight to 

 the bottom, arid if weights, of sufficient size 

 to sink the line were used they would be too 

 heavy to draw up without the cable giving 

 way under the strain. At last means were de- 

 vised for lowering an iron tube to the bottom 

 by means of a cannon-ball or other heavy 

 weight which was provided with a trip or 

 catch which released the weight when it 



