  The continuity with Huygens's theory of centrifugal force is important in another respect. In his brief defense of the first two laws of motion Newton remarks, “What has been demonstrated concerning the times of oscillating pendulums depends on the same first two laws and first two corollaries, and this is supported by daily experience with clocks” [P, 424]. In Huygens's Horologium Oscillatorium, the only place any counterpart to the second law surfaces is in the theory of centrifugal force and uniform circular motion. The theory Huygens presents extends to conical pendulums, including a conical pendulum clock that he indicates has advantages over simple pendulum clocks. In the 1670s Newton had used a conical pendulum to confirm Huygens's announced value of the strength of surface gravity as measured by simple cycloidal and small-arc circular  pendulums.[26] (Huygens himself had measured the strength of surface gravity with a conical pendulum, obtaining the same value to four significant figures as he had obtained with simple  pendulums.[27]) The precise agreement between these two theory-mediated measures of surface gravity, one of them predicated on Newton's first two laws of motion and the other not, in fact constituted the strongest evidence for the first two laws at the time the Principia was first published. For, the simple pendulum measure was known to be stable and accurate into the fourth significant figure. The evidence in hand for the first two laws, taken as a basis for measuring forces, was thus much stronger than has often been appreciated.