376 



NATURE S TEACHINGS. 



passes must be so constructed that if it be bent, or even 

 coiled, it retains its form, and does not become flattened. In 

 order to obtain this object, a very long thin wire is coiled 

 spirally to a suitable length. Over this wire is sewn the casing 

 of the tube, which is afterwards made waterproof with elastic 

 varnish. A still simpler mode is by enclosing a spiral wire 

 within a tube of vulcanised india-rubber. It will be seen, then, 

 that by the elasticity of the spiral wire the tube must always 

 retain its shape, no matter how much it may be bent. 



On the right hand of the illustration are shown the movable 

 Gas-lamp and tube, and a portion of the latter is given 



TRACHEA OF INSECT, 

 WITH ITS SPIRAL THREAD. 



TUBE OP GAS-LAMP, 

 WITH ITS SPIRAL WIRE. 



with its spiral wire partially unwound, in order to show its 

 structure. 



The large tubes which convey air to divers are made in the 

 same manner, as they would not only succumb to the pressure 

 of the water without the wire, but could not be dragged over 

 obstacles or round corners without collapsing. It often happens 

 that a diver is obliged, when surveying a sunken ship, to 

 traverse the whole of her interior, descending ladder after 

 ladder, and entering every cabin in the ship. This could 

 not be done but for the internal coil of wire within the tube. 

 Reference will presently be made to the subject of diving. 



On the left hand is seen an object that looks something like 

 a branch hollowed very thin. It is a magnified view of part 

 of the Trachea or breathing-tube through which air is con- 



