SCIENCE OF COMMON THINGS. 37 



Stems of grasses hollow. Limit to the size of ships. 



In order that they may possess the greatest strength, 

 and by their lightness assist in flying. 



213 Why are the stems of seeds and grain-bearing plants hollow tube? 1 



Because this disposition of matter gives to the stalk 

 its greatest strength, enables it to resist the action of 

 *the wind, and sustain, without breaking, the ripened 

 ear of grain or seed. 



314 Is a column with ridges projecting from it, stronger than one that 

 w perfectly smooth f 



It is. 



SIS Why is a hoUow tube of metal stronger than tfie same quantity of 

 metal as a solid rod f 



. Because its substance standing farther from the 

 centre, has a greater power of resisting a bending 

 force. 



S1G Of two bodies of similar shape, but of different sizes, which is pro- 

 portionably the weaker f 



The larger. That a large body may have propor- 

 tionate strength to a smaller, it must have a greater 

 proportionate amount of material ; and beyond a cer- 

 tain limit, no proportions whatever will keep it to- 

 gether ; but it will fall in pieces by its own weight. 



SIT* Why cannot trees attain an unlimited height of trunk f 



Because, beyond a certain limit, the weight of the 

 material will overcome the supporting strength of the 

 material. 



18 Why is it impracticable to build ships beyond a certain size f 



Because the weight of the timber and other materials 

 contained in them tends to cause them to fall apart. 



In 1825, two vessels, the largest ever constructed, were built in Ca- 

 nada, of 10,000 tons burden. They were found to be weak from their 

 size alone, and were both lost on their first voyage. 



