66 ALONG THE FLORIDA REEF. 



water and tossed it to Long John, to be placed 

 in the water-pail for security. 



"Here's an odd fellow," Tom called out a 

 moment after, stooping over the rock and bring- 

 ing up a curious-looking spider-crab. 



" That is a deep-water one," said the doctor ; 

 "some of his big relatives, measuring nearly 

 three feet across, have been hauled up in the 

 South Atlantic from a depth of nearly two 

 miles." 



" As deep as that ? " exclaimed Harry ; " why, 

 I should think the pressure was too great for 

 animals to live at such great depths." 



" Water is practically incompressible," ex- 

 plained the doctor ; " that is to say, it can not 

 be forced into a smaller compass, as solids can. 

 So, as all these creatures are filled with water, 

 the pressure is equalized. If you lower an empty 

 bottle two miles under water it will often remain 

 intact, and yet the pressure in deep water is 

 simply tremendous. A deep-water crab, for in- 

 stance, must withstand a pressure, at such depths 

 as two and a half miles, of a number of tons as 

 against the fifteen pounds' pressure which a fish 



