iv PURSUIT OF KNOWLEDGE 83 



explained that there was no sign of water, and that in 

 their opinion, it was useless to bore to a greater depth. 

 " Go on," was the quiet rejoinder, " you will come upon 

 water to-morrow. You are within two feet of it." 

 Next day it proved exactly as Prestwich had foretold. 

 He knew the exact level of the springs in the valley, and 

 that water must be found when that depth was reached. 



Another direction in which Prestwich's work found 

 practical application was in connection with British coal- 

 fields. When only twenty years of age, he produced a 

 memoir on the coalfield of Coalbrookdale which was a 

 model of completeness. He spent his holidays examining 

 the surface geology of the district, and descending pits 

 to see the underground structure of the coal seams, and 

 the fossil plants they contained. By minute and patient 

 study he produced a geological map covering an area 

 of about a hundred square miles on the scale of one inch 

 to a mile ; and it was so true to detail that when the 

 Geological Survey mapped the same district later no 

 very important differences were found from this work 

 done by a youth who was practically a City clerk when 

 he accomplished it. 



F. W. Bessel, renowned among astronomers for epoch- 

 making works in many departments of the astronomy 

 of precision, was also a merchant's clerk at Bremen when 

 he calculated the orbit of the celebrated comet of Halley. 

 This youth of twenty, self-taught and occupied with 

 commercial pursuits, solved unaided a problem of high 

 order in mathematical astronomy. He sent his work 

 to H. W. M. Olbers, a physician at Bremen, who for 

 half a century devoted all his spare hours to studies of 

 the skies and the movements of comets. Olbers received 

 Bessel's work with delight, and secured its publication. 

 Two years later he induced Bessel to accept a post as 



