Aids to Microscopic Inquiry. 47 



subjected to a great augmentation of water-pressure its air- 

 bladder will burst. 



The weight of large animals is so great in proportion to 

 their bulk, that ordinary atmospheric currents do not move 

 them, and have no tendency to tear them to pieces ; but small 

 insects can almost float in air, the gossamer spider needs only 

 his silk thread for his balloon or parachute, and minute germs 

 of life constructed of delicate materials can remain for an inde- 

 finite time the inhabitants of the atmosphere and the sport of 

 its winds. 



We have spoken of the effect of water in sustaining or 

 balancing weight to a much greater extent than air, and the 

 microscopist meets with thousands of instances in which this 

 principle is applied in nature' s work. Look in that pond, the 

 delicate branches of the myriophyllum are spread out in grace- 

 ful forms. You take the plant out of the dense water into the 

 light air, its spreading beauties have collapsed, and you see 

 only a mass of entangled green thread. In the water the 

 plant's light branches floated; in the air they fall. The influ- 

 ence of flotation is beautifully shown in the jelly-fish common 

 on all coasts. In the water the long tentacles are sufficiently 

 sustained to enable the muscular power of the animal to employ 

 them as its wants require ; but the moment it is uplifted in 

 the air they all fall together as a helpless and inert mass. 



This sustaining power enables whales and other sea mon- 

 sters to support their cumbrous forms, and it also enables 

 minute and delicate structures, too light and too thin for atmo- 

 spheric life, to preserve the shape and capacity for motion 

 on which their existence depends. The waters of the ocean 

 keep up the gigantic sea- weeds that rise like forest trees from 

 the shallows of tropical coasts, and they also keep up the 

 fine tufts of the Plumularia or other compound polyps, whose 

 delicate shrub-like abodes decorate marine rocks and pools. 

 In the water the Yorticellids can stand erect, elevate or depress 

 themselves at will. In the air they would require for the same 

 performances greater strength of material and more muscular 

 force. 



When water is still, its power of sustaining weight may be 

 employed, as we have seen, in relieving delicate structures of a 

 strain that would be too much for them ; but when in motion 

 it beats against obstacles — as other bodies do — with a force 

 compounded of its velocity and its weight. An air storm may 

 snap trees asunder, tear off the roofs of buildings and overthrow 

 high walls, but from the lightness of air, the highest velocity 

 which its currents are known to assume, fails to give it the 

 momentum requisite for carrying along the huge masses of 

 stone and rock which a rapid water torrent sweeps away in its 



