60 PROGRESS OF SCIENCE. 



1567 1643. Peri and Monteverde created the OPERA, 

 after attempts at reconstructing the melopeia of the Greeks. 



1571 1630. Kepler formulated the three LAWS OF 

 MOTION, together with Galileo's, the basis of Newton's 

 labours ; explained the tides, but it was reserved to Newton 

 to formulate the law which regulates them. 



1577 1643. Guldinus determined the CENTRE OF 

 GRAVITY a solution necessary for the progress of 

 mechanics. 



1577 1644. Castelli created HYDRAULICS, showing how 

 to measure running water. 



1577 1644. Van Helmont recognised numerous kinds of 

 GAS being the first of all writers to use the word ; called 

 carbonic acid " wood-gas" the gas rising from fermenting beer, 

 " gas sylvestre." His lectures on chemistry popularised that 

 science. 



15781657. Harvey discovered the CIRCULATION OF 

 THE BLOOD, the starting point of modern physiology ; 

 discovered also that the blood during its passage through the 

 lungs meets the air drawn in at each breath. 



As a biologist he made valuable discoveries about 

 generation : he observed the very beginnings of life in the in- 

 cubated egg of the fowl and in the embryos of mammalian 

 animals the result of which was the demonstration of the 

 famous dictum, " Omne vivum ex ovo " (all life comes from 

 an egg), which is true of all higher animals if not applicable 

 to plants. 



1591 1626. Snell measured the length of a DEGREE by 

 means of trigonometry the result confirming Kernel's calcu- 

 lations ; he also discovered the LAW OF REFRACTION (1621) 

 determining the index of refraction. Snell's law, as it is called, 

 is stated thus : TIu ratio between thz sines of the incident and 

 refracted rays is always the same for the same substance 

 the index of refraction for water being three-fourths. The 

 importance of this law may be conceived when it is re- 

 membered that the power of telescopes could not have been 

 regulated without it. 



16001610. Scarpi, Drebbel, Santorio invented an air 

 THERMOMETER each in 1600, 1609, J 6io respectively. They 



