price of corn; the seasons of fish, fowl, and insects; the physicians of the society are urged to give account of epidemic diseases; the effect of weather upon medicine; due consideration of the weekly and annual bills of mortality in London; that ‘instead of the vanity of prognosticating he could wish we would have the patience for some years of registering past times, which is the certain way of learning to prognosticate.’ He speaks of self-registering anemometers, thermometers, and hygrometers as being practicable. Many other things he might suggest which, if the design be once begun, he would most willingly submit upon occasion. He exhorts his hearers ‘not to flag in the design, since in a few years, at the beginning, it will hardly come to any visible maturity. … The Royal Society should plant crabstocks for posterity to graft on’ (ib. p. 221).
The mere enumeration of the subjects brought by Wren before the society occupies more than three pages of the ‘Parentalia.’ In 1663 he suggested the self-registering weathercock, designed to record the various meteorological variations which are now performed by photography (see Birch, i. 341); and in 1666 an exceedingly simple form of level ‘for taking the horizon every way in a circle,’ the main principle of which was a bowl having the lip accurately turned and provided with a ball-and-socket joint, so that when a drop of quicksilver was adjusted to the centre, the lip should lie level in every direction. He had probably found the want of some such instrument in his survey of London after the fire. In 1667 he reported his experiments on the force of gunpowder in lifting weights and bending springs; also a means of curing smoky chimneys. In the same year he showed methods of taking astronomical measures to seconds, and his pair of telescopes jointed for the same purpose. In 1668 he presented papers and showed experiments to illustrate the laws of motion deduced by him several years before from careful and varied observation of the effects produced by the collision of suspended balls under different conditions—equal, unequal, direct, and differential velocities and momentum. On this subject Newton, in the ‘Principia’ (p. 20), writes: ‘From these laws [i.e. the laws of motion] Dr. Christopher Wren, knight; John Wallis & Christian Huyghens, who are beyond comparison the leading geometers of this age, arrived at the laws of the collision and mutual rebound of two bodies; but their truth was proved by Dr. Wren by experiments on suspended balls in the presence of the Royal Society.’
In 1670 Wren showed to the society an improvement in the machinery for winding up weights by ropes from great depths (Royal Society Register, bk. iv. p. 99, with diagram). An identical arrangement has recently been brought into use. In 1679, Newton having written to the Royal Society to propose that an experiment should be made to give ocular proof of the earth's diurnal motion by letting a weight fall from a considerable height, which ought to fall to the eastward of the plumb-line, Wren proposed a still more effective test by ‘shooting a bullet upward at a certain angle from the perpendicular round every way’ to see if the bullet would fall in a perfect circle around the barrel. Bishop Sprat, speaking of the labours of the Royal Society in 1667, selects Wren's name alone for special mention. He refers to ‘his doctrine of motion’ which ‘Descartes had before begun, having taken up some experiments of this kind on conjecture and made them the first foundations of his whole system of nature, but some of his conclusions seeming very questionable because they were only derived from the gross trials of balls meeting one another at tennis, billiards, &c., Dr. Wren produced before the society an instrument to represent the effects of all sorts of impulses made between two hard globous bodies whether of equal or different bigness and swiftness, and following or meeting each other.’ Then he adds: ‘And because the difficulty of a constant observation of the air by night and day seemed invincible, he therefore devised a clock to be annexed to the weathercock, so that the observer, by the traces of a pencil on paper, might certainly conclude what had blown in his absence. After a like manner he contrived a thermometer to be its own register. He has contrived an instrument to measure the rain that falls, and devised many subtil ways for the easier finding the gravity of the atmosphere, the degrees of drought and moisture.’ He mentions also new discoveries in the pendulum—‘that in one descent and ascent it moves unequally in equal times, and that from the pendulum may be produced a natural standard for measure.’ Wren saw reason, however, to give up the latter proposal when it was found that the length of the degree varied in different latitudes. Dr. Sprat proceeds: ‘He has invented many ways to make astronomical observations more accurate and easy … has made two telescopes to open with a joint like a sector, by which distances can be taken to half minutes … devices to telescopes for taking small distances and diameters to seconds, apertures to take in more or less light the better to fit glass to crepusculine observations; has added much to the
