3 o 4 THE POPULAR SCIENCE MONTHLY. 



TIME-KEEPING IN PARIS. 



By EDMUND A. ENGLEE, 



WASHINGTON UNIVERSITY, ST. LOUIS. 



MANY of the discoveries of science which at the time are regarded 

 merely as refinements very interesting, bnt without practical 

 value sooner or later find their special uses in supplying wants before 

 unfelt. It is but one of the evidences of the advance of civilization 

 that exact methods of dividing and measuring time are now in de- 

 mand, not only by scientists and professional men as formerly, but 

 by persons in the most ordinary pursuits of life. To railroad-men 

 and watch-makers as a matter of necessity, to manufacturers and 

 business-men as a matter of economy, and to individuals as a mat- 

 ter of convenience, it has come to be highly important to know 

 what is the exact time of day to the second, in circumstances where 

 half a century ago it would have quite sufficed to know the minute 

 or even the hour. This may be due to the increased value of time 

 when measured by the number of events or the magnitude of opera- 

 tions which modern ingenuity is capable of crowding into a given 

 interval ; there can be no doubt that a second to-day records a greater 

 stride in the world's progress than did many hours in the days of our 

 ancestors. Of so great importance, for many evident reasons, has the 

 knowledge of the exact time become, that much thought of some of 

 the best heads has been devoted to methods of ascertaining it and 

 making it available by distribution for public use. 



The methods of obtaining the exact time by astronomical observa- 

 tions have long been well established, and are, except in minor details, 

 the same in all parts of the world. It will here be sufficient to say, in 

 explanation of the usual method, that time is determined by observing 

 the transit, over the meridian, of stars or other heavenly bodies 

 whose position is known by previous calculation verified by repeated 

 observation. The difference between the time of the calculated me- 

 ridian passage and the time indicated by the clock when the star was 

 observed to pass the meridian is the error of the clock. The face- 

 reading of the clock at the instant of transit corrected for this error 

 is the exact time at that instant.* 



* It is, perhaps, needless to say that the operation of taking time by the transit in- 

 strument is really far more complicated than would appear from the description above; 

 but the difficulties arise only from mechanical or physical imperfections, or from uncertain 

 or changing conditions. Thus, corrections must always be made in nice work for errors 

 in the instrument or its setting such as the level, azimuth, and collimation corrections 

 for personal equation of the observer, and for aberration ; these corrections, however, 

 ii dy aid the observer in ascertaining the exact instant when the star actually crossed his 

 meri Mai) and do not in any way affect the principle already given. For a full account 

 of the methods of making these corrections, the reader is referred to Chauvenet's 

 " Astronomy." 



