THE DOME. 



511 



of air, it will resist the pressure of the atmosphere for days 

 together. 



When it is remembered that the Receiver is deprived of its 

 internal air, and therefore has to resist a pressure equal to 

 fifteen pounds on every square inch of its surface, it may be 

 imagined how strong the Dome is. Were the top or either 

 side to be flat, it would be crushed as soon as a vacuum was 

 formed sufficient to deprive it of the support of the air within. 



A glance at the illustration will show how the Receiver is 

 modelled on the same plan as the Human Skull, the outlines 

 being curiously similar. It is this formation which imparts 

 such strength to so thin a set of bones as those which com- 



HUMAN SKULL. 



RECEIVER OF AIR-PUMP. 



pose the human skull as enables them to protect a sensitive 

 organ like the brain, on which both reason and life itself depend. 

 Eggs also form good examples of the wonderful strength 

 obtained by this principle, their thin shells protecting the yelk 

 and the white, as well as the chick through its progress to 

 maturity. 



The last subject in this chapter is a curious example of an 

 evidently accidental resemblance in form. 



The figure on the right of the accompanying illustration will 

 at once be recognised as one of those Salad-dressing Bottles 

 which try to conceal by their shape the small volume of their 

 contents. 



That on the left represents one of the many forms through 

 which the Medusa passes before it attains its perfect form. It 



