44 SCIENCE OF COMMON THINGS. 



Dovetailing. Construction of the skull. Egg-shells. 



the arch is such, that the force which would break a 

 horizontal beam or structure is made to compress all the 

 ^articles of the arch alike, and they are therefore in no 

 danger of being torn or overcome separately. 



S6O What is meant by dovetailing? 



It is a method of insertion in 

 which the parts are connected 

 ~by wedge-shaped indentations, 

 which permit them to be sepa- 

 rated only in one direction. (See 



F. 12. """ ' J 1 ^' *rv 



SGI What beautiful application of the arch exists in the human struc- 

 ture ? ~ 



In the skull, protecting the brain. The materials 

 are here so arranged as to present the greatest strength, 

 with the least weight. 



S6J3 Why is it difficult to break an egg by pressing directly upon its 

 ends ? 



Because the shell of the egg is constructed on the 

 principle of the arch, and is therefore capable of re- 

 sisting great pressure. 



SG3 Why is a dished or arched wheel of a carriage much stronger for 

 resisting all kinds of shocks than a flat wheel? 



In an arched or dished wheel, the extremity of a 

 spoke cannot be displaced inwards, or towards the car- 

 riage, unless the rim of the wheel be enlarged, or all 

 the other spokes yield at the same time ; and it cannot 

 be displaced outwards, unless the rim be diminished, 

 or the other spokes yield in an opposite direction. 



Now the rim, being strongly bound with a tire of iron, cannot suffer 

 either increase or diminution, and the strength of all the spokee is thus 

 conferred by it on each individually. In a flat wheel, a given degree of 

 displacement, outwards or inwards, of the extremities of a spoke, would 

 less affect the magnitude of the circumference, and therefore the rim of 

 such a wheel secures it much less firmly. 



S64 Why are the fore wheels of carriages smaller than the hind 

 wheels ? 



Because they facilitate the turning of the carriage. 

 The advantage of the wheel is proportioned to the mag- 



