6o ESSEX WORTHIES. 



regions, being that shown in the accompanying reduced woodcut. 

 Several others are depicted in his book. He particularly discusses 

 the effects of masses of iron ore in mountains and continents in 

 producing local perturbations or variations of the compass ; a matter 

 which has quite lately received fresh attention recently from the 

 magnetic surveys of Professors Riicker and Thorpe, in which they 

 have measured the perturbing effects of mountain-chains such as the 

 Malvern Hills, and have even been led to discover the existence of 

 underground mountains, one of which, for example, is in the neigh- 

 bourhood of Reading. Book VI. is of a more speculative character, 

 dealing with magnetic motions and cosmical systems : the main point 

 of interest in it being its frank acceptance of the astronomical 

 doctrines of Copernicus. 



These contributions to purely magnetic knowledge were of great 

 importance ; but far transcending them in interest is a short digres- 

 sion interpolated in the Second Book. This is the famous chapter 

 on Electricity which laid the foundation of that science. Prior to 

 his time the only known electrical facts w^ere two isolated observa- 



FiG. s. 



Gilbert's Electroscope, or Versorium. 



tions of prehistoric date. The mineral amber, or electron^ then of 

 great rarity and regarded as a gem, was known to acquire, when 

 rubbed, the magical property of attracting straws and other light 

 objects, A similar property had been recognised to exist in jet. 

 Amber was a substance about which there was something uncanny. 

 It was clear like glass, when of good quality, but was often found to 

 contain flies and other insects enclosed within itself — " shining," 

 says Gilbert, " in eternal sepulchres." Much had the ancients, 

 including Theophrastus and Pliny, written about it and the magical 

 properties which it exhibited after being rubbed. This peculiar 

 phenomenon was submitted to examination by Gilbert with an 

 industry and experimental sagacity thoroughly characteristic of the 

 man. He devised for facilitating the observation of feeble attractions, 

 a simple instrument, consisting of a light, stiff arm of metal, 

 resembling in shape, a compass needle, pivoted like a compass upon 



