Bishop Wilkins, in his _Mathematical Magic_, in the chapter relating to “such engines as did receive a regular and lasting motion from something belonging to their own frame, whether weights or springs, &c.,” quotes Pancirollus, “taken from that experiment in the multiplication of wheels mentioned in Vitruvius, where he speaks of an instrument whereby a man may know how many miles or paces he doth go in any space of time, whether or no he pass by water in a boat or ship, or by land in a chariot or coach. They have been contrived also into little pocket instruments, by which, after a man hath walked a whole day together, he may easily know how many steps he hath taken.” More curious is “the alarum, mentioned by Walchius, which, though it were but two or three inches big, yet would both wake a man and of itself light a candle for him at any set hour of the night. And those great springs, which are of so great force as to turn a mill (as some have contrived), may be easily applied to more various and difficult labours.” Occasionally, in these old curiosities, we trace anticipations of some of the scientific marvels of the present day. Thus, when the Grand Duke of Tuscany, in 1669, visited the Royal Society at Arundel House, he was shown “a clock, whose movements are derived from the vicinity of a loadstone; and it is so adjusted as to discover the distance of countries, at sea, by the longitude.” The analogy between this clock and the electrical clock of the present day is not a little remarkable. The Journal-book of the Society for 1669 contains many allusions to “Hook’s magnetic watch going slower or faster according to the greater or less distance of the loadstone, and so moving regularly in every posture.” On the occasion of the visit of illustrious strangers, this clock and Hook’s magnetic watches were always exhibited as great curiosities.[4] --------------------- Footnote 2: _Vulgar and Common Errors_, book iv. chap. xii. Footnote 3: _Time and Timekeepers._ By Adam Thomson, 1842. Footnote 4: See Weld’s _History of the Royal Society_, vol. i. pp. 220, 221. ---------------------------- PERIODS OF REST. The terrestrial day, and consequently the length of the cycle of light and darkness, being what it is, we find various parts of the constitution both of animals and vegetables which have a periodical character in their functions, corresponding to the diurnal succession of external conditions; and we find that the length of the period, as it exists in their constitution, coincides with the length of the natural day.