2i6 PROGRESS OF SCIENCE. 



water which presses on a fish is unfelt by it because the 

 pressure is equilibrated. Torricelli discovered and formulated 

 THE LAW of velocity of efflux by a theorem which bears his 

 name : " That water falls from a vessel with a velocity equal to 

 that with which a body would fall from the same height as the 

 level of the water above the tap " ; so that he was really the 

 founder of hydrostatics. 



1602 1686. Guericke, concurrently with Torricelli, 

 demonstrated atmospheric pressure. He invented the 

 MAGDEBURG HEMISPHERES, which fully proved the pressure 

 of air to be exerted on all sides equally. He also invented 

 the AIR-PUMP, to produce a vacuum the bell-glass, from 

 which the air is exhausted by a pump, being kept down by 

 the outside air pressure. He showed the elasticity of air. 

 He was the first to propose the freezing point of water as the 

 lowest limit of the scale of the thermometer the boiling 

 point being proposed by Renaldini (in 1694) as the opposite 

 limit. He constructed the FIRST ELECTRIC MACHINE 

 using a globe of sulphur and charging it with electricity by 

 turning it in a frame, and pressing it with a cloth which 

 caused the required friction. The electricity produced by 

 sealing-wax or sulphur is called resinous electricity. Guericke 

 found out several electrical phenomena : I, that a spark is 

 produced by electricity passing between two separate bodies ; 

 2, that two bodies charged with the same electricity repel each 

 other so that he was the practical continuator of Gilbert, and 

 the precursor of Muschenbroeck, Franklin, and Galvani. Not 

 long after him a further advance in this branch was made by 

 Du FAYE, who discovered that there exists another kind of 

 electricity. He obtained it by rubbing the end of a glass 

 rod with silk. This is called vitreous electricity in contradis- 

 tinction of Guericke's. Du Faye showed that each electricity 

 repels itself and attracts the other. It was for the purpose of 

 generating vitreous electricity that HAWKSBEE made (1740) 

 the SECOND ELECTRIC MACHINE, on Guericke's principle, in 

 which the sulphur was replaced by a glass globe and the 

 cloth by a piece of silk. We shall soon see how these facts 

 conduced to the electrical marvels of our time. 



1618 1663. Grimaldi discovered and demonstrated the 



