W. Oughtred, J. Hadley, G. Graham 95 



descended from them. Crabtree mentions the invention of 

 the micrometer in a letter to Horrocks, and the instrument 

 itself was exhibited by Townley at a meeting of the Royal 

 Society in 1667. Unfortunately it escaped the notice of 

 astronomers until Huygens had constructed a similar but 

 less perfect appliance, and Adrien Angout had produced a 

 micrometer in which Gascoigne's edges were replaced by 

 silk fibres. 



If one had to select the instrument which combines the 

 greatest simplicity with the highest precision, there is little 

 doubt that one's choice would fall on the sextant, the most 

 perfect appliance that has ever been invented. It is mainly 

 used on board ship, but it has been successfully employed 

 in the United States for accurate surveys on land. No one 

 who has not held a sextant in his hand, and seen how, after 

 a few days' practice, he could determine the local time to 

 the tenth part of a second, and the latitude to a few hundred 

 yards, can realize the beauty of the instrument and the sense 

 of power it gives to its user. The inventor, John Hadley, 

 was an instrument maker about whose life very little is 

 known, though the Royal Society recognized his merits by 

 electing him to their Fellowship, and ultimately made him 

 a Vice -President. His instrument, the circle of which only 

 covered 45, and which therefore ought more properly to b<j 

 called an " octant," was first shown to the Royal Society 

 in 1744. Hadley also revived the use of reflecting telescopes ; 

 the construction of which had shown little progress since 

 Newton's time. 



The accuracy of astronomical observations depends in 

 many cases on the excellence of the timekeepers employed 

 to record the instant at which a star passes the centre of 

 the telescopic field of view. Clocks used for the purpose 

 are regulated by the swing of a pendulum acting through a 

 mechanism called an escapement. The first efficient appli- 

 ance of its kind, the anchor escapement, was invented by 

 Robert Hooke, and improved upon by George Graham 

 (1675-1751), an ingenious clockmaker who was generally 

 interested in scientific matters. We owe to him, e.g., the 

 discovery of the diurnal variation of terrestrial magnetism. 

 In the construction of clocks he introduced an important 



