290 THE AMERICAN NATURALIST. [Vol. XXXVIII. 



The attraction of gravity upon all ponderable matters is, 

 however, opposed by the media in which they are. This is 

 implied by the statement of the value of gravity at the earth's 

 surface, for this statement specifies in vacuo. Every ponderable 

 body is bouyed up (or supported) by a force equal to the weight 

 of the fluid it displaces. This law of Archimedes concerns us 

 as it applies to the air and to both fresh and salt water. Besides 

 this we have the solidity and comparative impenetrability of 

 the earth itself to reckon with, for the soil is capable of 

 mechanically supporting much more than the weight of the 

 parts of plants and animals resting upon or within it. Gravity 

 is then opposed, partly or wholly, according to the medium in 

 which the attracted object is. 



The opposition to gravity varies from .0013 gr. per cc. in air 

 at 0° C, and at ordinary atmospheric pressure, through i.oo gr. 

 per cc. in pure water and 1.20 gr. in sea water, to much more 

 than the weight per cc. of any of the substances with which 

 we ordinarily have to deal. Thus if we have a plant or animal 

 or any other portion of matter weighing x grams in air, this 

 matter would have a weight in pure water equal to x minus 

 I gram for every cc. of volume. In sea water this would be 

 X — 1.2 y, in which y represents the number of cc. in the 

 portion of matter. To take a concrete instance, suppose we 

 have a block of wood, occupying a space of 9 cc. and weigh- 

 ing 10 gr. in air. This would weigh in water 10 — i x 9 = 1 

 gr.; in sea water this would be 10 — 1.2 x 9 =0.8 gr. or, in 

 other words, the block would float. In the same way the block 

 would be completely supported, gravity would be completely 

 offset, if the block were on or in the soil. But as the soil, 

 the water, and the air do not come into direct contact with 

 and are therefore not displaced by each individual part and par- 

 ticle of a portion of matter, there must be enough mechanical 

 strength within the portion of matter to resist the force of 

 gravity or the body would fall to pieces. We see then that 

 though a body as a whole may be bouyed up by a considerable 

 force which resists gravity, the component particles of the body 

 are not necessarily so bouyed up but are subjected to the full 

 attraction of gravity. Although the bouyancy of the medium 



