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 considerable 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 constructed is found in the “igloo,” or snow-hut of the Esquimaux, 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. Geneviève 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 “Receiver” of the Air-pump. It is made of glass not thicker than an ordinary tumbler, and yet, even when exhausted 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 compose the human skull as enables them to protect a sensitive organ like the brain, on which both reason and life itself depend.