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In his De Magnete William Gilbert claims that it is possible to make a magnet by hammering a red-hot bar of wrought iron arranged north–south on an anvil. This is contrary to modern ideas concerning the ‘Curie temperature’ (770°C for carbon steel), and to the recognized susceptibility of steel magnets to mechanical abuse. It has proved impossible to replicate Gilbert's technique experimentally. Only lengthy cold hammering of hardened carbon steel specimens on a large ferrous anvil produced weak permanent magnets.

Robert Hooke is commonly thought of as the inventor of ‘Hooke's joint’ or the ‘universal joint’. However, it is shown that this flexible coupling (based on a four-armed cross pivoted between semicircular yokes attached to two shafts) was in fact known long before Hooke's time but was always assumed to give an output exactly matching that of the input shaft. Hooke carefully measured the relative displacements of the two axes, and found that if one were inclined to the other, uniform rotation of the input produced a varying rate of rotation of the output. He also recognized that this variable rate exactly corresponded to the movement of the shadow of a gnomon across the face of a sundial, as generated by the projection of the uniform motion of the Sun around an inclined polar axis. He therefore proposed that a ‘mechanical sundial’ might be made by coupling a 24-hour clock movement (with its hour shaft at the appropriate inclination) to a pointer via a universal joint. This proposal has been investigated both practically and mathematically, and shown to be valid. Hooke's studies of the universal joint caused it to be identified with his name, and it has ultimately proved far more important as a rotary coupling than as a sundial analogue. More complex versions subsequently designed by Hooke included provision for two basic couplings to be linked by an intermediate shaft. With appropriate setting of phase and shaft angles this ‘double Hooke's joint’ could annul the variable output velocity characteristic of the single universal. It has proved invaluable for modern automotive transmissions.

Sir, To perform my late promise to you, I shall without further ceremony acquaint you, that in the beginning of the Year 1666 (at which time I applyed my self to the grinding of Optick glasses of other figures than Spherical,) I procured me a Triangular glass-Prisme, to try therewith the celebrated Phaenomena of Colours . . . THUS did Isaac Newton (1642-1727, F.R.S. 1672, P.R.S. 1703-1727) introduce his celebrated letter to Oldenburg on the prismatic spectrum and the compound nature of white light (1). Its production was stimulated by the favourable reception given by the Royal Society to his reflecting telescope (2), which led to his election as a Fellow on 11 January 1672 (3). The letter appeared, almost unaltered, in the Philosophical Transactions for 19 February 1672 (4), and was Newton’s first scientific paper. It was soon joined by replies to objections raised by Pardies, Hooke, Huygens, Linus and Lucas (5), but his second major paper on light and colours— although read in parts to the Society during 1675 and 1676— was never published in the Phil. Trans .

THE construction of accurate paraboloidal mirrors for reflecting telescopes has been the subject of much attention from physicists, practical opticians, and historians of science. However, the paraboloidal primary must be combined with a secondary mirror to give any form of working telescope other than the Herschelian design, and the quality of this secondary is every bit as important to the definition of the final image. In the Newtonian reflector the secondary mirror takes the form of a plane surface of elliptical outline. Historical specula therefore provide a practical record of attempts to achieve a perfectly flat solid surface— an undertaking which has often proved more difficult than making mirrors of a curved figure. Plane surfaces were subsequently required in many optical instruments besides telescopes, such as prisms and diffraction gratings. Then, in the early decades of the nineteenth century, another field of application was created when Maudslay, Whitworth and others realized the need for accurately flat surfaces in precision engineering and toolmaking. To test their products and ensure interchangeability they had to produce plane surfaces upon cast iron ‘surface plates’ and the hardened steel anvils of screw micrometers.

Most biographies of Isaac Newton (1642-1727; F.R.S. 1672; P.R.S. 1703-1727) mention a water clock that he is reported to have constructed as a boy when living at Woolsthorpe Manor in Lincolnshire. The source of this and other anecdotes concerning Newton’s boyhood (1) is William Stukeley, who made it his business to collect such memories upon retiring to live in nearby Grantham. In a letter (2) dated 26 June 1727 ( i.e. only some three months after Newton’s death) he writes: Moreover Sir Isaac’s water clock is much talked of. This he made out of a box he begged of Mr. Clark’s (his landlord) wife’s brother. As described to me, it resembled pretty much our common clocks and clock-cases, but less; for it was not above four feet in height, and of a proportionable breadth. There was a dial plate at top with figures of the hours. The index was turned by a piece of wood, which either fell or rose by water dropping. This stood in the room where he lay, and he took care every morning to supply it with its proper quantity of water; and the family upon occasion would go to see what was the hour by it. It was left in the house long after he went away to the University.