46 Aids to Microscopic Inquiry. 



would be improved;, because the water would buoy him up, 

 leaving much less for his muscular power to do. When a 

 creature lives in water so much of its weight is practically 

 taken away, that a comparatively small muscular force will 

 move it. A man feels this effect when he moves a submerged 

 stone, and the diver can carry in his arms, under water, masses 

 that he could not possibly lift into the air. 



But as water is denser than air, it requires more force to 

 move a given bulk through it, the shape being the same. In 

 moving any substance through a resisting medium, the form is 

 extremely important. A thin sheet of paper may be moved 

 edgewise through the air, with facility and safety; but, if 

 held so that its broad surface receives the full shock, the 

 force required to push it on will be much greater, and it will 

 be torn to pieces in the struggle. 



The preceding facts are known to every one of ordinary 

 education, and we only mention them in order to suggest a 

 train of thought having reference to microscopic pursuits. The 

 microscopist examines animals, and in some cases vegetables 

 that indulge in locomotion, and he should reflect upon the 

 amount of force required for this purpose. He should more- 

 over consider the two classes of resistance to be overcome — 

 the one arising from the weight of the thing moved, and the 

 other from the resistance of the medium through which its 

 motion is performed. Large animals are so much heavier than 

 air, that as we never see it lift them, we are apt to forget the 

 /pressure in all directions which that fluid exerts. A balloon 

 filled with light gas is forced up by the weight of the air all 

 around it, just as a cork introduced below water is forced up 

 to the surface. In practical mechanics we have to deal with 

 stiff materials, and we employ cranks and such like means to 

 'make a force turn round the corner, and do work at rigfht 

 angles to the direction in which we primarily exert it. The 

 fluid is its own crank, and the perpendicular pressure of the air 

 on each side of a balloon is turned into an ascending pressure 

 below it, shoving it up. Water acts in just the same way. 

 According to fluid laws, pressure is as depth, and at any given 

 depth, equal in all directions. The air presses on the earth's 

 surface with a force or weight nearly equal to fifteen pounds 

 per square inch. We are unconscious of this force, because it 

 acts in all directions, and the walls of our cavities are not forced 

 in because those cavities contain air that has just as much 

 tendency to force them out ; or because they contain fluids 

 still more capable than air of resisting the pressure to which 

 they are exposed. When a cavity of any kind does not 

 communicate freely with the air or other fluid outside it, an 

 increase of pressure is instantly felt, and if a fish be rapidly 



