170 Of Pneumatics,

Dr. Cotes has demonstrated, that if altitudesin the air be taken in arithmetical proportion?the rarity of the air will be in geometrical pro-portion. For instance.

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And hence it is easy to prove by calculation,fhat a cubic inch of such air as we breathe,would be so much rarefied at the altitude of 500miles, that it would fill a hollow sphere equal indiameter to the orbit of Saturn.

The weight or pressure of the air is exactlydetermined by the following experiment.

The To- Take a glass tube about three feet long, andricelUan open at one end; fill it with quicksilver, andexperi- putting your finger upon the open end, turnthat end downward, and immerse it into a small' ' .. . vessel