G8 THE BOTTOM OF THE SEA. 



wliich is blue, the wave will appear greenish-yellow. 

 Further, it is obvious that the difterent orientations 

 of the wave relative to the position of the sun must 

 contribute, as much as its changes of form, to vary 

 the play of light. This optic property of the waves 

 enables us to recognise afar off a change in the win*], 

 as it produces a different tint in the changing surface 

 of the sea. 



6. Measure of Temperature at the Bottom of the Sea. 



The attempt to ascertain the temperature at the 

 bottom of the sea at any given time is surrounded 

 with as many difficulties as the procuring of water 

 from certain depths. The thermometer is subjected 

 not merely to the influence of temperature, but to the 

 pressure of the water by which it is environed. Of 

 slight importance when the depth is inconsiderable, 

 this pressure becomes enormous when the descent is 

 measured by thousands of yards. 



M. Despretz has proposed to leave the thermo- 

 meter open, in such a manner that the pressure would 

 be sustained both by the interior and the exterior ot 

 the tube. It is generally preferred to enclose the in- 

 strument in some very solid envelope, and close it 

 hermetically. This envelope, formed of iron or cop- 

 per, is a good conductor of heat. The thermometer 



