THE BELL SPRING. 371 



The two figures beneath the wheel spring represent an 

 object very familiar to us, namely, a Spring Solitaire, one 

 figure showing it as open, and the other as closed. In this 

 article the clasp is held in its place by a spring, and is only 

 released by pressure. 



Below the solitaire is a very prosaic application of the 

 Spiral Spring, namely, that by which a house-bell is kept in 

 vibration after the force of the pull has ceased, and which 

 renders the bell, as Dickens happily remarks, so greedy to ring 

 after it has been pulled. 



I made and employed a spring of a similar character in 

 closing the door of my parrot's cage. Polly is a wonderfully 

 clever bird, and a capital talker. First, she had a cage with 

 upright bars, two of which could be slid upwards by way of 

 a door. She soon found out the trick of the bars, and used 

 to escape, carefully replacing the bars afterwards. 



When she was transferred to a metal cage, she discovered 

 that the door slid upwards, and began at her old tricks. So I 

 took a piece of galvanised iron wire, coiled it into a spiral 

 spring, fastened one end to the upper part of the door, and 

 the other by a hook to a staple at the bottom of the cage. 

 Consequently, when Polly lifted the door, and loosened her 

 grip for a fresh hold, the door closed itself again. So, after 

 awhile, Polly gave up the door, and now never tries to 

 open it. 



Passing to the upper right-hand corner of the illustration, 

 there is shown a portion of Moss as it appears when magnified, 

 and discharging its spores. When they are ripe a vast number 

 of little spiral springs are let loose, and shoot the sporules into 

 the air. 



Below the moss are four figures, which are, in fact, the 

 same object differently magnified, and seen from different 

 points of view. These peculiar organs are technically termed 

 " cnidae," from a Greek word which signifies a nettle. The 

 appropriateness of the name we shall presently see. 



I have already mentioned that the tentacles of various 

 marine animals are furnished with poison- cells. The object of 



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