510 NATURE'S TEACHINGS. 



The latter operation may be seen whenever a large fly gets 

 into the web. The Spider darts at it, bites it, and then, ejecting 

 a loose mass of fibres, rolls it up in a moment, as in a shroud, 

 carries it off and hangs it in a convenient place, and mends the 

 broken meshes of the web. But both kinds of the cords of the 

 net are made differently from the winding-up fibres, the former 

 being fixed together, and the latter kept separate. 



Principle of the Dome. 



We are all familiar with Domes, especially when the Dome 

 of St. Paul's is the most conspicuous object in our metropolis. 

 Few persons, however, except professional architects and 

 builders, seem to ask themselves the principle on which the 

 Dome is constructed. 



The strength of the arch is well known, and the Dome is 

 practically a number of arches, affording material support to 

 each other, and so enormously increasing the strength of the 

 edifice. 



A good idea of the Dome principle may be formed by taking 

 two croquet hoops, placing them at right angles to each other, 

 tying them together at the intersection, and pushing the ends 

 in the ground. Even by this very simple arrangement consider- 

 able strength can be obtained ; but, if the hoops be sufficiently 

 multiplied to form a close Dome, it will be evident that the 

 strength will be correspondingly increased. 



So strong, indeed, is the Dome, that it could be made without 

 mortar or cement, although, of course, its strength is increased 

 by their use. A very good example of a Dome thus con- 

 structed is found in the " igloo/' or snow-hut of the Esqui- 

 maux, which has already been described. 



As to the example which I have selected, it would have been 

 easy enough to have chosen one of the great Domes of the 

 world, such as St. Peter's at Rome, St. Maria del Fiore at 

 Florence, St. Paul's of London, or St. Genevieve or the 

 Invalides of Paris. 



I have, however, selected the present example on account of 

 the thinness of its walls, the fragility of its material, and the 

 enormous pressure which it has to undergo. This is the 

 " Eeceiver " of the Air-pump. It is made of glass not thicker 

 than an ordinary tumbler, and yet, even when exhausted 



