viii INTRODUCTION TO MECHANICS. 



compression is increased in the lower strata of air by the weight of the 

 upper strata, which rests upon them, and thus the air near the surface 

 of the earth is more dense than in the superior regions. The pressure 

 of the atmosphere has been compared to that of a pile of fleeces of wool, 

 in which the lower fleeces are pressed together by the weight of those 

 above ; these lie light and loose in proportion as they approach the up- 

 permost fleece, which receiving no external pressure is confined merely 

 by the power of its own gravity. 



There are some bodies which do not appear to gravitate : smoke and 

 steam, for instance, rise instead of falling, but it is still gravity which 

 produces their ascent. The air near the earth being heavier than smoke, 

 steam, or other vapours, not only supports these light bodies, but, by its 

 own tendency to sink below them, forces them to rise. The principle 

 is just the same as that by which a cork, or a drop of oil, if forced to the 

 bottom of a vessel of water, rises to the top as soon as it is set at liberty : 

 the only difference being, that, in the case of the atmosphere, the weight 

 or density continually diminishes ; and the ascending body therefore does 

 not rise through the whole extent of the atmosphere, but only till it reaches 

 a stratum of which the weight is equal to its own ; and there, if no other 

 changes take place, it remains stationary. Smoke ascends but a very little 

 way ; it consists of minute particles of fuel carried up by a current of 

 heated air from the fire below. Heat expands all bodies ; it consequently 

 rarefies air, and renders it lighter than the colder air of the atmosphere ; 

 the heated air from the fire carries up with it vapour and small particles 

 of the combustible materials which are burning in the fire. When this 

 current of hot air is cooled by mixing with that of the atmosphere, the 

 minute particles of coal or other combustible fall, and it is this which 

 produces the small black flakes which render the air and everything in 

 contact with it, in London, so dirty. 



Balloons ascend upon the same principle, the materials of which they 

 are made are heavier than the air ; but the air with which they are filled 

 is an elastic fluid of a different nature from the atmospheric air, and con- 

 siderably lighter ; so that, on the whole, the balloon is lighter than the air 

 which it displaces, and will consequently rise. Thus you see that it is 

 the resistance of the air alone which prevents bodies of different weight 

 from falling with equal velocities. Those which are lighter than the air 

 are forced by it to ascend ; those of an equal weight will remain stationary 

 in it, and those that are heavier will descend through it, and their descent 

 will be more or less retarded according to their weight. If you let fall a 

 crown piece and a piece of writing paper of exactly the same dimensions, 

 the crown piece will reach the ground much sooner than the paper, but if 

 you place the paper upon it so closely that no air shall intervene, the 

 paper will fall as rapidly as the crown piece. That bodies when not 

 supported by the atmosphere fall with equal velocities may be proved by 

 the air-pump, a machine by means of which the air may be expelled from 

 any close vessel placed upon it. Glasses of various shapes, called re- 

 ceivers, are employed for this purpose, and bodies of whatever size or 

 weight placed within them will fall from the top to the bottom in the same 

 space of time. The experiment is usually made with a guinea and^a 

 feather: they are placed on a brass plate in the upper end of the glass, 

 and as soon as the air is pumped out, by turning a screw the brass plate 

 is inclined, and the two bodies fall at the same moment, and reach the 

 ground at the same moment. 



