154 GREAT MEN OF SCIENCE 



of the original magnet. He further recognised, in investi- 

 gating the poles by the aid of a small suspended needle, that 

 they are never points, but always more or less extended 

 regions of the magnet, from which the attraction or repul- 

 sion proceed. These facts, which Coulomb also had to take 

 into account very closely in the course of his investigations, 

 and which altogether pointed to the need for great caution 

 when attempting to calculate with magnetisms or magnetic 

 fluids supposed to be located at definite points in space, and 

 to be the bearers of the magnetic force, could not be ex- 

 plained until more than 300 years later, after Ampere and 

 Faraday had arrived at a better understanding of the mat- 

 ter; their perfectly clear revelation at so early a period by 

 Gilbert is thus all the more admirable. The same is also 

 true of his recognition of the fact that the earth is a great 

 spherical magnet, which at once allows us to form a general, 

 accurate, and comprehensible picture of the behaviour of the 

 magnetic needle, including its inclination (deviation from 

 the horizontal, when hung up at its centre of gravity) at all 

 points of the earth's surface. 



In the matter of electrical phenomena, Gilbert's only con- 

 tribution to progress lay in his finding a number of other 

 substances beside amber - for example precious stones such 

 as diamond and sapphire, glass, sulphur, resin - which are 

 also electrified by friction, and in recognising clearly not only 

 the similarity between the electric and magnetic forces, but 

 also their definite points of difference. 



Seventy years later followed Guericke's first step towards 

 the construction of an electrical machine, Leibniz's observa- 

 tion of electric sparks produced by it, Guericke's discovery 

 that electrical repulsion also exists as well as attraction, and 

 Boyle's proof that electric and magnetic force also acts in the 

 vacuum of the air-pump. Thenceforward, thunderstorms 

 were regarded with increasing certainty as electrical 

 phenomena. 



