Sir

Jorge Hirsch's index for measuring research achievement, as described in your News Story “Index aims for fair ranking of scientists” (Nature 436, 900; 2005) was used by the geophysicist Harold Jeffreys for recording his cycling prowess, n being the highest number of days on which he had cycled n or more miles. I think he told me, some 35 years ago, that his n was 70 and that he first had the idea from his fellow cyclist, the astrophysicist Arthur Eddington.