AMERICAN INSTITUTE. 469 



on liis body. At three hundred and sixty-one feet, one hundred 

 and eighty-one pounds ; at six hundred and six feet, two hundred 

 and eighty-six pounds j at four thousand two hundred and six 

 feet, eighteen hundred and thirty-one pounds to the square inch; 

 at six tliousand feet, over one ton. Whales sometimes descend 

 into the depths of the ocean four thousand nine hundred feet, 

 when they sustain considerably over two hundred thousand tons, 

 nearly, if not quite, one hundred and thirty-eight tons to each 

 square foot of surface exposed. 



A column of sea water thirty-three feet in height is about the 

 same weight as a column of air of an equal base, extending from 

 the earth's surface to the atmosphere's limit. When a tremendous 

 gale of wind passes over the ocean, fish will find it perfectly tran- 

 quil at the depth of two hundred and fifty feet. By diminishing 

 the friction of the wind by the use of oil, or ice, you may make 

 the surface of the sea quite smooth. 



The Leech {Hirudo Medicinalis.) — This genus of suctorial ani- 

 mals I placed in two ponds some ten years since; they were from 

 Sweden ; their habits are aquatic, and they are supplied with a 

 sucker at both ends. After studying their characteristics for a 

 long time, I found that they deposited their eggs, and then col- 

 lected them into small balls and covered them with an excretion, 



I have counted in a leech eighty-five soft rings, by which means 

 it gathers itself up, and swims with great agility. The head is 

 small, and skin black, edged with a narrow line of yellow on the 

 side, and yellow spots on the back; the abdomen is red, inter- 

 spersed with yellow spots. The mouth is armed with three car- 

 tilaginous jaws, so constructed as to form three radi of a circle, 

 each having two rows of sharp serai-circular saws, with which 

 they cut the skin by a sawing motion. This accounts for the tri- 

 radiate form of their bite. Their digestion is very slow; a single 

 meal suffices for a year. They have but one intestine, a stomach 

 with ccBcal sacs, and an sesophagus. I have been able to detect 

 eight eyes, situated on the sucking surface, above the mouth, but 

 I believe they have more. 



Leeches are becoming scarce, from the fact that many errors 

 are committed in the usual method adopted for preserving them. 



