I find it curios that Newton did not indicate how to measure force directly. Hook's law dates to around 1660, some thirty year before Newton's Principia , which refers to some of Hook's astronomical measurements but not to Hook's law. Since Newton and Hook were contemporaries and occasionally corresponded (with no love lost between them), Newton presumably knew Hook's law. Consistent with Newton's Principia, Euler's influential works Mechanica (1736), on point mechanics, and Theoria Motus Corporum Solidorum seu Rigidorum (1765) , on rigid bodies, do not seem to invoke Hookes's law or any other method to measure forces, and only briefly discuss distance and time measurements. (It is perhaps of interest to note that the first commercial spring balance may have been made in England around 1760, long after Hook's discovery.) In a cursory examination of the physics literature on F=ma , the earliest reference I have found that treats force as a primary quantity is Maxwell's 1876 proposal to measure force with elastic threads.