242 THE WORLD MACHINE 



throughout space, as most good folk suppose, then what was 

 it which enabled the planets to overcome the resistance which 

 they must meet ? Poseidonius and Al-Hazen could make their 

 calculations as to the height of the earth's atmosphere : there 

 was nothing to prove that they were right. 



A very few experiments with the barometer, its revelation 

 of the decrease of air pressure with the vertical height, sufficed 

 for a new calculation as to the height the atmosphere might 

 extend. This agreed fairly well with the estimates made from 

 the appearance of the sun and the stars when they are still 

 below the horizon. Men could then rise to a conception that 

 was wholly new that, save for material bodies, space is empty. 



After that a propelling force for the planets was no longer 

 needful. Speeding through a vacuum with no resistance to over- 

 come, it was clear that once set going they would go on for ever. 



It was the development of these new ideas which made it 

 possible for the human mind to conceive of action at a distance, 

 that is to say, to conceive of a force which could reach through- 

 out all space. It was this which opened the way for Isaac 

 Newton, and let his understanding pass to conclusions where 

 Kepler, Galileo, and Descartes had failed. 



The human mind was reaching at last to clear and practical 

 representations of force what we to-day call energy. Instead of 

 the vague notions of levitation and gravitation which they had 

 found in the physics of Aristotle, Kepler and Galileo had sub- 

 stituted definite notions of a power acting always towards the 

 centre of the earth, a force which could exert a certain pull, 

 which acts upon all bodies alike. Guericke had demonstrated 

 that Galileo's law as to falling bodies applied equally to all 

 bodies regardless of their weight, that a cannon-ball and a 

 feather dropped in a vacuum reach the bottom together. For 

 mystical notions regarding nature's predilections as to a vacuum, 

 they had established a clear conception that the air is a fluid 

 which may exert a pressure precisely as does water. With the 

 establishment of the practical identity in their actions of air 

 and water had come a mechanics of fluids. In a very little 

 while Boyle and Mariotte had discovered the law which governs 

 the " spring " of air, that is, that air occupies a volume directly 

 proportional to the pressure exerted upon it. Galileo's primitive 

 thermoscopes were being developed into accurate thermometers ; 

 they were making the first crude measures of the energy of heat. 



