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 



1 Drinkwater's Galileo, p. 90. 



F2 



