36 



PNEUMATICS. 



more than 16,000 times rarer than at the earth's surface. 

 At the tops of high mountains, through the rarity of the 

 air, a most violent oppression is felt at the chest, some- 

 times attended with spitting of blood, and with blood 

 gushing from the ears and nose : the external air being 

 unable to counterbalance the internal pressure. 



That the air has weight, was granted by the ancients ; 

 but it has been proved beyond a doubt by the air-pump, 

 which is a modern invention, and also by the barometer, 

 and the exact quantity of its weight has been ascertained. 

 Because the atmosphere will balance in an hermetically 

 sealed tube a column of quicksilver of about thirty inches 

 in height, as also it will sustain in a sucking pump a 

 column of water of about thirty-two or thirty-three feet 

 high, it follows that the weight or pressure of the atmos- 

 phere is equal to a column of quicksilver or water of the 

 above height and of a like base, and this is found to be 

 in the proportion of fifteen pounds nearly on every 

 square inch of surface,* or of one ton on every square 

 foot ; whence it has been computed that the whole weight 

 of the atmosphere is equal to a globe of lead of sixty 

 miles in diameter. 



Air, like other fluids, presses in all directions, both per- 

 pendicularly, laterally, and upwards. The upward pres- 

 sure of the air may be shown as follows. Fill a wine- 

 glass with water, and place over it a piece of writing- 

 paper ; if the paper be pressed firmly over the glass by 

 the palm of the hand and the glass be inverted, the water 

 will still remain suspended in the glass by the pressure 

 of the atmosphere, although the hand be removed. 



Air is the support of animal and vegetable life, but if 

 it be closely pent up for a considerable time it will be- 

 come vitiated by being deprived of its vivifying principle. 

 Any means whereby the oxygen or pure part of the air 

 is destroyed, must tend to render it unfit for respiration ; 

 whence it is highly unwholesome to sleep, or to remain 

 shut up, in a newly plastered or whitewashed room, from 

 the lime imbibing the oxygen. When it is considered 

 that a man consumes or fixes more than a cubic foot of 



* A Cubic foot of air weighs very nearly one ounce and a quarter, 

 and its specific gravity compared with water is as 800 to one. 



