MECHANICAL PRINCIPLES OF FLUIDS. 67 



to him in 1630, says 2 , "If we were in a vacuum, 

 the weight of the air above our heads would be 

 felt." Descartes also appears to have some share 

 in this discovery; for, in a letter of the date of 

 1631, he explains the suspension of mercury in 

 a tube, closed at top, by the pressure of the column 

 of air reaching to the clouds. 



Still men's minds wanted confirmation in this 

 view : and they found such confirmation, when, in 

 1647, Pascal showed practically, that if we alter 

 the length of the superincumbent column of air by 

 going to a high place, we alter the weight which 

 it will support. This celebrated experiment was 

 made by Pascal himself on a church-steeple in 

 Paris, the column of mercury in the Torricellian 

 tube being used to compare the weights of the air ; 

 but he wrote to his brother-in-law, who lived near 

 the high mountain of Puy de Dome in Auvergne, 

 to request him to make the experiment there, 

 where the result would be more decisive. "You 

 see," he says, " that if it happens that the height of 

 the mercury at the top of the hill be less than at 

 the bottom, (which I have many reasons to believe^ 

 though all those who have thought about it are 

 of a different opinion,) it will follow that the weight 

 and pressure of the air are the sole cause of this 

 suspension, and not the horrour of a vacuum : since 

 it is very certain that there is more air to weigh 

 on it at the bottom than at the top; while we 



3 Drink water's Galileo, p. 90. 



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