294 PRESSURE IN THE AIR, 



be placed under the receiver of an air-pump, 

 without exhausting the air out of them, and 

 double the quantity of air be thrown into the 

 same space, it will require a double force to 

 separate them, so that a double atmosphere 

 has the same advantage over a single one, as a 

 single one has over an exhausted one.* 



2. In order to prove that the rupture of the 



* It is to GALILEO, a celebrated mathematician of Pisa, 

 in Italy, that the error of measuring air by weight is to 

 be referred. He advanced (and maintained it to be an unde- 

 niable truth) both in philosophy and mechanics, that no heavy 

 body ascended, without another heavy body descending as a 

 counterpoise to it ; he was the first who supported and suggest- 

 ed the doctrine, that preternatural ascent of fluids arose from a 

 regular and natural cause in the weight and pressure of the air. 

 It was this hint which led TORRICELLIUS, of Florence, in 

 1643, to perform a variety of experiments, with a view of 

 ascertaining the weight of the air, and to form the first rough 

 model of a barometer ; he constructed a tube 60feet hi length, 

 which he afterwards reduced to 40, by suspending it in water, 

 and by means of a sucker attached to the upper part of it, on 

 exhausting the air out of it he found that the water followed 

 the sucker, and rose to 32 33 feet, but no art whatever could 

 elevate it beyond 38 or 40 feet. Such was the unweildiness 

 of the instrument, that it required the sails of a windmill to 

 invert the tube : he therefore availed himself of the mean and 

 relative weight which mercury bore to water, so that by sub- 

 stituting mercury for water, he was enabled to reduce the 

 length of the instrument from 40 feet to 33 inches. 



