Jan. 23, 1885] 



♦ KNOWLEDGE . 



69 



Were it possible to suddenly destroy all clocks and watches 

 in any givt n centre vi population among civilised nations,' 

 while uU other surioumiings of modem development re- 

 mained as before, we can scarcely conceive of the endless 

 confusion that would arise. Only by contemplating the 

 results of such a catastrophe can we fully understand what 

 an important part the knowledge of accurate time plays in 

 our every -day all'airs. 



Man shares with the inferior animals the knowledge and 

 the use of the simplest and earliest division of time into 

 day and night, and in a more restricted sense into sea-ons. 

 The division ot" the day into minor parts has been developed 

 by man as necessity or convenience required. It has not 

 been many years since wntches were made with hour-hands 

 only, and the general use of the finer di\ isions into miniitts [ 

 and secoi ds is almost entirely the outgrowth cf the require- 

 ments of modern civilisation. Astronomical time-keeping 

 is not here considered. By the Babylonian system of 

 dividing the day, which was used by the Jews and other 

 Oriental nations, the time between sunrise and sunset was 

 portioned into twelve equal parts at all seasons of tlie year, 

 the hour varying in length with the season. If this method 

 of division still prevailed, the hours in New York city 

 •would vary in length from about forty-six to about seventy- 

 five of our present minutes. In tbe Arctic regions the 

 inapplicability of this system to general use would reach 

 its climax of absurdity. 



The general facts upon which all systems of timekeeping 

 are based are commonly understood, but the details are 

 seldom referred to. 



The most primitive kind of timepiece is a sundial. 

 Reduced to its simplest form, a sun-dial consists of a 

 straight pcle erected upon a permanently fixed circular 

 plate, the shadow of the pole indicating midday when it 

 coincides with a line drawn due north from the base of the 

 pole, the pole being erected upon a line parallel with the 

 axis of the earth. The other hours of the day arc 

 indicated by marks upon the circular plate upon which the 

 shadow of the pole successively falls. 



When the sun dial was invented cannot be stated. It 

 was of very ancient origin, and is mentioned in the thirty- 

 eighth chapter of Isaiah. The clepsydra, or water-clock, 

 and the hour glass, although very ancient, must from their 

 nature have been invented subsequent to the sun-dial. 

 But sun-dials, of which there are about a dozen ditlerent 

 kinds, although common, were never in such general use as 

 clocks are in modern times, and were philosophical rather 

 than popular instruments. The clock was invented 

 about 1.379, and the pendulum as a regulating power in 

 1657. 



The rapid development of the science of horology in the 

 present century has been almost coincident with and in no 

 small degree dependent upon the constiiiction and operation 

 of railway and telegraph lines. The needs of the-e great 

 engines of modem civilisation created a general demand for 

 exactness in time-reckoning which had never existed before. 

 It was required both for the use of their employ's and for 

 the public which patronised their lines. 



A sundial being stationary, when properly made and 

 adjusted, exhiVjited solar time correctlv, and a watch regu- 

 lated from the dial by the equation of time would also be 

 correct for that particular spot, Vjut the moment the owner 

 of the watch began to move east or west, his timepiece no 

 longer registered correct time, and when he travelled with 

 the speed of a railway train the error was rapidly 

 exaggerated. 



The necessity for exactness before mentioned, and the 

 impossibility of adhering to locil time, early attracted the 

 attention of railway managers, and caused them much per- 



plexity and annoyance. With the rapid construction of 

 railway lines, the commingling of tlic various local standards 

 soon became decidedly intrieat(\ Travellers were greatly 

 inconvenienced by the lack of knowlcdgo of tlio standard 

 upon which the time of trains us advertised was based, and 

 to such the situation was full of difficulties. Some of 

 these difficulties were stated in an "open letter" published 



in"TheC(!ntury" 

 for September, 

 1883. The sub- 

 ject in its prac- 

 tical aspect also 

 attracted the 

 attention of 

 scientists and 

 scientific socie- 

 ties. It became 

 a prominent topic 

 of discussion at 

 meetings of the 

 American Metro- 

 logical Society, 

 the Association 

 for the Advance- 

 ment of Science, 

 and the Society 

 of Civil Engi- 

 neers. Although 

 astronomers use 

 sidereal time, 

 based upon the 

 position of the 

 stars, and not of 

 the sun, in com- 

 mon with many 

 other scientists 

 they were gene- 

 rally warmly in- 

 terested in the 

 subject. 



Tbe local time 

 kept by clocks 

 is an average of 

 solar time, and 

 is properly desig- 

 nated " mean 

 time," as distin- 

 guished from the 

 variable time 

 shown by the 

 sun - dial. No 

 clock or watch 

 can be made ta 

 keep the time as 

 shown by the 

 sxindial, and this- 

 new system of 

 time- keeping, 

 therefore, be- 

 came necessary 

 when clocks and 

 watches were 

 invented. The relation between mean and apiiarent. 



time, and what is meant by the " equation of time," 

 may be seen at a glance by reference to the accom- 

 panying diagram. Mean time being represented by 

 the right line graduated for the several months of 

 the J ear, the variation of apparent time is shown 

 by the curved line entwined around it. In otlier 



jiu;. 1. — Diaj^ram sliowiiiff comparison of 

 mean (or clock) time witli solar (or ap- 

 parent) time, at the several seasons of the 

 year. The perpendicular central line re- 

 presents mean time, and the curved line 

 solar time, at mean noon. 



