[94] Introduction to the Commentaries on the Motions of Mars.
[95] Substantially the filar micrometer of modern astronomy.
[96] Galilei, at the end of his life, appears to have thought of contriving a pendulum with clockwork, but there is no satisfactory evidence that he ever carried out the idea.
[97] In modern notation: time oπf oscillation = 2π√(l∕g).
[98] I.e. he obtained the familiar formula (v2)∕r, and several equivalent forms for centrifugal force.
[99] Also frequently referred to by the Latin name Cartesius.
[100] According to the unreformed calendar (O.S.) then in use in England, the date was Christmas Day, 1642. To facilitate comparison with events occurring out of England, I have used throughout this and the following chapters the Gregorian Calendar (N.S.), which was at this time adopted in a large part of the Continent (cf. chapter II., [§ 22]).
[101] From a MS. among the Portsmouth Papers, quoted in the Preface to the Catalogue of the Portsmouth Papers.
[102] W. K. Clifford, Aims and Instruments of Scientific Thought.
[103] It is interesting to read that Wren offered a prize of 40s. to whichever of the other two should solve this the central problem of the solar system.