348 HISTORY OF MECHANICS. 



Still men's minds wanted confirmation in this view ; and thev 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 cele- 

 brated 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 horror 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 can- 

 not say that nature abhors a vacuum at the foot of a mountain more 

 than on its summit." — M. Perrier, Pascal's correspondent, made the 

 observation as he had desired, and found a difference of three inches 

 of mercury, " which," he says, " ravished us with admiration and 

 astonishment." 



When the least obvious case of the operation of the pressure and 

 weight of fluids had thus been made out, there were no further diffi- 

 culties in the progress of the theory of Hydrostatics. When mathe- 

 maticians began to consider more general cases than those of the 

 action of gravity, there arose differences in the way of stating the 

 appropriate principles : but none of these differences imply any differ- 

 ent conception of the fundamental nature of fluid equilibrium. 



Sect. 2. — Discovery of the Laws of Motion of Fluids. 



The art of conducting water in pipes, and of directing its motion 

 for various purposes, is very old. When treated systematically, it has 

 been termed Hydraulics : but Hydrodynamics is the general name of 

 the science of the laws of the motions of fluids, under those or other 

 circumstances. The Art is as old as the commencement of civilization : 

 the Science does not ascend higher than the time of Newton, though 

 attempts on such subjects were made by Galileo and his scholars. 



When a fluid spouts from an orifice in a vessel, Castelli saw that 

 the velocity of efflux depends on the depth of the orifice below the 



