420 ffo Barometer. [Book V, 



afcendcd towards the fummit j and alfo, on the contrary, 

 that it role again in the fame proportion as he defcend- 

 ed : the difference was found to be three inches and one 

 line between the height of the mercury at the fummit 

 and the bafe. This experiment, fuggeited by Pafchal, 

 and repeated feveral times, always produced the fame 

 refult; whence it was concluded, that mercury was 

 fuftained above its level in the Torricellian tube, by 

 the prefTure of the atmofphere upon the rdtrvoir, 

 fince the mercury in the tube was obferved to fall, when 

 the column- of air which had the refervoir for its bafe 

 was diminilhed in height. Thefe experiments, in 

 proving incontrovertibly the weight of air, have au- 

 thentically reftored to this fluid a great number of na- 

 tural properties and effects, which were before attri- 

 buted to a caufe merely chimerical. 



M. Pafchal afterwards repeated the lame experi- 

 ment with water, wine, oil, &c. and the heights of the 

 columns of thefe liquors were always found to be 

 proportional to their denfities ; an evident proof that 

 they were counterpoifed by a weight, which could in 

 thofe cafes be no other than the preffure of the air. 



Many philofophers afterwards, having procured 

 'Torricellian tubes, placed them according to the man- 

 ner of M. Perrier, upon a fcale graduated into inches 

 and lines, and by frequent obfervations they perceived, 

 that the height of the mercury in the tube often varied. 

 They concluded, therefore, that the preffure of the 

 air, which was the caufe of the iufpenfion of the co- 

 lumn of mercury, was fome times greater and fome- 

 times leis, and confequently that it acted more or Jefs 

 forcibly upon the human frame. From thefe caufcs 

 and effects the idea was fuggefted, of making from 'the 

 Torricellian tube a new meteorological inftrumcnt, the 



, fame 



