April 12, 1907] 



SCIENCE 



571 



of these shorter intervals with the follow- 

 ing results : 



The average probable error of one of 

 these individual determinations is 0^009. 



All the men except M had observed with 

 a transit circle some years before the ob- 

 servations here discussed were begun, yet 

 with one pair of experienced observers 

 there is quite a decided change in the rela- 

 tive personal equation within a few months. 



For the same periods and the same pairs 

 of observers the following differences be- 

 tween the largest and smallest value for a 

 single night were obtained. 



The general mean of all these numbers 

 is 0^085. 



In attempting to adopt a mean clock 

 correction for the entire night and to test 

 the irregularity of the running of the clock 

 from day to day, this number is certainly 

 large enough to be annoying. If it is due 

 to irregularities in the running of the clock 

 during the night, a new clock is needed. 

 If it is inherent in the old key method of 

 observing transits, something should be 

 done to improve the method. 



To test the clock for periodic daily ir- 

 regularity another Riefler clock, No. 82, 



was compared with it every morning and 

 afternoon and frequently twice during the 

 night from January 19 to February 2, 

 1906, and a second time from February 

 23 to March 9. Between these two periods 

 Riefler No. 70 was dismounted, cleaned and 

 set up again. The probability that any 

 periodic irregularity that might exist in 

 these two clocks would be in the same phase 

 in both series of comparisons is not very 

 large. 



The relative daily rate of the two clocks 

 was in the first period 



0M426 + 0'.00052 t 



and in the second period 



0".0298 + 0".00032 t. 



The residuals arising from the compari- 

 son of the observed and computed differ- 

 ences between the two clocks were then 

 grouped with respect to the tenth of the 

 day at which the clock comparison was 

 made with the following results: 



Januaet 19 to Febettaet 2, 1906 



