PHYSICAL OCEANOGRAPHY 247 



this litre were brought up to the surface, it would expand so 

 that its volume would be increased by 18 cubic centimetres; 

 subtracting the 18 c.c. and weighing the remaining litre we 

 find a weight of 1028 grams. Thus even at a depth of 4000 

 metres the difference caused by pressure is not great. 



Now, what is the effect of this increase of density on a solid Sinking of a 

 body lowered into the sea ? Let us suppose a piece of solid ^^^''^ ^""^y* 

 iron, weighing 1000 grams in the air, to be sent down to 4000 

 metres at Station 63. When it is lowered just beneath the 

 surface it becomes lighter by 131 grams, thus weighing 869 

 grams. When it has reached a depth of 4000 metres the 

 buoyancy is 134 grams, so that the piece of iron there weighs 

 866 grams — a difference in weight of 3 grams for a piece of 

 iron weighing 1000 grams in air. This is merely 0.3 per cent 

 of the weight, and consequently quite insignificant. In other 

 words, metals and other solid substances are practically just as 

 heavy in deep water as they are at the surface, and will sink as 

 rapidly there as in shallow water. This may be proved by 

 direct observation, for if a messenger is sent down to close a 

 water-bottle at a depth of 2000 metres it will be found to take 

 four times as long as when sent down to 500 metres. 



But suppose that, instead of a massive piece of iron, we take sinking of 

 a perfectly tis^ht capsule of thin iron filled with air, and lower it ^n air-fiiied 



i. J <j X ' C3.DSUIg. 



down to 4000 metres ; in the course of the descent the pressure 

 increases, forcing the walls of the capsule together. The 

 volume of air within the capsule may be so large that it only 

 just sinks at the surface, its total specific gravity being then 

 very little greater than that of the water ; but when it has 

 reached a depth of 10 metres the air is compressed to half its 

 original volume, granted that the capsule is collapsible, and the 

 weight of the iron then acting more freely, the capsule will sink 

 faster and faster ; when it reaches a depth of 4000 metres it is 

 exposed to a pressure of 400 atmospheres, and the compressed 

 air having hardly any buoyancy left, the capsule will sink almost 

 as fast as if it had been made of solid iron throughout. 

 Collapsible solid bodies containing air will accordingly sink 

 faster in deep water than at the surface. A piece of wood 

 floats at the surface because it contains a large amount of air, 

 but there is nothing to prevent it from sinking when it is sent 

 down into deep water ; therefore wood and cork are not 

 suitable for floats at great depths. It is the same with the dead 

 bodies of marine animals, etc., for when the air is compressed 

 they will easily sink. 



