166 



WELLS'S NATURAL PHILOSOPHY. 



What law reg- 

 ulates till' den- 

 sity of the at- 

 mosphere ? 



Fig. 155. 



. . The question, therefore, naturally occurs in this connection, 



What limits . ^^. ,,..,,, • , , , 



the atmosphere Tiz. : II air exijands unlimitedly, when unrestricted, wliy does 



to the earth? j^^j- q^j. atmosphere leave the earth and difluse itself through- 

 out space indefinitely ? Tiiis it would do were it not for the action of gravi- 

 tation. Tiie particles of air, it must be remembered, possess weight, aud by 

 gravity are attracted toward the center of the earth. This tendency of gravity to 

 condense the air upon the earth's surface, is opposed by the mutual repulsion 

 existing between the particles of air. These two forces counterbalance eaoh 

 ether : the atmosphere will therefore expand, that is, its particles will separata 

 from one anotlicr, until the repulsive force is diminished to such an extent as ta 

 render it equal to the weight of the particles, or what is the same thing, tc 

 the force of the attraction of gravitation, when no further expansion can take 

 place. "We may therefore conceive the particles of air at the upper surface of 

 the atmosphere resting in equilibrium, under the influence of two opposite 

 forces, viz., their own weight, tending to carry them downward, and the 

 mutual repulsion of the particles, which constitutes the elasticity of air, tend, 

 ing to drive them upward. 



371. The density of the air, or the quantity 

 contained in a given bulk, decreases with the 

 altitude, or height above the surface of the 

 earth. 



This is owing to the diminished pressure of the air, and 

 the decreasing force of gravity. Those portions directly 

 incumbent upon the earth are most dense, because tliey bear 

 the weight of the superincumbent portions; thus, the hay 

 at the lower part of the stack bears the weight of that 

 above, and is therefore more compact and dense. (See Fig, 

 155.) This idea may be conveyed by the gradual shading 

 of the figure, which indicates the gradual diminution in the 

 density of the atmosphere in proportion to its altitude. 



wiien is air 372. Air is Said to be rarefied 



said to be rare- -, . , . , , , -, 



ficd? when it IS caused to expand and occupy a 



greater space. 



Generally, when we speak of rarefied air, we mean air that is expanded to 

 a greater degree, or is thinner, than the air at the immediate surface of the 

 earth. 



373. The great law governing the compressibility of air, which is known 

 irom its discoverer as " Mariotte's Law," may be stated as follows: 



Tlie vohime of space which air occupies is in- 

 versely as the pressure upon it. 



If the compressing force be doubled, the air which is compressed ■will 

 occupy one half of the space: if the compressing force be increased in a three- 

 fold proportion, it will occupy one third the space ; if fotiifold, one fourth the 

 space, and so on. 



■What is Ma- 

 riotte' s Law T 



