374 



NATURE 



{Feb. 14, 1889 



i 



of indicating the space passed over, such as hands, bells, 

 or marks on paper. 



About the eleventh century the motive-power of a 

 stream of water or sand was replaced by a falling weight ; 

 and in the early part of the sixteenth century, Peter Hele, 

 of Niirnberg, substituted a coiled-up spring for the weight. 

 As is often noticed when a foreign clock is wound up, the 

 motive-power of such a spring varies very much as it 

 amcoils. This difficulty was overcome in 1525 by the 

 invention of the fusee, the increased leverage of which 

 compensates for the decreased power of the partly un- 

 coiled spring. In modern going-barrel watches reliance 

 is placed on the careful adjustment of the regulating 

 machinery; while in chronometers a very long spring is 

 wound up so frequently that it never uncoils beyond a 

 very small extent. In 1840, Wheatstone proposed a 

 method of conveying the motion of a standard clock to 

 sev-eral others by a current of electricity, and the electric 

 •current has since been used both as a motive power and 

 as a regulator. 



But little is known about the early methods of regu- 

 lating clocks and watches, but, according to Shakespeare, 

 the result does not seem to have been satisfactory, though 

 some may consider his testimony invalidated by the 

 accompanying libel ("Love's Labour Lost," iii. i, 191). 

 Biron speaks : 



'* What I, I love, I sue, I seek a wife ! 

 A woman, that is like a German clock, 

 Still a-repairing, ever out of frame, 

 And never going aright, being a watch, 

 But being watched, that it may still go right." 



The use of clocks in observatories (1500), and fof 

 finding longitudes at sea (1530), caused a demand for 

 better instruments which was only slowly met. 



Galileo is said to have discovered the isochronism of 

 the pendulum before about 1590, by observing a lamp 

 ■swinging in the Cathedral at Pisa, but the discovery, 

 though used by him, was not published until 1639, and it 

 is doubtful if he applied it to clocks. In 1673, Huyghens 

 proved the isochronism of the cycloidal pendulum, and 

 showed that a pendulum could be caused to vibrate in a 

 cycloid by making the upper portion of the suspending 

 arrangement of steel springs or silk fibres, which wrap 

 round cycloidal cheeks. The cycloidal cheeks are not 

 found to answer in practice, but many makers use one or 

 two parallel steel springs, which causes the bob to de- 

 scribe a curve which falls within the circle, and adds a 

 positive and negative accelerating force at the com- 

 mencement and end of each swing. 



The time,/, of one swing of a simple circular pendulum 

 of length /, at a place where the acceleration due to 

 gravity is g, is — 



+a 



\2 vers e 



+ 



;:^)X^r^H^/i 



where 6 is half the angle through which the swing passes. 

 When 6 is very small, vers 6 vanishes, and the swing is 

 isochronous. If ^ = 2°, the error is about 1/13333, or 

 two seconds in three days. If <9 = 8% vers 6 = 0-00973, 

 and the second and third terms become o'ooi22 and 

 0000003 respectively, or the time of oscillation is about 

 I 833 longer than it would be if the arc were indefinitely 

 small. 



Increase of temperature causes / to become longer, and 

 therefore the clock to go more slowly. This cause of 

 •error is minimized by making the rod of some substance, 

 such as varnished pine, which expands but little, or com- 

 pensated for by some device, such as Graham's mercurial 

 pendulum (1722), Harrison's gridiron pendulum (1725), or 

 Baily's astronomical pendulum, in which expansion away 

 from the axis of suspension is neutralized by an equal 

 expansion towards it, so that the effective length of the 

 pendulum remains unaltered. 



The spring balance-wheel, which consists essentially of 

 a heavy horizontal wheel, to which an oscillating motion 

 is given by a long fine hair- spring, was invented by 

 Hooke in 1660, and perfected by Huyghens in 1674. 

 The difficulty of expansion is got over by dividing the 

 wheel into two semicircles, each attached by one end 

 only to the diameter, and made of two strips of metal of 

 different coefficients of expansion, so that each curves 

 inwards to compensate for the expansion of the radius 

 which carries it. 



Extremely short intervals of time have to be accurately 

 measured in various scientific and practicarresearches,such 

 as those connected with the science of astronomy and the 

 art of gunnery. Many forms of the chronograph usid 

 for this purpose are extremely complicated, but the prin- 

 ciple on which they all act is simple. A cylinder covered 

 with paper is driven round by clockwork, at the rate, say, 

 of a turn per minute, and a point connected with a pen- 

 dulum beating half-seconds divides the circumference 

 into 120 equal spaces. Suppose that by pressing a key 

 an electric current causes a pen to press against the 

 paper. So long as the key is down a line is traced, and 

 the length of it, measured by the half-second pricks, 

 determines how long the key has been down. Usually 

 the cylinder is also caused to move along its axis, so as 

 to throw the two circles of pricks and lines into spirals. 

 It is said that i 1000 of a second can be estimated by 

 this method. 



The need of accurate measures of time has had great 

 effect upon, if it did not absolutely originate, the science 

 of astronomy, and in many of the most important 

 physical laws time is either directly or indirectly a most 

 important factor. Thus, Sir William Thomson has found 

 that, by a long-continued stress, the elastic resilience of 

 a body may diminish, and has proposed for this curious 

 fact tiie name of elastic fatigue ; Harcourt and Esson and 

 other chemists have investigated the circumstances which 

 cause the rate at which certain chemical changes take 

 place to vary ; Berthelot and Uixon have measured the 

 velo:;ity of propagation of explosion waves ; the time 

 taken for sensation to pass through nerve-fibre and for 

 other physiological phenomena has been carefully studied. 



In 1830, Lyell, following up the work of Smith, Hutton, 

 Murchison, and Sedgwick, showed that the history of the 

 earth is continuous, and was governed by the same laws 

 in the past as it is now, and hence that the rates at which 

 changes are now going on are measures of the rates at 

 which they have gone on in the past. Great doubt was 

 thus thrown on the current view that the world has 

 only existed for about 6000 years. For suppose chalk 

 is now being formed at the bottom of the Atlantic at the 

 rate of one-fifth of an inch per annum, and that the 

 chalk formations in England, which are known to be 

 more than 3500 feet thick, were formed in the same way 

 at about the same rate ; the time required for the mere 

 formation of this series of beds would be, not 6000 years, 

 but more nearly 3500 X 12 X 5, or 210,000 years ! And 

 we must reckon, in addition, the time required to form all 

 the other beds below and above the chalk and to bring 

 them all into their present positions and conditions. 



Advanced geologists, then, convinced by the arguments 

 of Lyell, postulated a world history of many millions of 

 years, but their results were ignored or ridiculed by those 

 who had not taken the trouble to investigate the proofs 

 upon which the theory rested. In 1859, the publication 

 of the " Origin of Species " brought this, among many 

 other questions, prominently before the public. The 

 admirable style and careful manner in which facts and 

 theories, old and original, were shown by Darwin to 

 point to the great law of evolution as opposed to the 

 theory of special creations, threw what were previously 

 the arcana of science open to all, and caused the acri- 

 monious discussion of the duration, not only of each 

 living or extinct type, but of the world itself. The fiercest 



