58 Britain's Heritage of Science 



Halley (1656-1742), who plays an important and interesting 

 part in the history of science. The son of a soap-boiler, 

 and educated at St. Paul's School and Queen's College, 

 Oxford, Halley, at the early age of nineteen, invented an 

 improved method for determining the elements of planetary 

 orbits. Finding that more accurate measurements of the 

 positions of fixed stars were necessary to the progress of 

 astronomy, and that this task was being satisfactorily 

 carried out at Greenwich for the northern heavens, he planned 

 a journey to catalogue some of the southern stars. Through 

 the good offices of the East India Company he obtained a 

 passage to St. Helena, but disappointed with the weather 

 conditions, he returned to England after having registered the 

 positions of about 300 stars. He was an ardent supporter 

 of Newton, and it was in great part due to Halley's efforts 

 that the " Principia " were published. 



Halley was the first to take a comprehensive view of the 

 subject of Terrestrial Magnetism. Some advances had been 

 made in that subject since Gilbert's time, notably by Edward 

 Gunter (15811621), one of the early professors of astronomy 

 at Gresham College, who had taken regular observations of the 

 angle between the direction in which the magnetic needle 

 sets and the geographical north, and found a progressive change 

 in its amount. When the first observation was taken in 

 England, the needle pointed to the east of north; in 1657 

 it pointed due north, and the declination then gradually 

 increased towards the west. Henry Gellibrand (1597-1637) 

 continued and extended these observations. 



In order to explain these slow changes called " the 

 secular variation of terrestrial magnetism," Halley formed 

 the theory that the earth is divided into an outer crust 

 and an inner nucleus, each part possessing its own inde- 

 pendent magnetic poles. A fluid layer was supposed to 

 separate the shell and the core, and Halley imagined the 

 latter to revolve with a slightly smaller velocity than the 

 former about a common axis. It is easy to see that if 

 we accept the premises, a suitable adjustment of the mag- 

 netic axes of the inner and outer parts of the earth would 

 lead to a slow revolution of the resulting magnetic axis. 

 This theory was recently renewed and extended by Henry 



