THE RENAISSANCE in 



elementary machine, and all other machines as modi- 

 fications and complications of it. 



Leonardo recovered Archimedes' conception of the 

 pressure of fluids. He showed that liquids stand at 

 the same level in communicating vessels, while, if 

 different liquids fill the two arms, their heights will be 

 inversely as their densities. He deals also with 

 hydrodynamics the efflux of water through orifices, 

 its flow in channels, the propagation of waves over its 

 surface. From waves on water he passes to waves in 

 air and the laws of sound, while he saw that light 

 showed many analogies which suggested that here too 

 a wave theory was applicable. The reflection of an 

 image is the echo of the source ; as with a ball thrown 

 against a wall, the angle of reflection is equal to the 

 angle of incidence. 



In the realm of astronomy Leonardo conceived of 

 a celestial machine conforming to definite laws, in 

 itself a remarkable advance on the prevalent Aristo- 

 telian ideas that the heavenly bodies are divine, incor- 

 ruptible, essentially different from our world subject to 

 change and decay. He calls the earth a star, not differ- 

 ent from the others, and proposes in his projected book 

 to show that it would reflect light like the moon. 

 With errors in detail, Leonardo's astronomy is true in 

 spirit, and with him modern astronomy appears. 



As things are older than writings, the earth bears 

 trace of its history before the records of books. Fossils 

 now on high inland mountains were produced in sea- 

 water, and could not have reached their present 

 position in the forty days of the Noachian deluge; 

 indeed the whole waters of the world, clouds, rivers 

 and ocean, could not cover the mountains of the earth. 



