TIME- KEEPING IN PARIS. 



305 



But experience has shown that no clock, however fine its mechan- 

 ism, will run without change of error ; so that, although for a par- 

 ticular instant the error of a clock is known by astronomical observa- 

 tion, it is by no means certain what will be its error for any subse- 

 quent instant. Its error for this instant can be determined with pre- 

 cision only by another observation. An approximation to its error al 

 any instant can, however, be obtained by simple calculation, based 

 upon two assumptions : first, that the change in error between the 

 last two (or any previous two) observations was uniformly distributed 

 over the interval of time between those observations, thus making it 

 possible to determine a rate of change; second, that the rate of 

 change in error since the last observation has been uniformly the 

 same as during the previous interval. The reliability of this approxi- 

 mation is evidently entirely dependent upon an empirical knowledge 

 of the clock. Cloudy weather sometimes makes this method the only 

 resource. 



In order that a clock should be used as an indicator of time, it is 

 not enough that its error at every instant should be known ; its error 

 imtst be continually corrected so that its face-reading shall always 

 indicate true time. And, in order that a clock should be used as 

 a distributor of time, it must be provided with apparatus, distinct 

 from the mechanism which keeps the time and in no way inter- 

 fering with it, which is capable of sending time to other clocks. The 

 methods and instruments in use in Paris for the accomplishment of 

 these two objects will be described in this paper. 



Fig. 1. Regulator of Paris Observatory Pendulum Contact-Plates. 



At the Paris Observatory a very fine standard clock or astronom- 

 ical regulator is kept running on correel mean time by transit obser- 

 vations, being provided with the most approved self-compensating 



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VOL. XX. 



