THE DEVELOPMENT OF MECHANICS 241 



it perfectly clear that there is a complete analogy between the 

 weight and pressure of water and the weight and pressure of 

 air that their effects are the same. But he goes a step further. 



In the course of his experiments Pascal had noted that the 

 pressure of water in a vessel is greater towards the bottom than 

 at the top. The idea came to him that the same should be 

 true of any considerable body of air. Doubtless he had deeply 

 studied optics and the refraction of light ; probably he knew 

 of the various attempts that had been made to determine the 

 height of the earth's atmosphere. It had been measured by 

 the Arabian Al-Hazen in the eleventh century and by Poseidonius 

 a thousand years before that. If the analogy between water 

 and air is complete, then at the top of a high mountain the 

 pressure of the air will be less than at its base the column 

 of mercury will sink. Possibly the slight but curious variation 

 from day to day in the height of the mercury column, first 

 observed by Torricelli, had already given him a prescience of 

 the discovery he was to make. At any rate, he sends Perier 

 up to the top of a mountain with a column of mercury to test 

 the question. It turns out just as he had foreseen. The baro- 

 meter in its present-day sense had been discovered. 



Six years later Otto von Guericke, burgomaster of Magdeburg, 

 had invented the air-pump. With it he had shown further 

 that a pair of copper hemispheres, when tightly fitted together 

 and the air pumped out, could not be separated by teams of 

 horses pulling in opposite directions. Nature's " horror of a 

 vacuum " was simply the pressure of the air. A new science 

 aerostatics, pneumatics was born. 



It is easy to see what an impulse towards mechanical ideas 

 and conceptions all these new discoveries must give. They 

 had a curious effect in another way ; they emptied the spaces 

 of the inter-planetary ways of air ; they left the heavens, save 

 for the planets, comets, meteorites which we may see, a void. 

 This was a great step. One of the most puzzling problems in 

 celestial physics, as we have seen, was the force which urges 

 the planets, the earth as well, in their rapid flight. Galileo, 

 by the force of his genius, could rise to a clear and definite con- 

 ception of inertia, but he had no thought of applying it to the 

 movement of the planets. There was nothing to suggest such 

 an application. It was clear enough that bodies shot through 

 the air meet with a resistance. If the air extends indefinitely 



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