BALLOONS. 437 



ignite a balloon when at a great height. The second was the 

 perpetual labour required in keeping the fire alight. Straw 

 burns very rapidly, and so the aeronaut had no opportunity of 

 making those meteorologic observations in which consist almost 

 the entire value of the balloon. 



Then it was thought that hydrogen gas, being about four- 

 teen times lighter than ordinary air, would answer the purpose, 

 and such has proved to be the case. Formerly the gas was 



GOSSAMER SPIDER. 



made at great expense from sulphuric acid and zinc, but it 

 is now found that the common coal-gas is quite as efficient, 

 very much cheaper, and fills the balloon much more rapidly. 



The same principle, though not the same form, is found in 

 Nature. 



There are certain tiny spiders called Gfossamers, which have a 

 curious power of floating in the air. They have been seen on 

 the tops of lofty spires, and they are sometimes so numerous 

 that the air is full of their floating webs, and the ground is 

 white with those that have descended. 



Their mode of ascent is this. They climb to the top of some 

 elevated object, if it be only a grass-blade. They then pour 

 out a long, slender, thread-like web, which shortly begins to 

 tend upwards. As soon as the Spider feels the pull, it crawls 

 upon the web, and sails away into the air. The duration and 



